Bacteriology 102:
General Overview of the
Lactic Acid Bacteria

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Overview of the Lactics
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By John Lindquist. Based on material in the Laboratory Manual for the Food Microbiology Laboratory (1998 edition, edited by John L. and published at the University) which was used in Bacteriology/Food Science 324 at the University of Wisconsin – Madison.
This page is relevant to Experiment 12 in the UW-Madison Bacteriology 102 Course and is not intended to be authoritative for applied or research purposes. A major revision/modernization should be coming soon!

The lactic acid bacteria consist of several genera which include Streptococcus, Enterococcus, Lactococcus, Leuconostoc, Lactobacillus, Pediococcus and Aerococcus. Based on similarities in physiology, metabolism and nutritional needs, these genera are grouped together, but the long-used taxonomic grouping in "Family Lactobacillaceae" is no longer applied. A primary similarity is that all members produce lactic acid as a major or virtually sole end product of the fermentation of sugars. The lactic acid bacteria are of paramount importance in the food industry, both as beneficial organisms and as spoilage organisms. They are used in the production of fermented milk products such as yogurt, sour cream, cheese and butter, and in the production of sausage, pickles and sauerkraut. The result of these fermentations are more shelf-stable products with characteristic aromas and flavors. If the growth of lactic acid bacteria is not in some way controlled, they can be a major cause of food spoilage. The souring of milk and the greening of meat are common examples of spoilage resulting from the unchecked activity of these organisms.

Taxonomic changes have been taking place in this group of bacteria, particularly in the genus Streptococcus. Traditionally, the "Sherman criteria" along with the "Lancefield antigens" have been used to help in the characterization of species and groups of species within Streptococcus. Within the past two decades, based most definitively on nucleic acid studies, the genera Enterococcus and Lactococcus have been spun off from Streptococcus. On occasion, one may still note the use of outdated terminology such as Streptococcus faecalis and Streptococcus lactis; these species are now called Enterococcus faecalis and Lactococcus lactis, respectively.

All lactics are gram-positive organisms. Streptococcus, Enterococcus, Lactococcus and Leuconostoc are oval cocci commonly arranged in pairs or chains. Pediococcus and Aerococcus are cocci found in tetrads. Lactobacillus cells are rod-shaped and arranged in chains. In characterizing the morphology of lactic acid bacteria, it may be difficult, at times, to distinguish a short rod from an ovoid coccus. Determining the arrangement (pairs versus tetrads of cocci, for example) can be challenging as well. One must take the time to look at these organisms carefully and repeatedly.

With occasional exceptions, the lactics are aerotolerant anaerobes which means that they possess the fermentative type of metabolism associated with anaerobes and are also indifferent to the presence of oxygen. They are incapable of any form of respiration. Iron porphyrin-containing compounds are not produced by the lactics. They are, therefore, unable to synthesize the true catalase enzyme. Catalase is an enzyme that respiring organisms use to detoxify H2O2 formed during aerobic growth. Many lactics do produce pseudocatalase, an enzyme that mimics true catalase in its activity. This enzyme is inactivated by acid, so one must take care to avoid the over-production of acid during growth of the lactics, if the enzyme is to be detected. Hence the use of a very low-glucose-containing medium for this test. Brain Heart Infusion Agar works well, as it is also buffered to some extent.

As obligate fermenters of carbohydrates, the lactics are universally able to ferment glucose. The lactics can be divided into two groups based on the end-products formed during the fermentation of glucose. Homofermentative lactics produce lactic acid as the sole or major end product, while the heterofermentative lactics produce equivalent amounts of lactic acid, CO2 and ethanol. The hot-loop test can be used to differentiate these two groups. As CO2 is very soluble in water, a Durham tube is unable to detect its production. Rather, one can culture the test organism in a medium containing glucose and, after good growth is obtained, plunge a red-hot loop into the broth. This will cause release of CO2 which will be seen as an eruption of very fine bubbles rising to the surface of the broth and forming a noticeable ring. APT Broth is often used in this test, but as this medium contains citrate, an occasional false-positive reaction may be seen; certain homofermenters (e.g., Lactococcus lactis subsp. diacetilactis) can metabolize citrate, forming CO2 among other products.

Nutritionally the lactic acid bacteria are extremely fastidious. A medium that will support their growth must contain a fermentable carbohydrate and many growth factors. Brain Heart Infusion and APT are two media that commonly support the growth of the lactics as they contain glucose and are very rich in growth factors.

Enrichment for the lactics often makes use of the ability of these organisms to initiate growth at low pH and to grow in the presence of sodium azide (NaN3). Generally the use of NaN3 is more effective. This chemical acts to inhibit iron porphyrin synthesis and therefore cytochrome activity. As the lactics do not respire and do not synthesize or use these compounds, they are not inhibited by the presence of NaN3. Brain Heart Infusion Broth and APT Broth – each supplemented with 0.02% NaN3 – are two media used to enrich selectively for lactic acid bacteria. A rich, azide-containing plating medium incubated aerobically virtually guarantees exclusive isolation of lactic acid bacteria.

Some of the lactics have the ability to produce levans or dextrans from sucrose. This involves, in the case of dextran production, the enzyme dextran sucrase. Its activity on sucrose results in the production of a very slimy polymer of glucose called dextran. Dextran sucrase produces slime from sucrose, but will not produce slime from glucose even though dextran is a polymer of glucose.

Further information can be obtained from sources such as Bergey's Manual, The Prokaryotes and the Manual of Clinical Microbiology.

Selected Groups of Bacteria
Bacteriology 102 Website
Additional notes on "lactics" & "enterics"
Page last modified on 1/7/00 at 1:15 PM, CST.
John Lindquist, Department of Bacteriology,
University of Wisconsin – Madison