Lactic acid bacteria

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The Lactic Acid Bacteria (LAB) comprise a clade of Gram positive, low-GC, acid tolerant, non-sporulating, non-respiring rod or cocci that are associated by their common metabolic and physiological characteristics. These bacteria produce lactic acid as the major metabolic endproduct of carbohydrate fermentation. This trait has historically linked LAB with food fermentations as acidification inhibits the growth of spoilage agents. Proteinaceous bacteriocins are produced by several LAB strains and provide an additional hurdle for spoilage and pathogenic microorganisms. Furthermore, lactic acid and other metabolic products contribute to the organoleptic and textural profile of a food item. The industrial importance of the LAB is further evidenced by their generally regarded as safe (GRAS) status, due to their ubiquitous appearance in food and their contribution to the healthy microflora of human mucosal surfaces. LAB are also widely used as silage inoculants. The genera that comprise the LAB are at its core Lactobacillus, Leuconostoc, Pediococcus, Lactococcus, and Streptococcus as well as the more peripheral Aerococcus, Carnobacterium, Enterococcus, Oenococcus, Teragenococcus, Vagococcus, and Weisella; these belong to the order Lactobacillales.

[edit] LAB metabolism

There are two main hexose fermentation pathways that are used to classify LAB genera. Under conditions of excess glucose and limited oxygen, homolactic LAB catobolize one mole of glucose in the Embden-Meyerhof-Parnas (EMP) pathway to yield two moles of pyruvate. Intracellular redox balance is maintained through the oxidation of NADH, concomitant with pyruvate reduction to lactic acid. This process yields two moles ATP per glucose consumed. Representative homolactic LAB genera include Lactococcus, Enterococcus, Streptococcus, Pediococcus and group I lactobacilli.

Heterofermentative LAB utilize the pentose phosphate pathway, alternatively referred to as the pentose phosphoketolase pathway. One mole Glucose-6-phosphate is initially dehydrogenated into 6-phosphogluconate and subsequently decarboxylated to yield one mole of CO₂. The resulting pentose-5-phosphate is cleaved into one mole glyceraldehyde phosphate (GAP) and one mole acetyl phosphate. GAP further is metabolized into lactate as in homofermentation, with the acetyl phosphate reduced to ethanol via acetyl-CoA and acetaldehyde intermediates. Theoretically, end products (including ATP) are produced in equimolar quantities from the catabolism of one mole glucose. Obligate heterofermentative LAB include Leuconostoc, Oenococcus, Weissella, and group III lactobacilli.

[edit] Streptococcus reclassification

In 1985, members of the diverse genus Streptococcus were reclassified into Lactococcus, Enterococcus, Vagococcus, and Streptococcus based on biochemical characteristics as well as molecular features. Historically, streptococci were segregated primarily based on serology, which has proven to correlate well with the current taxonomic definitions. Lactococci (formerly Lancefield group N streptococci) are used extensively as starter innocula in dairy fermentations, with humans estimated to consume 10¹⁸ lactococci annually. Partly due to their industrial relevance, both Lactococcus lactis subspecies (lactis and cremoris) are widely used as generic LAB models for research. L. lactis ssp. cremoris, used in the production of hard cheeses, is represented by the laboratory strains LM0230 and MG1363. Similarly, L. lactis ssp. lactis is employed in soft cheese fermentations, with the workhorse strain IL1403 ubiquitous in LAB research laboratories. In 2001, Bolotin et al sequenced the genome of IL1403 which coincided with a significant shift of resources to understanding LAB genomics and related applications. Currently, there are two L. lactis ssp. cremoris genomes (MG1363 and SK11) that have been sequenced that have yet to be publicly released.

[edit] References

• Holzapfel, WH; Wood, BJB (eds.). (1998). The genera of lactic acid bacteria, 1st ed., London Blackie Academic & Professional. ISBN 0-7514-0215-X.

• Salminen, S.; von Wright, A; and Ouwehand, AC (eds.). (2004). Lactic Acid Bacteria: Microbiological and Functional Aspects, 3rd ed., New York: Marcel Dekker, Inc.. ISBN 0-8247-5332-1.