IMPORTANT CHARACTERISTICS OF COMMON LACTIC ACID BACTERIA

The important lactic acid bacteria can be classified in to four genus as follows.

I. Lactococcus

Formerly known as streptococci, the lactococci form the majority of mesophilic organisms (optimum temperature of growth between 25 and 30°C) which are used mainly for acid production in the dairy industry. Among all the bacteria in this group, Lactococcus lactis subspecies lactis is an important organism known for its versatile applications. It is gram positive, cocci, usually occur in chains, though single and paired occurrence is not uncommon. They are homofermentative in nature and when grown in milk, lactic acid (L form) forms 95% of the end product.

There are two subspecies available viz. Lactococcus lactis subspecies lactis and Lactococcus lactis subspecies cremoris, the former being more heat resistant and salt tolerant than the latter. There is another variant of Lactococcus lactis subspecies lactis known as Lactococcus lactis subspecies lactis biovar diacetylactis which converts citrate present in milk in to diacetyl, acetoin and 2,3 butylene glycol along with carbon dioxide and other compounds, responsible for the characteristic aroma.

Diacetyl is the principal flavour compound in cultured buttermilk, curd or dahi, butter, cottage cheese and it is commonly referred to as butter flavour. Lactococcus lactis subspecies cremoris is mainly used as Cheddar cheese starters.

II. Streptococcus

The only species of importance to the dairy industry under the genus Streptococcus is Streptococcus thermophilus. It is relatively heat resistant and grows well at high temperatures up to 52°C. It is generally used in combination with Lactobacillus bulgaricus in the preparation of yoghurt, mozzarella and emmental cheese where the temperature of incubation generally exceeds 40°C. Known for its ability to have associative interaction with Lactobacillus bulgaricus, the extracellular polysaccharide synthesizing species are used in the manufacture of high viscosity stirred type of yoghurt.

III. Leuconostoc

Two species of leuconostocs are important as dairy starters. They are Leuconostoc mesenteroides subspecies cremoris and Leuconostoc mesenteroides subspecies lactis. These organisms are heterofermentative in nature and produce carbon dioxide and ethanol in addition to lactic acid as end products. The problem with leuconostoc is that they are slow growers when cultured alone in milk. Their growth can be influenced by the metabolic end products of other bacteria when co cultured along with them. They are usually grown along with lactococci resulting in the production of diacetyl, carbon dioxide in addition to lactic acid.

IV. Lactobacillus

These are organisms consisting of physiologically and genetically diversified group, gram positive and rod shaped in nature. Majority of the organisms falling under the category are homofermentative (converting lactose in to lactic acid). E.g. Lactobacillus delbrueckii subspecies bulgaricus and Lactobacillus delbrueckii subspecies helveticus. They are called thermodurics since they are capable of growing at higher temperatures (40°C) and considered the most acid tolerant in the lactic acid bacteria. The growth of these bacteria is generally initiated at a pH of 5.5 to 6.2 and the pH is lowered to below 4.0 once when they establish in the substrate. In general, the lactobacilli are slow growers when cultured alone and hence they are usually co cultured with streptococci especially, Streptococcus thermophilus.

Thus the lactic acid bacteria play a great role in the fermentation process of the dairy industry.

Learn more about Probiotic properties of Lactobacilli

The health claims of ingesting live cells of lactobacilli could be due to several possible mechanisms, which may include restoration of normal intestinal flora and inhibitory effect towards human pathogens, accumulation of their metabolites including organic acids in the intestine and enhancement in the normal functioning of the digestive tract.