Beer spoilage bacteria and hop resistance
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International Journal of Food Microbiology 89 (2003) 105 – 124
www.elsevier.com/locate/ijfoodmicro
Review article
Beer spoilage bacteria and hop resistance
Kanta Sakamoto a,b,*, Wil N. Konings b
a
Fundamental Research Laboratory, Asahi Breweries, Ltd. 1-21, Midori 1-chome, Moriya-shi, Ibaraki 302-0106, Japan
b
Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30,
9751NN Haren, The Netherlands
Received 4 September 2002; accepted 15 March 2003
Abstract
For brewing industry, beer spoilage bacteria have been problematic for centuries. They include some lactic acid bacteria such
as Lactobacillus brevis, Lactobacillus lindneri and Pediococcus damnosus, and some Gram-negative bacteria such as
Pectinatus cerevisiiphilus, Pectinatus frisingensis and Megasphaera cerevisiae. They can spoil beer by turbidity, acidity and the
production of unfavorable smell such as diacetyl or hydrogen sulfide. For the microbiological control, many advanced
biotechnological techniques such as immunoassay and polymerase chain reaction (PCR) have been applied in place of the
conventional and time-consuming method of incubation on culture media. Subsequently, a method is needed to determine
whether the detected bacterium is capable of growing in beer or not. In lactic acid bacteria, hop resistance is crucial for their
ability to grow in beer. Hop compounds, mainly iso-a-acids in beer, have antibacterial activity against Gram-positive bacteria.
They act as ionophores which dissipate the pH gradient across the cytoplasmic membrane and reduce the proton motive force
(pmf). Consequently, the pmf-dependent nutrient uptake is hampered, resulting in cell death. The hop-resistance mechanisms in
lactic acid bacteria have been investigated. HorA was found to excrete hop compounds in an ATP-dependent manner from the
cell membrane to outer medium. Additionally, increased proton pumping by the membrane bound H+-ATPase contributes to
hop resistance. To energize such ATP-dependent transporters hop-resistant cells contain larger ATP pools than hop-sensitive
cells. Furthermore, a pmf-dependent hop transporter was recently presented. Understanding the hop-resistance mechanisms has
enabled the development of rapid methods to discriminate beer spoilage strains from nonspoilers. The horA-PCR method has
been applied for bacterial control in breweries. Also, a discrimination method was developed based on ATP pool measurement
in lactobacillus cells. However, some potential hop-resistant strains cannot grow in beer unless they have first been exposed to
subinhibitory concentration of hop compounds. The beer spoilage ability of Pectinatus spp. and M. cerevisiae has been poorly
studied. Since all the strains have been reported to be capable of beer spoiling, species identification is sufficient for the
breweries. However, with the current trend of beer flavor (lower alcohol and bitterness), there is the potential risk that not yet
reported bacteria will contribute to beer spoilage. Investigation of the beer spoilage ability of especially Gram-negative bacteria
may be useful to reduce this risk.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Beer; Spoilage; Bacteria; Detection; Hop; Antibacterial; Resistance
* Corresponding author. Fundamental Research Laboratory, Asahi Breweries, Ltd. 1-21, Midori 1-chome, Moriya-shi, Ibaraki 302-0106,
Japan. Tel.: +81-297-46-1504; fax: +81-297-46-1506.
E-mail address: kanta.sakamoto@asahibeer.co.jp (K. Sakamoto).
0168-1605/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0168-1605(03)00153-3106 K. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124
1. Introduction detecting beer spoilage bacteria in breweries is still
traditional incubation on culture media. A number of
Beer has been recognized for hundreds of years as selective media have been developed since Louis
a safe beverage. It is hard to spoil and has a remark- Pasteur published in 1876 ‘Études sur La Biére
able microbiological stability. The reason is that beer (Studies on beer)’ (Pasteur, 1876). It usually takes a
is an unfavorable medium for many microorganisms week or even longer for bacteria to form visible
due to the presence of ethanol (0.5 – 10% w/w), hop colonies on plates or to increase the turbidity in
bitter compounds (approximately 17 –55 ppm of iso- nutrients broths. Consequently, the products are often
a-acids), the high content of carbondioxide (approx- already released for sale before the microbiological
imately 0.5% w/w), the low pH (3.8 –4.7), the ex- results become available. If a beer spoilage bacterium
tremely reduced content of oxygen ( < 0.1 ppm) and is then detected and identified in the beer product it
the presence of only traces of nutritive substances needs to be recalled from the market. This will cause
such as glucose, maltose and maltotriose. These latter serious commercial damages to the brewery. Most
carbon sources have been substrates for brewing yeast microbiologists have focused on developing more
during fermentation. As a result, pathogens such as specific and rapid methods for the detection of beer
Salmonellae typhimurium and Staphylococcus aureus spoilage bacteria than using traditional culture meth-
do not grow or survive in beer (Bunker, 1955). ods. However, a method is needed to determine
However, in spite of these unfavorable features, a whether the bacteria detected are capable of growing
few microorganisms still manage to grow in beer. in beer or not. Up to date, the only available method is
These, so-called beer spoilage microorganisms, can the so-called ‘forcing test’ in which the detected
cause an increase of turbidity and unpleasant sensory bacterium is re-inoculated and incubated in beer. It
changes of beer. Needless to say that these changes usually takes 1 month or even longer to detect visible
can affect negatively not only the quality of final turbidity in the inoculated beer, meaning that this
product but also the financial gain of the brewing method is not very practical. A rapid method to
companies. predict beer-spoiling ability is therefore urgently
A number of microorganisms have been reported needed and the development of such a method will
to be beer spoilage microorganisms, among which be essential for understanding the nature of the beer-
both Gram-positive and Gram-negative bacteria, as spoiling ability.
well as so-called wild yeasts. Gram-positive beer Among the components of beer, hop compounds
spoilage bacteria include lactic acid bacteria belong- have received a lot of attention for reason of their
ing to the genera Lactobacillus and Pediococcus. preservative values and their bitterness. For centuries
They are recognized as the most hazardous bacteria it was generally believed that hops protect beer from
for breweries since these organisms are responsible infection by most organisms, including pathogens, but
for approximately 70% of the microbial beer-spoilage it was only in the 20th century that Shimwell
incidents (Back, 1994). The second group of beer (1937a,b) showed that hop compounds only inhibit
spoilage bacteria is Gram-negative bacteria of the growth of Gram-positive bacteria and not of Gram-
genera Pectinatus and Megasphaera. The roles of negative bacteria. His findings had a great impact
these strictly anaerobic bacteria in beer spoilage have because many pathogens such as Salmonella species
increased since the improved technology in modern are Gram-negative bacteria. Feature(s) such as low pH
breweries has resulted in significant reduction of and alcohol content undoubtedly have a negative
oxygen content in the final products. Wild yeasts do effect on growth of these pathogens in beer. Among
cause less serious spoilage problem than bacteria but Gram-positive bacteria, some species of lactic acid
are considered a serious nuisance to brewers because bacteria are less sensitive to hop compounds and are
of the difficulty to discriminate them from brewing able to grow in beer. Insight in the mechanism of
yeasts. resistance of lactic acid bacteria to hop compounds is
Considerable effort has been made by many mi- important for understanding their beer-spoiling ability.
crobiologist to control microbial contamination in The antibacterial activity of hop compounds and the
beer. The most commonly used method today for hop resistance of lactic acid bacteria have beenK. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124 107
investigated. On the other hand, little studies have Table 1
been done on the beer-spoiling ability of Gram- Beer spoilage bacteria
negative bacteria. Rod-shaped Cocci
This report reviews the currently available infor- Gram-positive Lactobacillus spp. Pediococcus spp.
mation about beer spoilage bacteria, their growth and bacteria Lb. brevis P. damnosus
Lb. brevisimilis P. dextrinicus
spoilage activity in beer.
