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.
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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.
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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 genusStreptococcus. 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.
Metabolism
The essential feature of LAB metabolism is efficient carbohydrate fermentation coupled to substrate-level phosphorylation. Adenosine triphosphate (ATP) generated is subsequently used for biosynthesis. LAB as a group exhibit an enormous capacity to degrade different carbohydrates and related compounds. Generally, the predominant end product is lactic acid (>50% of sugar carbon). However, LAB adapt to various conditions and change their metabolism accordingly. This may lead to significantly different end-product patterns.
Based on sugar fermentation patterns, two broad metabolic categories of LAB exist: homofermentative and heterofermentative. The first category,homofermentative LAB, includes some lactobacilli and most species of enterococci, lactococci, pediococci, streptococci, tetragenococci, and vagococci, that ferment hexoses by the Embden-Meyerhof (E-M) pathway. The second category, heterofermentative LAB, includes leuconostocs, some lactobacilli, oenococci, and weissella species. The apparent difference on the enzyme level between these two categories is the presence or absence of the key cleavage enzymes of the E-M pathway (fructose 1,6-diphosphate) and the PK pathway (phosphoketolase).
Homolactic Fermentation
Under conditions of excess glucose and limited oxygen, homolactic LAB catabolize one mole of glucose in the Embden-Meyerhof 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 of ATP per glucose consumed. Representative homolactic LAB genera include Lactobacillus, Lactococcus, Enterococcus, Streptococcus and Pediococcus species.
The transport and phosphorylation of sugars occur by (1) transport of free glucose and phosphorylation by an ATP-dependent hexose kinase (other sugars, such as mannose and fructose, enter the major pathways at the level of glucose-6-phosphate or fructose-6-phosphate after isomerization or phosphorylation or both); or (2) the phosphoenolypyruvate (PEP) sugar phosphotransferase system (PTS), in which PEP is the phosphoryl donor for the uptake of sugar. Some species of LAB use the PTS for transport of galactose only; others use the PTS for all sugars.
