Lactic Acid Bacteria
Lactic acid bacteria
Lactic acid bacteria compose a group of bacteria that degrade carbohydrate (e.g., fermentation ) with the production of lactic acid. Examples of genera that contain lactic acid bacteria include Streptococcus, Lactobacillus, Lactococcus, and Leuconostoc.
The production of lactic acid has been used for a long time in food production (e.g., yogurt, cheese, sauerkraut, sausage,). Since the 1970s, the popularity of fermented foods such as kefir, kuniss, and tofu that were formally confined to certain ethnically oriented cuisines, has greatly increased.
Generally, lactic acid bacteria are Gram-positive bacteria that do not form spores and which are able to grow both in the presence and absence of oxygen. Another common trait of lactic acid bacteria is their inability to manufacture the many compounds that they need to survive and grow. Most of the nutrients must be present in the environment in which the bacteria reside. Their fastidious nutritional needs restrict the environments in which lactic acid bacteria exist. The mouth and intestinal tract of animals are two such environments, where the lactic acid bacterium Enterococcus faecalis lives. Other environments include plant leaves (Leuconostoc, Lactobacillus, and decaying organic material.
The drop in pH that occurs as lactic acid is produced by the bacteria is beneficial in the preservation of food. The lowered pH inhibits the growth of most other food spoilage microorganisms . Abundant growth of the lactic acid bacteria, and so production of lactic acid, is likewise hindered by the low pH. The low pH environment prolongs the shelf life of foods (e.g., pickles, yogurt, cheese) from contamination by bacteria that are common in the kitchen (e.g., Escherichia coli, or bacteria that are able to grow at refrigeration temperatures (e.g., Listeria. The drop in the oxygen level during lactic acid fermentation is also an inhibitory factor for potential food pathogens. Research is actively underway to extend the protection afforded by lactic acid bacteria to others foods, such as vegetables.
The acidity associated with lactic acid bacteria has also been useful in preventing colonization of surfaces with infectious bacteria. The best example of this is the vagina. Colonization of the vaginal epithelial cells with Lactobacillus successfully thwarts the subsequent colonization of the cell surface with harmful bacteria, thus reducing the incidence of chronic vaginal yeast infections.
Lactic acid bacteria produce antibacterial compounds that are known as bacteriocins. Bacteriocins act by punching holes through the membrane that surrounds the bacteria. Thus, bacteriocins activity is usually lethal to the bacteria. Examples of bacteriocins are nisin and leucocin. Nisin inhibits the growth of most gram-positive bacteria, particularly spore-formers (e.g., Clostridium botulinum. This bacteriocin has been approved for use as a food preservative in the United States since 1989. Leucocin is inhibitory to the growth of Listeria monocytogenes.
Lactic acid bacteria are also of economic importance in the preservation of agricultural crops. A popular method of crop preservation utilizes what is termed silage. Silage is essentially the exposure of crops (e.g., grasses, corn, alfalfa) to lactic acid bacteria. The resulting fermentation activity lowers the pH on the surface of the crop, preventing colonization of the crop by unwanted microorganisms.
See also Economic uses and benefits of microorganisms