Lb. buchneri P. inopinatus
Lb. casei
Lb. coryneformis Micrococcus sp.
2. Beer spoilage bacteria Lb. curvatus M. kristinae
Lb. lindneri
Lb. malefermentans
Beer is a poor and rather hostile environment for
Lb. parabuchneri
most microorganisms. Its ethanol concentration ranges Lb. plantarum
from 0.5% to 10% (w/w) and is usually around 4– 5%. Gram-negative Pectinatus spp. Megasphaera sp.
Beer is usually acid with pH’s ranging from pH 3.8 to bacteria P. cerevisiiphilus M. cerevisiae
4.7, which is lower than most bacteria can tolerate for P. frisingensis
P. sp. DSM20764
growth. Furthermore, the high carbondioxide concen-
Selenomonas sp. Zymomonas sp.
tration (approximately 0.5% w/w) and extremely low S. lacticifex Z. mobilis
oxygen content ( < 0.1 ppm) makes beer a near to Zymophilus sp.
anaerobic medium. Z. raffinosivorans
Beer also contains bitter hop compounds (approx-
imately 17 – 55 ppm of iso-a-acids), which are toxic,
especially for Gram-positive bacteria. The concen- of species and genera of Gram-positive bacteria. Only
trations of nutritive substances, such as carbohy- a few of these lactic acid bacteria are beer-spoiling
drates and amino acids, are very low since most organisms. Most hazardous for the brewing industry
have been consumed by brewing yeasts during are those belonging to the genera Lactobacillus and
fermentation. Pediococcus. In the period 1980– 1990, 58– 88% of
Only a few bacteria are able to grow under such the microbial beer-spoilage incidents in Germany
inhospitable conditions and are able to spoil beer (see were caused by lactobacilli and pediococci (Back et
Table 1). These bacteria include both Gram-positive al., 1988; Back, 1994). Also in Czech all beer spoilage
and Gram-negative species. Gram-positive beer spoil- bacteria detected in the breweries belonged to lactic
age bacteria belong almost always to the lactic acid acid bacteria (Hollerová and Kubizniaková, 2001).
bacteria. They are regarded as most harmful for The situation in other countries seems to be similar
brewing industry and are the cause of most of bacte- although for commercial reasons little statistical in-
rial spoilage incidents. Only a few Gram-negative formation has been supplied. These lactic acid bacte-
bacteria are known to cause beer spoilage. Some of ria spoil beer by producing haze or rope and cause
these belong to the acetic acid bacteria and had unpleasant flavor changes such as sourness and atyp-
received most attention. Today, these aerobic bacteria ical odor.
do not present a serious problem in beer spoilage
anymore since improved brewing technology has led 2.1.1. Lactobacilli
to a drastic reduction of the oxygen content in beer. The genus Lactobacillus is the largest genus of
Instead, strictly anaerobic bacteria, typically Pectina- lactic acid bacteria and includes numerous species.
tus spp. and Megasphaera cerevisiae, have become They are widely used in various fermentation pro-
serious beer spoilage bacteria. cesses, including food products such as beer, wine,
yoghurt and pickles. In contrast to the general believe
2.1. Gram-positive bacteria that all lactobacilli can grow in beer, only a few
species have been reported to be capable of beer
Almost all the beer spoilage Gram-positive bacteria spoilage (Rainbow, 1981; Priest, 1987, 1996; Jes-
belong to lactic acid bacteria. They are a large group persen and Jakobsen, 1996). Lactobacillus brevis108 K. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124
appears to be the most important beer spoiling Lac- (0.15 ppm) than of lactic acid (300 ppm) (Hough et
tobacillus species and is detected at high frequency in al., 1982). Diacetyl is also produced during normal
beer and breweries. More than half of the bacterial fermentation by yeast and too high levels of diacetyl
incidents were caused by this species (Back et al., are produced when yeast is not properly removed
1988; Back, 1994; Hollerová and Kubizniaková, from young beer (Inoue, 1981). The pathway of
2001). Lb. brevis is an obligate heterofermentative diacetyl formation by lactic acid bacteria has been
bacterium. It is one of the best-studied beer spoilage studied in detail (Speckman and Collins, 1968, 1973;
bacteria and grows optimally at 30 jC and pH 4 –6 Jönsson and Pettersson, 1977).
and is generally resistant to hop compounds. It is Lactobacillus brevisimilis (Back, 1987), Lb. mal-
physiologically versatile and can cause various prob- efermentans, Lb. parabuchneri (Farrow et al., 1988),
lems in beer such as super-attenuation, due to the Lb. collinoides and Lb. paracasei subsp. paracasei
ability to ferment dextrins and starch (Lawrence, (Hallerová and Kubizniaková, 2001) have also been
1988). The antibacterial effects of hop compounds reported to be beer spoilage species.
and the mechanism(s) responsible for hop resistance Recently, few additional newly discovered species
have been studied in detail in this species (Simpson, of lactobacilli have been added to the list of beer
1991, 1993a,b; Simpson and Smith, 1992; Fernandez spoilers. According to its 16S rRNA gene sequence, a
and Simpson, 1993; Simpson and Fernandez, 1994; strain called BS-1 is related to Lb. coryneformis, but
Sami et al., 1997a,b, 1998; Sakamoto et al., 2001, its narrow fermentation pattern (only limited to glu-
2002; Suzuki et al., 2002). cose, mannose and fructose) differs significantly not
The second most important beer spoiling lactoba- only from that of Lb. coryneformis but also from other
cillus, Lactobacillus lindneri is responsible for 15 – Lactobacillus spp. (Nakakita et al., 1998). Two other
25% of beer-spoilage incidents (Back et al., 1988; novel lactobacillus species were found in spoiled beer
Back, 1994). The physiology of Lb. lindneri is very with significantly different taxonomical properties
similar to that of Lb. brevis and only recently has Lb. from those of other Lactobacillus spp. (Funahashi et
lindneri been recognized on the basis of its 16S rRNA al., 1998). One species, LA-2, with a 16S rRNA gene
gene sequence as a phylogenetically separate species sequence 99.5% similar to that of Lb. collinoides, has
in the genus Lactobacillus (Back et al., 1996; Anon., a strong beer soiling ability. The other species, LA-6,
1997). Lb. lindneri is highly resistant to hop com- has a weak beer-spoiling ability an did not show any
pounds (Back, 1981) and grows optimally at 19 –23 significant homology to the other Lactobacillus spp.
