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12-1 An introduction to lactic acid bacteria

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Lactic acid bacteria are Gram-positive usually non-motile, non-spore-forming rods and cocci. They lack the ability to synthesize cytochromes and porphyrins (components of respiratory chains) and therefore cannot generate ATP by creation of a proton gradient. The lactics can only obtain ATP by fermentation, usually of sugars. Since they do not use oxygen in their energy production, lactic acid bacteria happily grow under anaerobic conditions, but they can also grow in oxygen's presence. They are protected from oxygen byproducts (e.g. H2O2) because they have peroxidases. These organisms, as defined in experiment 3, are aerotolerant anaerobes. They are differentiated from other organisms by their ability to ferment hexoses to lactic acid, hence the name. Lactic acid bacteria can be divided into two groups based upon the products produced from the fermentation of glucose. Homofermentative organisms ferment glucose to two moles of lactic acid, generating a net of 2 ATP per mole of glucose metabolized. Lactic acid is the major product of this fermentation. Heterofermentative lactic acid bacteria ferment 1 mole of glucose to 1 mole of lactic acid, 1 mole of ethanol, and 1 mole of CO2. One mole of ATP is generated per mole of glucose, resulting in less growth per mole of glucose metabolized. Because of the low energy yields, lactic acid bacteria often grow more slowly than microbes capable of respiration, and produce smaller colonies of 2-3 mm.

It is easy to determine whether a lactic acid bacteria has a homo- or heterofermentative metabolism by the hot-loop test. A major end-product of heterofermentation is CO2. In a medium containing glucose this gas is highly soluble at high pH and will stay in solution. If, however, the temperature of the solution is increased, CO2 will become insoluble and will be released in the gaseous form. The hot-loop test consists of growing a test isolate to saturation in a medium containing glucose. After incubation, a wire loop (inoculating loop) is heated to redness and plunged into the broth culture. This causes the liquid around the loop to heat up. If a test organism is heterofermentative, CO2 bubbles will evolve close to the loop.

The lactic acid bacteria have limited biosynthetic ability, requiring preformed amino acids, B vitamins, purines, pyrimidines and typically a sugar as carbon and energy source. A rich medium is usually employed when cultivating lactics. These multiple requirements restrict their habitats to areas where the required compounds are abundant (animals, plants, and other multicellular organisms). Lactic acid bacteria can grow at temperatures from 5-45°C and not surprisingly are tolerant to acidic conditions, with most strains able to grow at pH 4.4.

The genera of lactic acid bacteria

Lactics are classified by the fermentation pathway used to ferment glucose and by their cell morphology. Lactobacillus are rod shaped organisms that can be either hetero- or homofermentative. They are widespread and can be isolated from many plant and animal sources. Lactobacilli are more tolerant to acid than the other genera of lactic acid bacteria and this property makes them important in the final phases of many food fermentations when other organisms are inhibited by the low pH.

Leuconostoc are ovoid cocci, often in chains. All bacteria of this genus have a heterofermentative mode of metabolism. When grown in media containing sucrose, copious amounts of a slimy polysaccharide (dextran) are produced. Dextran has found uses in medicine as a plasma extender and in biotechnology.

Pediococcus are cocci often found in pairs and tetrads that are strictly homofermentative. Their habitat is restricted mainly to plants. P. cerevisiae has been used as a starter culture for the fermentation of some sausages with great success. Streptococcus are cocci in chains that are distinguished from the Leuconostoc by their strictly homofermentative metabolism. These organisms can be isolated from oral cavities of animals, the intestinal tract, skin, and any foods that come in contact with these environments. While the other genera of lactic acid bacteria rarely cause disease, Streptococcus pyogenes is a common, troublesome pathogen, causing strep throat and rheumatic fever.

Enterococcus and Lactococcus are two recent taxonomic divisions of lactic acid bacteria. These were created to reorganize the large and divergent Streptococcus genus into smaller, more related groups of bacteria. Enterococcus are gram-positive cocci that form pairs or chains. They are distributed widely in the environment, particularly in feces of vertebrates. Strains can grow in the presence of 6.5% NaCl and with 40% bile present.

Lactococcus includes strains that are gram-positive, spherical cells occurring in pairs or chains. They have a strictly homofermentative metabolism and are found in dairy and plant products. For centuries lactic acid bacteria have been used to produce fermented food products including pickles, sauerkraut, sausage, yogurt, cheese, buttermilk, soy sauce, and more. Some examples include Streptococcus thermophilus along with Lactobacillus bulgaricus that are used in the production of yogurt. Also, Lactococcus lactis and S. thermophilus are two strains often used as starter cultures in the production of cheese. Finally, Lactobacillus and Leuconostoc are useful in the fermentation of cabbage to sauerkraut.