Lactic Acid Bacteria: About, Testing and Considerations
What are Lactic Acid Bacteria (LAB)?
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. Classification: Bacilli.
Where Are Lactic Acid Bacteria Found?
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. Because they are so beneficial they are often the only or the major component of many probiotic formulations marketed as cure-alls for any number of common ailments.
What do Lactic Acid Bacteria do for Food?
LAB in Wine Production
When most people think of microorganisms and alcohol, yeast immediately comes to mind. It turns out that LAB play an important role as well, specifically in the production of wines, via a process called malolactic fermentation. Malolactic bacteria (MLBs), which are a subset of lactic acid bacteria, are able to convert the malic acid found in most grapes into lactic acid. This has profound effects on the total acidity (TA and/or pH) of the wine, but more importantly on its flavor profile. Wine gurus refer to the flavor changes as “reduction in sharpness of the acid” and “imparting of a buttery quality” among a slew of other flavor benefits. Malolactic bacteria occur naturally on grapes however in modern winemaking these natural flora are typically killed off and a particular strain is added back in during the primary fermentation. The timing of the addition is tricky because MLBs are sensitive to high alcohol concentrations and will be killed if added too late in the process.
Role in Fermented Food Products
Beyond winemaking, 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 like yogurt, cheese, cultured butter, sour cream, sausage, cucumber pickles, olives and sauerkraut. Although many LABs inhibit food spoilage by producing growth-inhibiting substances and large amounts of lactic acid, some species may be spoilage agents themselves, especially in matrices like beer, wine, and processed meats.
Lactic Acid Bacteria Testing
Lactic acid bacteria are typically enumerated using standard agar plate count methods. Specifically, the method described in the Compendium of Methods for the Microbiological Examination of Foods (Chapter 19 - Acid Producing Microorganisms) occasionally the method described in Chapter 8.070 of Standard Methods for the Examination of Dairy Products.
The methods primarily use two agars MRS agar and APT agar for enumeration of lactic acid bacteria. Both media are designed for the recovery of lactic acid bacteria and contain peptone and dextrose, which supply nitrogen, carbon and other elements necessary for growth. Polysorbate 80, acetate, magnesium and manganese provide growth factors for culturing a wide variety of lactobacilli. APT agar is primarily used for the recovery of heterofermentative lactic acid bacteria from meats, while MRS agar is used for general recovery of lactic acid bacteria. In practice, MRS is most widely used.
As the requirements for recovering LABs from various food matrices can be complex, there are many modifications available for both MRS and APT, or even additional media, to best tailor the test to the matrix.
The lactic acid bacteria are a large and diverse group and while their preferred and optimum growth conditions are similar they do vary some, and media nutrient profile along with incubation time and temperature can and do impact recovery.
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