Microbiology and Immunology

Lactic acid bacteria WORLD OF MICROBIOLOGY AND IMMUNOLOGY

336

•

Various biochemical tests are utilized in a microbiology

laboratory. The ability of a microbe to utilize a particular com-

pound and the nature of the compound that is produced are

important in the classification of microorganisms, and the diag-

nosis of infections. For example, coliform bacteria were tradi-

tionally identified by a series of biochemical reactions that

formed a presumptive-confirmed-completed triad of tests.

Now, media have been devised that specifically support the

growth of coliform bacteria, and Escherichia coliin particular.

Various laboratory tests are conducted in animals to

obtain an idea of the behavior of microorganisms in vivo. One

such test is the lethal dose 50 (LD50), which measures the

amount of an organism or its toxic components that will kill 50

percent of the test population. The lower the material neces-

sary to achieve the LD50, the more potent is the disease com-

ponent of organism.

See alsoAntibiotic resistance, tests for; Blood agar, hemoly-

sis, and hemolytic reactions; Microscopy; Qualitative and

quantitative analysis in microbiology

LLactic acid bacteriaACTIC ACID BACTERIA

Lactic acid bacteriacompose 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 lac-

tic acid bacteria is their inability to manufacture the many com-

pounds 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 faecalislives. Other environments

include plant leaves (Leuconostoc, Lactobacillus, and decaying

organic material.

The drop in pHthat occurs as lactic acid is produced by

the bacteria is beneficial in the preservation of food. The low-

ered 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 contaminationby

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 pro-

tection 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 infec-

tious 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 yeastinfections.

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-for-

mers (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 alsoEconomic uses and benefits of microorganisms

LLactobacillusACTOBACILLUS

Lactobacillusis the name given to a group of Gram-negative

bacteriathat do not form spores but derive energy from the

conversion of the sugar glucose into another sugar known as

lactose. The name of the genus derives from the distinctive

sugar use. Lactobacillushas a number of commercial uses,

especially in aspects of dairy production, including the manu-

facture of yogurt. As well, Lactobacillusis part of the normal

microbial population of the human adult vagina, where it

exerts a protective effect.

Prominent examples of the genus include Lactobacillus

acidophilus, LactobacillusGG, Bifidobacterium bifidum, and

Bifidobacterium longum.

A distinctive feature of the members of the genus

Lactobacillusis the formation of lactic acid from glucose.

This is the property that confers the sour taste to natural,

Lactobacillus-containing yogurt. As well, the lactic acid low-

ers the pHof the environment that the bacteria dwell in. In the

case of the vagina, this acidic change can inhibit the growth of

other, harmful invading bacteria. Consistent with this, the use

of suppositories containing Lactobacillusspecies has been

successful in controlling recurrent bacterial vaginal infections.

Similarly, use of the bacterium has been promising in the con-

trol and prevention of recurrent urinary tract infections.

Aside from the exclusion of bacteria due to the pH alter-

ation in the vagina or urinary tract, Lactobacillusalso adheres

to cells lining the vagina and the urinary tract, and colonizes

womi_L 5/6/03 3:31 PM Page 336