[Ishani7]

Lactic acid bacteria (LAB) are gram positive, non sporing, cocci or rods that produce lactic acid as the end product of sugar fermentation. The taxonomic boundaries of Lactic acid bacteria have been controversial. In fermented foods like yoghurt and lassi, lactic acid bacteria play a vital role in flavour development and preservation as well.

The classification of lactic acid bacteria into different genera is based on morphology, ability grow at harsh conditions, mode of glucose fermentation, chemotaxonomic markers, DNA sequence data etc. These include eight genera; Lactobacillus, Leuconostoc, Pediococcus, Streptococcus, Carnobacterium, Enterococcus, Lactococcus, Vagococcus of which the latter four genera were originally classified under Streptococci. These organisms are broadly divided into two groups as homofermenters who produce lactic acid as the major end product of glucose fermentation and heterofermenters who produce Carbon dioxide, Ethanol apart from lactic acid. Pediococcus, Lactococcus, Vagococcus belong to homofermenters and Leuconostoc, Carnobacterium and some Lactobacilli belong to heterofermenters.

Lactic acid bacteria cause some negative effects such as development of acidity in milk and spoilage of some other food such as meat etc. There is strong evidence that when consumed in sufficient quantities Lactic acid bacteria exhibit prophylactic and therapeutic effects. About one million viable cells are required to obtain these health beneficial effects.
Probiotics concept was introduced in clinical nutrition in 1980’s. This emphasized the positive physiological role of certain Lactic acid bacteria and Bifidobacteria. Probiotic lactic acid bacteria and Bifidobacteria are capable of passing through upper gastrointestinal tract and colonizing in the large intestine. Health benefits of LAB as probiotics include:

• Reduce the risk of diarrheal infections
• Enhance immune function
• Reduce the severity of lactose intolerance
• Reduce the population of harmful microorganisms in the colon
• Reduce the incidence of colon cancer
• Lower serum cholesterol level
• Lower blood pressure
• Improve mineral absorption
• Restore gut flora during or after antibiotic therapy
• Improve balance of microorganisms in the colon
• Improve bowel function by increasing stool frequency, increasing stool weight and increased production of short chain fatty acids (SCFA)

Dysbiosis is the condition where the harmful microorganisms in the gut overpopulate the beneficial bacteria. This is the major reason for many dietary diseases. The reasons for dysbiosis can be harmful pesticide residues remaining in food accumulated during a long period of time, food allergies and other disease conditions which can be overcome with the use of probiotics. Probiotic bacteria bring about these beneficial effects by competitive exclusion, production of bacteriocins and organic acids and altered absorption of the intestinal mucosa.

It was shown that incorporation of indigestible polysaccharides that escape digestion in the upper gastrointestinal tract are very effective in boosting the probiotic effects. These are known as prebiotics and increase the proportion of beneficial bacteria in the colon. Food entering the colon serves as substrate for endogenous colon bacteria, thus indirectly providing the host with energy, metabolic substrate and essential micronutrients. Prebiotic concept was introduced by Gibson et al. Cereal dietary fibre render prebiotic effects. Cereal based fermented foods carry both live bacteria and prebiotic dietary fibre. These dietary fibres should be soluble, hydrolysable and fermentable by the gut microflora. A prebiotic should,

• Be neither hydrolyzed nor absorbed in the upper Gastrointestinal tract
• Selectively stimulate the growth of potentially beneficial bacteria in the colon

End products of these fibres include short chain fatty acids ( acetic acid, propionic acid, butyrate acid etc.) , Carbon dioxide, Hydrogen Sulphide, Methane and Hydrogen. Gases are either excreted or metabolized. Short chain fatty acids are quickly absorbed in to the blood. Especially propionates and acetates have been reported to lower hepatic cholesterol synthesis. Resistant starch, unabsorbable sugars, oligo and polysaccharides (soybean oligosaccharides-stachyose and raffinose), β glucan. Arabinoxylans are few examples for prebiotics. These are also called as ‘colonic food’.

Synbiotics are the metabolites produced by the synergistic action of prebiotics and probiotics such as short chain fatty acids, amino acids, peptides, growth factors, signal molecules etc.