jC (Priest, 1987; Back et al., 1996) but survives (Funahashi et al., 1998).
higher thermal treatments than other lactic acid bac-
teria (Back et al., 1992). All Lb. lindneri strains, tested 2.1.2. Pediococci
so far, are capable of beer spoilage, while other Pediococci are homofermentative bacteria which
lactobacilli comprise both beer spoiling and nonspoil- grow in pairs and tetrads. They were originally known
ing strains (Rinck and Warkerbauer, 1987a,b; Stor- as ‘sarcinae’ because their cell organization was
gårds et al., 1998). confused with that of true sarcinae. Beer spoilage,
Lactobacillus buchneri, Lb. casei, Lb. corynefor- caused by cocci and characterized by acid formation
mis, Lb. curvatus and Lb. plantarum are less common and buttery aroma of diacetyl, was therefore called
beer-spoiling bacteria than the two species described ‘sarcinae sickness’. Pediococcus spp. produce rope
above (Priest, 1996). Lb. buchneri resembles closely and extensive amounts of diacetyl like Lb. casei. They
Lb. brevis but differs in its ability to ferment melezi- are found at many stages in the brewing process from
tose. In addition, some Lb. buchneri strains require wort till finished beer. Several Pediococcus spp. have
riboflavin in their growth medium (Sharpe, 1979; been found in breweries: Pediococcus acidilactici,
Back, 1981). Lb. casei can produce diacetyl, which Pediococcus damnosus P. dextrinicus, P. halophilus
gives beer an unacceptable buttery flavor. Diacetyl which recently is classified as Tetragenococcus hal-
appears to be more potent in that respect than lactic ophilus (Collins et al., 1990), P. inopinatus, P. parvu-
acid, the major end-product of lactic acid bacteria. lus and P. pentosaceus (Back, 1978; Back and
The threshold value of diacetyl in beer is much lower Stackebrandt, 1978). Among them is P. damnosus,K. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124 109
the most common beer spoiler. It was responsible for stain, DSM20764, isolated from spoiled beer that
more than 20% of all bacterial incidents in the period differs considerably in genotype from the two other
1980 –1987 (Back, 1980; Back et al., 1988) but only species (Weiss, personal communication, 1987). Its
for 3 – 4% in 1992 –1993 (Back, 1994). The incidence 16S rRNA gene sequence is distinctly different from
of beer spoilage by pediococci has decreased most that of the other two species (Sakamoto, 1997).
likely as a result of improved sanitation conditions in Pectinatus spp. are nonspore-forming motile rods with
breweries. P. damnosus is generally resistant to hop lateral flagella attached to the concave side of the cell
compounds. It is interesting that P. damnosus is body. They swim actively and appear X-shaped when
commonly found in beer and late fermentations, but cells are young and snake-like and longer when cells
seldom in pitching yeast. In contrast, P. inopinatus is are older. They possess some features that are char-
frequently detected in pitching yeast but rarely in the acteristic for Gram-positive bacteria (Haikara et al.,
other stages of beer fermentations (McCaig and 1981) and are regarded as being intermediate between
Weaver, 1983; Priest, 1987). P. inopinatus and P. Gram-positive and Gram-negative bacteria. Growth
dextrinicus can grow in beer at pH values above 4.2 takes place between 15 and 40 jC with an optimum
and with low concentrations of ethanol and hop around 32 jC (Chelak and Ingledew, 1987), between
compounds (Lawrence, 1988). P. pentsaceus and P. pH 3.5 and 6 with an optimum at 4.5 and in media
acidilactici have never been reported to cause any containing up to 4.5% (w/w) of ethanol (Chelak and
defect in finished beer (Simpson and Taguchi, 1995). Ingledew, 1987; Watier et al., 1993). During growth,
considerable amounts of propionic and acetic acids
2.1.3. Other Gram-positive bacteria are produced as well as succinic and lactic acids and
In additional to Lactobacillus and Pediococcus acetoin. Pectinatus spp. can also ferment lactic acid.
species, also, a species from the genus Microcococcus Since lactic acid is the sole source of carbon in SMMP
has been reported to be occasionally responsible for medium (see Section 3.1), this medium is used for the
beer spoilage. M. kristinae can grow in beer with low selective isolation of Pectinatus spp. and Megaspaera
ethanol and hop compounds at pH values above 4.5 spp. (Lee, 1994). The most characteristic feature of
(Back, 1981). Micrococci are usually strictly aerobic spoilage caused by Pectinatus spp. is extensive tur-
but M. kristinae can grow also under anaerobic bidity and an offensive ‘rotten egg’ smell brought by
condition (Lawrence and Priest, 1981). It produces a the combination of various fatty acids, hydrogen
fruity atypical aroma in beer (Back, 1981). sulfide and methyl mercaptan (Lee et al., 1978,
1980; Haikara et al., 1981). This spoilage activity
2.2. Gram-negative bacteria can cause serious damages for breweries.
2.2.1. Pectinatus 2.2.2. Megasphaera
Pectinatus spp. are now recognized as one of the Megasphaera has emerged in breweries along with
most detestable beer spoilage bacteria. They play a Pectinatus and is responsible for 3 –7% of bacterial
major role in 20– 30% of bacterial incidents, mainly in beer incidents (Back et al., 1988; Back, 1994). They
nonpasteurized beer rather than in pasteurized beer are nonspore-forming, nonmotile, mesophilic cocci
(Back, 1994). Pectinatus species were long thought to that occur singly or in pairs and occasionally as short
be Zymomonas spp. because of their phenotypical chains. This genus includes two species, M. elsdeni
similarities. The first isolate was obtained from brew- and M. cerevisiae. Since the first isolations in 1976,
eries in 1971 (Lee et al., 1978) and so were all only M. cerevisiae has been blamed to be responsible
subsequent isolates (Back et al., 1979; Haukeli, for beer spoilage (Weiss et al., 1979; Haikara and
1980; Kirchner et al., 1980; Haikara et al., 1981; Lounatmaa, 1987; Lee, 1994). M. cerevisiae grows
Takahashi, 1983, Soberka et al., 1988, 1989). The between 15 and 37 jC with an optimum at 28 jC and
natural habitat of the Pectinatus species are still at pH values above 4.1. The growth is inhibited at
unknown (Haikara, 1991). Two species are found in ethanol concentrations above 2.8 (w/v) but is still
this genus: Pectinatus cerevisiiphilus and Pectinatus possible up to 5.5 (w/v) (Haikara and Lounatmaa,
frisingensis (Schleifer et al., 1990). There is also one 1987; Lawrence, 1988). It is the most anaerobic110 K. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124
species known to exist in the brewing environment fermentation process. Beer produced with yeasts con-
(Seidel et al., 1979). Beer spoilage caused by this taminated with H. protea has a parsnip-like or fruity
organism results in a similar extreme turbidity as odor and flavor (van Vuuren, 1996). Abnormally high
Pectinatus and the production of considerable quan- levels of diacetyl and dimethly sulfide were detected
tities of butyric acid together with smaller amounts of in beer produced from wort contaminated by R.
