Lactic Acid Bacteria (page 1)
(This chapter has 5 pages)
© 2009 Kenneth Todar, PhD
Lactic Acid
Bacteria
Lactobacillus acidophilus The
bacterium is a member of the normal flora of humans, found in the
oral cavity, the small intestine, and the vaginal epithelium, where it
is thought to play a beneficial role. The organism is generally the
first bacterium listed as present in probiotic concoctions.
Lactic Acid Bacteria (LAB) are
Gram-positive, non-sporeforming cocci,
coccobacilli or rods with a DNA base composition of less than 53mol%
G+C. They generally are non respiratory and lack catalase. They ferment
glucose primarily to lactic acid, or to lactic acid, CO2
and ethanol. All LAB grow anaerobically, but
unlike most anaerobes, they grow in the presence of O2
as "aerotolerant
anaerobes". Although they lack catalase, they possess
superoxide
dismutase and have alternative means to detoxify peroxide radicals,
generally through peroxidase enzymes.
Although many genera of bacteria produce lactic acid as a primary or
secondary end-product of fermentation, the term Lactic Acid Bacteria is
conventionally reserved for genera in the order Lactobacillales, which
includes Lactobacillus, Leuconostoc,
Pediococcus,
Lactococcus and
Streptococcus, in addition to Carnobacterium,
Enterococcus,
Oenococcus, Tetragenococcus, Vagococcus,
and Weisella.
Because they obtain energy only from the metabolism of sugars, lactic
acid bacteria are
restricted to environments in which sugars are present. They have
limited biosynthetic ability, having evolved in environments that are
rich in amino acids, vitamins, purines and pyrimidines, so they
must be cultivated in complex media that fulfill all their nutritional
requirements. Most are free-living or live in beneficial or harmless
associations with animals, although some are opportunistic pathogens.
They are found in milk and milk products and
in decaying plant materials. They are normal flora of humans in the
oral cavity, the intestinal tract and the vagina, where they play a
beneficial role.
A few LAB are pathogenic for animals, most notably some members of the
genus
Streptococcus. In humans, Streptococcus pyogenes is a major
cause of
disease (strep throat, pneumonia, and other pyogenic infections,
scarlet fever and other toxemias), Streptococcus
pneumoniae causes
lobar pneumonia, otitis media and meningitis; some viridans and
nonhemolytic oral streptococci play a role in dental caries and may be
an insidious cause of endocarditis. The pathogenic streptococci are
dealt with
elsewhere in the text. This chapter deals primarily with LAB in
association with food and dairy microbiology, to a lesser extent with
LAB as beneficial components of the human normal flora and probiotics.
Lactic acid bacteria are among the most important groups of
microorganisms used in food fermentations. They contribute to the taste
and texture of fermented products and inhibit food spoilage bacteria by
producing growth-inhibiting substances and large amounts
of lactic acid. As agents of fermentation LAB are involved in making
yogurt, cheese, cultured butter, sour cream, sausage, cucumber pickles,
olives and sauerkraut, but some species
may
spoil beer, wine and processed meats.
Differential
characteristics of lactic acid bacteria based on morphology and
physiology
Characteristic
|
|
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|
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Morphology
|
rods
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cocci
|
cocci
|
cocci
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cocci in
tetrads
|
cocci
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|
CO2 from glucose*
|
±
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−
|
−
|
+
|
−
|
−
|
Growth
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|
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|
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at 10°C
|
±
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+
|
+
|
+
|
±
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−
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at 45°C
|
±
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+
|
−
|
−
|
±
|
±
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|
in 6.5% NaCl
|
±
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+
|
−
|
±
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±
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−
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at pH 4.4
|
±
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+
|
±
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±
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+
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−
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|
at pH 9.6
|
−
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+
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−
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−
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−
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−
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Lactic acid
configuration
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D, L, DL
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L
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L
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D
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L, DL
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L
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+ positive; - negative; ± varies
between species
*test for homo- or
heterofermentation of glucose: - homofermentation
+ heterofermentation
(chapter continued)
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