acetic, isovaleric, valeric and caproic acids as well as aquatilis (van Vuuren, 1996).
acetoin (Seidel et al., 1979). Like Pectinatus, the Recently, a novel strictly anaerobic Gram-negative
production of hydrogen sulfide causes a fecal odor bacterium was isolated from a brewery (Nakakita et
in beer (Lee, 1994), which makes this bacterium one al., 1998). It is a rod-shape bacterium with no flagella
of the most feared organisms for brewers. that can grow in beer at pH values above 4.3 and does
not produce propionic acid. Genetic and phenotipical
2.2.3. Other Gram-negative bacteria studies indicated that this bacterium is different from
In addition to the two genera described above, Pectinatus, Zymomonas and Selenomonas spp.
some other Gram-negative bacteria have been found
to cause problems in the brewing industry. Anaerobic
Zymomonas spp. have been found in primed beer to 3. Detection of beer spoilage bacteria
which sugar was added and in ale beer. Zymomonas
mobilis is an aerotolerant anaerobe and grows above 3.1. Culture media
pH 3.4 and at ethanol concentrations below 10% (w/v)
(van Vuuren, 1996). There is no report of Z. mobilis It has been seen above that only a limited number
spoilage in lager beer, probably because of its selec- of bacterial species are responsible for beer spoilage
tive fermentation character (nonfermentative of malt- and that only a few species are major beer spoilage
ose, maltotriose but fermentative of glucose, fructose bacteria. For quality assurance of finished beer it is
and sucrose). It produces high levels of acetaldehydes usually practically sufficient to control potential con-
and hydrogen sulfide. Also another Zymophilus spp., taminations by Lactobacillus brevis, Lb. lindneri,
Z. raffinosivorans, has been reported as a beer spoiler Pediococcus damnosus, and Pectinatus spp. Most
(Schleifer et al., 1990; Seidel-Rüfer, 1990). The genus studies on beer spoilage bacteria have focused on
Zymophilus is phylogenetically close to the genus the taxonomical classification of these bacteria. These
Pectinatus. Zymophilus spp. can grow in beer, like studies have made it possible to identify the bacteria
Pectinatus spp., at pH values above 4.3 – 4.6 and detected in beer and breweries and to take the proper
ethanol concentrations below 5% (w/v). Also, their measures to control them.
beer spoilage activity is similar to that of Pectinatus Along with the taxonomical studies, a number of
spp. (Jespersen and Jakobsen, 1996). selective culture media for beer spoilage bacteria have
Another Gram-negative bacterium Selenomonas been developed and recommended by various brew-
lacitifex has also been reported to play a role in ery organizations as shown in Table 2. European
certain beer spoilage incidents but this species has Brewery Convention recommends three media for
hardly been studied (Schleifer et al., 1990). Histori- the detection of lactobacilli and pediococci: MRS
cally, a lot of attention of the brewing industry has (de Man, Rogosa and Sharpe) agar supplemented with
been given to aerobic Gram-negative bacteria. Acetic cycloheximide to prevent growth of aerobes such as
acid bacteria, i.e., Gluconobacter and Acetobacter yeasts and moulds, Raka-Ray medium supplemented
used to be well known to breweries. They convert with cycloheximide and VLB S7-S (Versuchs- und
ethanol into acetate, which results in vinegary off- Lehranstalt für Brauerei in Berlin). Other optional
flavor of beer. For reasons explained above, such media are UBA (Universal Beer Agar) supplemented
aerobes are no longer important in modern breweries. with cycloheximide, HLP (Hsu’s Lactobacillus and
Hafnia protea, formerly Obesumbacterium prote- Pediococcus medium), NBB (Nachweismedium für
us, and Rahnella aquatilis, formerly Enterobacter bierschädliche Bakterien), WLD (Wallerstein Labora-
agglomerans, have been detected in pitching yeasts tory Differential Medium), Nakagawa medium, SDA
but never in finished beer. They can retard the (Schwarz Differential Agar) and MRS modified byK. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124 111
Table 2 American Society of Brewing Chemists recommends
Examples of culture media for the detection of beer spoilage UBA and Brewer’s Tomato Juice Medium for general
bacteria
microbial detection and other media including LMDA
Media Bacteria Recommended bya
(Lee’s Multi-Differential Agar), Raka-Ray, BMB
b
MRS (de Man, LAB EBC, ASBC, BCOJ (Barney – Miller Brewery Medium) and MRS for the
Rogosa and Sharpe)
detection of lactic acid bacteria and SMMP (Selective
Raka-Ray LAB, G( )c EBC, ASBC, BCOJ
VLB S7-S (Versuchs- LAB EBC, BCOJ Medium for Megasphera and Pectinatus) (The Tech-
und Lehranstalt für nical Committee and the Editorial Committee of the
Brauerei in Berlin) American Society of Brewing Chemists, 1992). Brew-
HLP (Hsu’s Lactobacillus LAB EBC, BCOJ ery Convention of Japan also recommends the use of
and Pediococcus
some of those media (Brewery Convention of Japan,
medium)
WLD (Wallerstein LAB EBC, BCOJ 1999). These culture media are listed in Table 2.
Differential) The conventional detection method based on cul-
Nakagawa LAB EBC, BCOJ turing of the organisms in these media has the
SDA (Schwarz LAB EBC, BCOJ significant disadvantage that is very time-consuming.
Differential Agar)
One week or even longer is needed to obtain visible
Concentrated MRS G( ) EBC, BCOJ
PYF (Peptone, Yeasts G( ) EBC, BCOJ colonies on plates or turbidity in broths. Consequent-
extract and Fructose) ly, the products are often already released for sale
Thioglycolate Medium G( ) EBC before the microbiological results become available.
LL-Agar G( ) EBC, BCOJ Another problem is that these media are not species-
UBA (Universal Beer Agar) LAB, G( ) EBC, ASBC, BCOJ
specific. Media for the detection of beer spoilage
NBB (Nachweismedium für LAB, G( ) EBC, BCOJ
bierschädliche Bakteriën) lactic acid bacteria allow also growth of nonbeer-
Brewer’s Tomato LAB, G( ) ASBC spoilage species such as Lactobacillus delbrueckii
Juice Medium and P. acidilactici. If the selectivity is increased by
LMDA (Lee’s Multi- LAB ASBC the addition of specific chemicals to these media, even
Differential Agar)
longer detection times might be required.
BMB (Barney – Miller LAB ASBC
Brewery Medium)
SMMP (Selective Medium G( ) ASBC, BCOJ 3.2. Identification methods
for Megasphaera and
Pectinatus) Following the bacterial detection in these media,
a
EBC, European Brewery Convention; ASBC, American species identification is needed. Besides the basic tests
Society of Brewing Chemists; BCOJ, Brewery Convention of such as colony morphology, cell morphology, Gram-
Japan.
b staining and catalase assays, also biochemical tests
LAB, lactic acid bacteria.
c
G( ), Gram-negative bacteria. such as sugar fermentation pattern and chromato-
graphic analysis of organic acids can be performed.
Also, specific detection and identification methods are
addition of maltose and yeast extract at pH 4.7. None used such as immunoassays with polyclonal or mono-
of these media are suitable for detecting all strains of clonal antibodies (Claussen et al., 1975, 1981; Dolezil
lactobacilli and pediococci but a combination of some and Kirsop, 1976; Haikara, 1983; Gares et al., 1993;
of these media yields the best results. For the detec- Sato et al., 1994; Whiting et al., 1992, 1999a,b; Ziola
tion of Pectinatus and Megasphaera, the following et al., 1999, 2000a,b), DNA – DNA hybridization,
media are recommended: Concentrated MRS broth, DNA sequencing (Doyle et al., 1995) and polymerase
PYF (Peptone, Yeast extract and Fructose) and Thio- chain reaction (PCR) (Tsuchiya et al., 1992, 1993,
glycolate Medium for enrichment of beer, LL-Agar 1994; DiMichele and Lewis, 1993; Thompson et al.,
for growth in Lee tube, and UBA, NBB and Raka– 1994; Vogesser et al., 1955a,b; Yasui, 1995; Stewart
Ray for routine analysis at breweries. For Zymomonas and Dowhanick, 1996; Yasui et al., 1997; Sakamoto,
spp., Zymomonas Enrichment Medium is also recom- 1997; Sakamoto et al., 1997; Satokari et al., 1997,
mended (European Brewery Convention, 1992). 1998; Juvonen and Satokari, 1999; Motoyama and112 K. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124
Ogata, 2000; Bischoff et al., 2001). These modern strain with registered genotypes can provide impor-
methods have been reviewed (Barney and Kot, 1992, tant intervention.
Schmidt, 1992; Dowhanick and Russell, 1993; Dow- Another approach is to determine the common
hanick, 1995; Schofield, 1995; Hammond, 1996; physiological properties responsible for beer-spoiling
Schmidt, 1999). ability. For beer spoilage lactic acid bacteria the
common physiological denominator is hop resistance,
3.3. Discrimination of beer spoilage bacteria from which allows growth of these bacteria in beer. How-
nonspoilers ever, measuring hop resistance by culturing in hop
containing medium, is time-consuming.
After detection and identification it is for some
species necessary to identify the bacterium as an
actual beer spoiler. While all strains belonging to 4. Antibacterial activity of hop compounds
Pectinatus spp. and M. cerevisiae have been reported
to be capable of spoiling beer (Haikara, 1991), lactic 4.1. History of hop usage in beer
acid bacteria include both beer spoilage and non-
spoilage strains. Among Lb. brevis and P. damnosus, The comfortable bitterness experienced in beer
most of the strains are capable of spoiling beer and drinking is characteristic for and is mainly caused
only a few strains are not. On the other hand, the by hop compounds. These hop compounds are present
number of beer spoilage strains in Lb. casei, Lb. in the flowers of the hop plant, Humulus lupulus, L.,
coryneformis and Lb. plantarum is limited. Excep- which are added to the wort. This plant has been
tionally, all strains of Lb. lindneri have been reported known for thousands of years. However, its use in
to be capable of spoiling beer (Rinck and Wackerba- beer is not old as the history of beer itself (5000 – 7000
uer, 1987a,b; Storgårds et al., 1998). years). A few descriptions of hops as a beer additive
Before the beginning of 1990s, the only method as well as a decoration for gardens were found in
available for judging the beer spoiling potential of a documents of the 6th century BC. German monks in
bacterium was the so-called ‘forcing test’. In this test, the 12th century often used hops in beer making. In
the bacterium was re-inoculated into beer or beer those days, as in ancient times, it was popular to use a
enriched with concentrated nutrient medium. Howev- variety of fruits, herbs and spices to flavor beer (so-
er, this test has proven to be far from practical for called gruit beer). Initially, the bitter taste from hops
quality assurance since a few months are needed to was not particularly appreciated. However, when in
obtain conclusive results. the 14th century beer production increased and beer
More rapid procedures have been developed. The was exported, the importance of hops in beer was
identification at the strain level can now be done at the gradually more and more appreciated, not only for its
genome level. Ribotyping, based on Southern hybrid- contribution to beer flavor but also for its contribution
ization with a ribosomal gene as a probe, has been to the stability. Hopped beer can be preserved signif-
successfully introduced (Motoyama et al., 1998, 2000; icantly longer than gruit beer. In 1516, Wilhelm IV,
Satokari et al., 2000; Suihko and Haikara, 2000; the lord of Bayern, enacted the ‘Reinheitsgebot (Pu-
Barney et al., 2001). Fully automated ribotyping rity Law)’ which ordered that beer must be made from
machines now commercially available and only 8 h barley, water and hops. Since then, the use of hops
are needed to obtain conclusive results. Amplified became more popular and standard. Many aspects of
Fragment Length Polymorphisms (AFLP) (Perpete et this law were adopted by other countries, which made
al., 2001) and Random Amplified Polymorphic DNA hops indispensable for the brewing industry.
(RAPD-PCR) (Savard et al., 1994; Tompkins et al.,
1996) have also successfully been applied for bacte- 4.2. Hop plant
rial strain identification as well as for identification
of brewing yeasts. In these methods, the genotype of The hop plant is a vine, belonging to the family of
each beer spoilage strain is registered in a database. hemp. It is dioecious and blooms yearly. Nowadays,
A comparison of the genotype of a newly detected it is mainly cultured for the brewing industry. OnlyK. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124 113
the female flowers, the so-called cones, are used for rangement or isomerization to iso-a-acids, which are
beer. The mature cones contain golden resinous much more soluble and bitterer than the original
granules, the lupulin, which are the most important compounds. This conversion, which is very important
part of the flower for the bitterness and preservation in hop chemistry, was advanced first in 1888 (Hay-
of beer. Hop resins are extracted and fractionated as duck). Wieland et al. (1925) suggested that the hy-
shown in Fig. 1. drolysis of humulone to humulinic acid proceeded via
an intermediate. Windisch et al. (1927) investigated
4.3. Antibacterial compounds in hops the humulone boiling products under alkaline condi-
tions and isolated a resinous and bitter oil termed
Hop chemistry has been developed since 19th ‘‘Resin A’’ with chemical properties similar to the
century and has been extensively reviewed (Verzele, intermediate and isomeric with humulone. Around
1986; Moir, 2000). Research has especially been 1950, Rigby and Bethune, (1952, 1953) showed that
focused on the antibacterial properties of hop com- a-acid fraction is a mixture of three major com-
pounds and the bitter substances derived from hops. pounds: humulone, cohumulone and adhumulone
This research goes back to 1888 when Hayduck (Fig. 2). The bittering compounds of beer were found
showed first that antiseptic properties of the hops to comprise three major analogues of these three a-
are due to the soft resins (Hayduck, 1888). In the acids, which are now known as iso-a-acids: isohumu-
Institute of Brewing of the United Kingdom, Walker lone, isocohumulone and isoadhumulone (Fig. 2).
conducted from 1922 till 1941 a long-term investiga- Stereoisomers (cis- and trans-) exist for each iso-a-
tion on ‘the preservative principles of hops’ (Pyman et acid. Finally, the chemical structure and configuration
al., 1922; Walker, 1923a,b, 1924a,b, 1925, 1938, of naturally occuring ( )-humulone (De Keukeleire
1941; Hasting et al., 1926; Hastings and Walker, and Verzele, 1970) and isohumolones (De Keukeleire
1928a,b, 1929; Walker and Hastings, 1931, 1933a,b; and Verzele, 1971) were elucidated. The isomerization
Walker et al., 1931, 1932, 1935, 1940; Walker and yield of a-acids during wort boiling process is low
Parker, 1936, 1937, 1938, 1940a,b). The study fo- (typically of the order of 30%) (Hughes, 2000) due to
cused on the antiseptic properties of a-acids fraction relatively acidic condition of wort (ca. pH 5.2) and the
(humulone) and h-acid fraction (lupulone). adsorption to the wort coagulum during boiling and
The a-acid fraction is a mixture of homologous fermentation.
compounds, the a-acids, which are not transferred as h-acids or lupulones in hops are very poorly
such to beer. During the wort boiling stage in the soluble in wort and beer and cannot undergo the same
brewing process, a-acids are converted by a rear- isomerization processes as a-acids. Consequently,
Fig. 1. Fractionation of hop resins (Hough et al., 1971).114 K. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124
Fig. 2. Chemical structures of hop compounds. The name of each a-acid (I) and h-acid (II) is dependent on its side chain. During wort boiling
process, a-acids (naturally R-body; III) are isomerized to result in stereoisomers of trans-isohumulones (IV) and cis-isohumulones (V).
they are not transferred to beer and have no direct humulone, isohumulone and humulinic acid) were
value in brewing. found to cause leakage of the cytoplasmic membrane
of Bacillus subtilis, resulting in the inhibition of active
4.4. Antibacterial mechanism of hop compounds transport of sugar and amino acids (Teuber and
Schmalreck, 1973). Subsequently, inhibition of respi-
The antibacterial activities of a-acid (humulone) ration and synthesis of protein, RNA and DNA was
and h-acid (lupulone) have been studied before 1950. also observed. Since the iso-a-acids are mainly pres-
Their antibacterial activities are higher than of iso-a- ent in beer among the hop resins and their derivatives,
acids but they dissolve to less extent in beer and water. a precise investigation of the antibacterial activity of
Studies of the antiseptic properties of hopped wort and iso-a-acids was needed for understanding the preser-
hop boiling product showed that they inhibit growth vation or bacterial stability of beer. The molecular
of Gram-positive bacteria but not of Gram-negative mechanism of antibacterial activity of iso-a-acids and
bacteria (Shimwell, 1937a; Walker and Blakebrough, the effects of pH of the growth medium and other
1952). It was first reported by Shimwell (1937a) that variables on the antibacterial activity of hop com-
the antiseptic potency of hop increases at lower pH. pounds were investigated after 1990 (Simpson and
Interestingly, he predicted that the antiseptic potency Smith, 1992). Hop compounds are weak acids and the
of hop is associated with permeability changes of the undissociated forms are mainly responsible for inhi-
bacterial cell wall Shimwell, (1937b). The ‘bacterio- bition of bacterial growth (Fig. 3).
static power’ was also studied of hop compounds, In Lb. brevis (Simpson, 1993b), trans-isohumulone
including humulone and the humulone boiling prod- reduces the uptake of leucine and causes slow leakage
uct (Walker and Blakebrough, 1952). The humulone of accumulated leucine. trans-Isohumulone dissipates
boiling product had less bacteriostatic potency in malt effectively the transmembrane pH gradient (DpH) of
extract (pH 5.5) and wort (ph 5.2) than the original the proton motive force but not the transmembrane
humulone, while its potency was the same at pH 4.3, electrical potential (Dw). Inhibition of H+-ATPase
the pH of beer. The hop constituents (lupulone, activity was not observed. Potentiometric studiesK. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124 115
naturally occurring analogues (Price and Stapely,
2001).
5. Hop resistance in lactic acid bacteria
Beer spoiling lactic acid bacteria have to be hop
resistant in order to grow in beer. The understanding
Fig. 3. Dissociation of trans-isohumulone (Fernandez and Simpson,
1995b). and elucidation of the mechanism of hop resistance is
not only of scientific interest but is also important for
the microbial control in brewing industry to predict
revealed that undissociated trans-isohumulone acts the beer-spoiling ability of lactic acid bacteria.
most likely as an ionophore, catalyzing electroneutral
influx of undissociated isohumulone, internal dissoci- 5.1. Variation of hop resistance
ation of (H+)-isohumulone and efflux of the complex
of isohumulone with divalent cations such as Mn2 +. The extent of hop resistance varies between bacte-
This cation is known to be present at high concen- ria. Among beer-spoiling lactic acid bacteria, Lb.
trations in lactobacillus cells (Archibald and Frido- brevis is so far the most resistant to hop compounds.
vich, 1981a,b; Archibald and Duong, 1984). The The degree of hop resistance varies among the differ-
results of this activity is a decrease of the pH gradient ent strains of Lb. brevis (Hough et al., 1957; Harris
across the membrane. It was reported that the anti- and Watson, 1960; Simpson and Fernandez, 1992;
bacterial activity of trans-isohumulone can be influ- Fernandez and Simpson, 1993). Hop resistance of
enced by the presence of cations in the medium. lactobacilli decreases upon prolonged serial subcultur-
Protonophoric activity of trans-isohumulone requires ing in the absence of hop compounds (Shimwell,
the presence of monovalent cations such as K+, Na+ or 1936; Yamamoto, 1958; Richards and Macrae,
Rb+ and increases with the concentration of these 1964). Hop resistance was thought to be caused by
monovalent cations (Simpson and Smith, 1992). immunity acquired by prolonged contact with hop
trans-Isohumulone cannot bind K+ unless a divalent compounds under brewing conditions (Shimwell,
cation, such as Mn2 +, Mg2 +, Ni2 + and Ca2 + or a 1937a). The necessity of beer spoiling lactic acid
trivalent cation, such as Li3 + and Al3 +, is present in bacteria to acclimatize to beer or hop compounds in
the medium (Simpson et al., 1993; Simpson and order to reproduce in beer (Yamamoto, 1958) was
Hughes, 1993). Thus, the ability of hop compounds solidly documented by Richards and Macrae in 1964.
to bind simultaneously two or more cations may be Hop resistance increased 8- to 20-fold in strains of
crucial for their antibacterial action but the reason has lactobacilli upon serial subculturing in media contain-
been still unclear. ing increasing concentrations of hop compounds,
The properties of other hop acids are similar to while subculturing of resistant populations in the
those of trans-isohumulone and it is likely that the absence of hop compounds resulted gradually in
mechanism of their antibacterial activities is also decreased hop resistance. It took about 1 year of
similar. Some strains of lactic acid bacteria, which subculturing in unhopped beer to maximally reduce
are sensitive to trans-isohumulone, are also sensitive hop resistance of lactobacilli, indicating that the
to ( )-humulone and colupulone and other strains acquired hop resistance can be a very stable property
resistant to trans-isohumulone are also resistant to the (Shimwell, 1936). However, organisms isolated from
related compounds (Fernandez and Simpson, 1993). spoiled beer frequently fail to grow upon reinocula-
The antibacterial activities of six naturally occurring tion in beer. Preculturing in the presence of subinhib-
iso-a-acids, five chemically reduced iso-a-acids and a itory concentrations of isohumulone is needed in order
reduced iso-a-acids mixture were higher at lower pH to make growth in beer possible (Simpson and Fer-
values while several hydrophobic reduced iso-a-acids nandez, 1992). The stability of hop resistance in Lb.
were found to be far more antibacterial than their brevis appears to vary from strain to strain. The hop116 K. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124
resistance in Lb. brevis strain BSO310 could not be resistance to ( )-humulone and colupulone (Fernan-
altered by plasmid curing or mutation induced with dez and Simpson, 1993), suggesting a common mech-
ultraviolet light (Fernandez and Simpson, 1993), sug- anism of resistance against a broad range of hop acids.
gesting that it may be generally a stable character, trans-Isohumulone and ( )-humulone have three
both phenotypically and genetically. The Lb. brevis hydrophobic side chains while colupulone has four.
strain ABBC45 can develop hop resistance in the The side chains are attached to a five-membered ring
same way as observed by Richards and Macrea (Sami of trans-isohumulone, but to a six-membered ring of
et al., 1997a). Hop resistance increased with the copy ( )-humulone and colupulone (Fig. 2). On the other
number of plasmid pRH45. When Lb. brevis ABBC45 hand, resistance was not observed to other ionophores
was cured from this plasmid by serial subculturing in (nigericin, A23187, CCCP, monesin), weak acid food
the absence of hop compounds, the degree of hop preservatives (sorbic acid, benzoic acid), solvents
resistance decreased (Sami et al., 1998; Suzuki et al., (ethanol) or antibiotics (ampicillin, cefamandole, van-
2002). This plasmid contains the horA gene that codes comycin) (Fernandez and Simpson, 1993). The resis-
for a polypeptide that is 53% identical to LmrA, the tance mechanism might be specific for the h-triketone
lactococcal ATP-binding cassette (ABC) multidrug group of the hop acids, which plays an essential role
transporter (van Veen et al., 1996). HorA protein in the antibacterial action (Simpson, 1991).
was expressed heterologously in Lactococcus lactis Microorganisms have developed various ways to
and found to function as an ABC-type multidrug resist the toxicity of antibacterial agents:
transporter and to excrete hop compounds (Sakamoto (i) enzymatic drug inactivation. A well known
et al., 2001). example is beta-lactamase which hydrolyzes the be-
ta-lactam ring into innocuous substrates. In hop-resis-
5.2. Features of hop resistance tant strains of Lb. brevis, neither conversion nor
inactivation of trans-isohumulone was found (Simp-
The pattern of resistance or sensitivity of several son and Fernandez, 1994).
species of lactobacilli to isohumulone is similar to that (ii) target alteration. Cellular targets can be altered
of the related hop acids humulone and lupulone by mutation or enzymatic modification in such a way
(Richards and Macrae, 1964; Fernandez and Simpson, that the affinity of the target for the antibiotics is
1993). Fernandez and Simpson (1993) compared the reduced. In the case of trans-isohumulone, the target
properties of hop-resistant strains of lactobacilli and site is the cell membrane (Teuber and Schmalreck,
pediococci with those of sensitive strains. No obvious 1973; Schmalreck et al., 1975; Simpson, 1993a,b). A
correlation was found between hop resistance and cell ‘sake (Japanese rice wine)’ spoilage bacterium, Lb.
morphology, colony morphology, pH range for heterohiochii, contains extremely long-chain fatty
growth, sugar utilization profile, products of metabo- acids in its membrane (Uchida, 1974), which may
lism, manganese requirement and sensitivity to super- play a role in ethanol resistance (Ingram and Buttke,
oxide radicals, expression of cellular proteins and 1984). It is possible but not yet investigated that hop-
resistance to various antibacterial agents. However, resistant Lb. brevis has also a changed lipid compo-
differences were found in the transmembrane pH sition of its membrane to lower the permeability to
gradient (DpH) and the cellular ATP pool. Because hop compounds.
hop compounds act as protonophores that dissipate (iii) inhibition of drug influx. The outer membrane
the DpH across the cellular membrane (Simpson, of Gram-negative bacteria restricts the permeation of
1993a,b), these differences are of great importance lipophilic drugs, while the cell wall of the Gram-
for understanding the mechanism of hop resistance. positive mycobacteria has been found to be an excep-
tionally efficient barrier. The permeation of hop com-
5.3. Mechanisms of hop resistance pounds might also be affected by the presence of a
galactosylated glycerol teichoic acid in beer spoilage
The molecular structure of antibacterial agents lactic acid bacteria (Yasui and Yoda, 1997b).
might supply an insight in the mechanism of resis- (iv) active extrusion of drugs. The presence of
tance. Resistance to trans-isohumulone also results in (multi) drug resistance pump in the cytoplasmicK. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124 117 membrane of many bacteria has been extensively of hop-resistant strains to produce large amounts of documented (Putman et al., 2000). A multidrug resis- ATP in the cell is needed for the increased activity of tance pump HorA (Sakamoto et al., 2001) has been the H+-ATPase and for the hop extruding activity of found in Lb. brevis ABBC45, which is overexpressed HorA. It is interesting that these responses occurred when exposed to hop compounds (Sami, 1999). In also in hop-sensitive strains at subinhibitory concen- addition, a proton motive force-dependent hop excre- tration of trans-isohumulone (Simpson, 1993a; Simp- tion transporter was suggested in this strain (Suzuki son and Fernandez, 1994). However, at higher et al., 2002). concentrations, both the DpH and the pool decreased (v) other mechanisms to tolerate the toxic effects of in the hop-sensitive strains, but not in the hop-resis- drugs. Since hop compounds act as protonophores and tant strains. A role of HitA in hop resistance was dissipate the transmembrane pH gradient (DpH), the suggested (Hayashi et al., 2001). It is about 30% cells could respond by increasing the rate at which identical to the natural resistance-associated macro- protons are expelled. The hop-resistant strains main- phage proteins (Nramp), which function as divalent- tain a larger DpH than hop-sensitive strains (Simpson cation transporters in many prokaryotic and eukary- and Fernandez, 1993) and Lb. brevis ABBC45 otic organisms. HitA could play a role in transport of increases its H+-ATPase activity upon acclimatization divalent cations, while isohumulone has been claimed to hop compounds (Sakamoto et al., 2002). The ATP to exchange protons for cellar divalent cations such as pool in hop-resistant strains was also found to be a Mn2 +. Thus, a simple mechanism of hop resistance larger than in hop-sensitive strains (Simpson and does not appear to exist. The resistance mechanisms Fernandez, 1994; Okazaki et al., 1997). The ability found so far in Lb. brevis are illustrated in Fig. 4. Fig. 4. Mechanisms of hop resistance. Hop compounds act as ionophores that exchange protons for cellular divalent cations. In a hop-sensitive cell, hop compounds (Hop-H) invade the cell and dissociate into hop anions and protons due to the higher internal pH. Hop anions trap divalent cations such as Mn2 + and diffuse out of the cell. The ionophoric action together with the diffusion of the hop – metal complex results in an electroneutral exchange of cations. Release of protons from hop compounds decreases the intracellular pH and results in a dissipation of the transmembrane proton gradient (DpH) and the proton motive force (pmf). Consequently, pmf-driven uptake of nutrients will be decreased. In hop-resistant cells, hop compounds can be expelled from the cytoplasmic membrane by HorA (a) (Sakamoto et al., 2001) and probably also by a pmf-dependent transporter (b) (Suzuki et al., 2002). Furthermore, overexpressed H+-ATPase increases the pumping of protons released from the hop compounds (c) (Sakamoto et al., 2002). More ATP is generated in hop-resistant cells than in hop-sensitive cells (Simpson and Fernandez, 1994). Galactosylated glycerol teichoic acid in the cell wall (Yasui et al., 1997) and a changed lipid composition of the cytoplasmic membrane of beer spoilage lactic acid bacteria may increase the barrier to hop compounds.
118 K. Sakamoto, W.N. Konings / International Journal of Food Microbiology 89 (2003) 105–124
6. Beer-spoiling ability in lactic acid bacteria or its homologues in a wide range of lactobacilli
(Sami et al., 1997b). Most horA-positive strains were
Hop resistance is crucial to beer-spoiling ability of found to have beer-spoiling activity, indicating that
lactic acid bacteria. In many bacteria, hop-resistance this is a very useful prediction method. Another
mechanisms need to be induced before growth in beer prediction method is based on ATP pool measure-
is possible. For this induction, some bacteria need to ments in lactobacillus cells (Okazaki et al., 1997).
be exposed to subinhibitory concentrations of hop Polyclonal and monoclonal antibodies specific on-
compounds (Simpson, 1993a,b). ly for beer spoilage strains have been reported. A
series of antisera were made against the Lactobacillus
6.1. Factors affecting the growth in beer group E antigen, a cell wall glycerol teichoic acid
beneath the S-layer protein (Yasui et al., 1992, 1995;
Beer spoilage and bacterial growth depend on the Yasui and Yoda, 1997a) and known to be present in
strain and the type of beer. The ability of 14 hop- Lb. brevis, Lb. buchneri, Lb. delbrueckii subsp. lactis
resistant lactic acid bacterial strains, including Lb. and subsp. bulgaricus (Sharpe, 1955; Sharpe et al.,
brevis and P. damnosus strains, was investigated for 1964). When the beer spoilage strain Lb. brevis 578
their capacity to grow in 17 different lager beers with was used as an antigen, the resulting antiserum
the biological challenge test (Fernandez and Simpson, reacted specifically with other beer spoilage strains
1995a). A statistical analysis of the relationship be- of Lb. brevis. Surprisingly, this antiserum also reacted
tween spoilage potential and 56 parameters of beer with beer spoilage strains of P. damnosus, but not with
composition revealed a correlation with eight parame- any strains of Lb. linderi (Yasui and Yoda, 1997a).
ters: pH, beer color, the content of free amino nitrogen, Galactosylated glycerol teichoic acid was found to be
total soluble nitrogen content and the concentrations of the most likely epitope that presumably selectively
a range of individual amino acids, maltotriose, undis- increases the cell barrier to hop compounds (Yasui
sociated SO2 and hop compounds. The effects of and Yoda, 1997b). Three monoclonal antibodies spe-
dissolved carbon dioxide (CO2) and phenolic com- cific for beer spoilage ability of lactic acid bacteria
pounds including catechin, gallic, phytic and ferulic were obtained by immunizing mice with cells cultured
acids on beer spoilage were also investigated (Ham- in beer (Tsuchiya et al., 2000). The monoclonal
mond et al., 1999). CO2 was found to inhibit the antibody raised against Lb. brevis reacted with all
growth of lactobacilli at the concentrations present in beer spoilage strains of Lb. brevis and several beer
typical beer but stimulate at the lower concentrations. spoilage strains of P. damnosus, but not with non-
Among the phenolic compounds, ferulic acid, a com- spoilage strains of Lb. brevis, P. damnosus and other
ponent of barley cell wall and hence present in all lactic acid bacteria. The monoclonal antibody raised
beers, exerted a stronger antibacterial activity after against P. damnosus reacted significantly with all beer
enzymatic conversion into 4-vinyl guaiacol. Organic spoilage strains of P. damnosus and weakly with many
acids in beer may also influence bacterial growth but of the beer spoilage strains of Lb. brevis. On the other
this aspect has hardly been studied. hand, the monoclonal antibody raised against Lb.
lindneri reacted specifically only with Lb. lindneri.
6.2. Prediction of beer spoilage by lactic acid The reactivity of the still unknown antigens did not
bacteria change regardless of the presence of hop compounds
in their culture media.
A number of attempts have been made to develop D-lactate dehydrogenase (LDH) of 60 strains of Lb.
methods to predict beer-spoiling ability. For lactic brevis, including 44 beer spoiling strains and 16
acid bacteria, hop resistance is the key factor. As nonspoiling strains, was also investigated. The strains
described above, some factors have been identified could be divided in five groups (A, B, C, D and E) on
to cause hop resistance. Rapid procedures for detect- the basis of the mobility of their D-LDH in native
ing these factors would be very beneficial for micro- polyacrylamide gels (Takahashi et al., 1999). Forty
bial control in breweries. A set of PCR primers have out of forty-four beer spoilage strains were classified
been made that can specifically detect the horA gene to the group B, suggesting a relationship between theYou can also read
