Lactobacilli and Bifidobacteria are Gram-positive lactic acid-producing bacteria that constitute a major part of the normal intestinal microflora in animals and humans. These friendly bacteria play a key role in enhancing resistance to colonization by exogenous, potentially pathogenic organisms.
Lactobacilli are Gram-positive, non-spore forming rods or coccobacilli. They have complex nutritional requirements and are strictly fermentative, aerotolerant or anaerobic, aciduric or acidophilic. Lactobacilli are found in a variety of habitats where rich, carbohydrate-containing substrates are available, such as human and animal mucosal membranes, on plants or material of plant origin, sewage and fermenting or spoiling food.
Bifidobacteria constitute a major part of the normal intestinal microflora in humans throughout life. They appear in the stools a few days after birth and increase in number thereafter. The number of bifidobacteria in the colon of adults is 1010 – 1011 CFU/gram, but this number decreases with age. Also demographic differences affect the number and species of bifidobacteria. Bifidobacteria are nonmotile, nonsporulating Gram-positive rods with varying appearance. Most strains are strictly anaerobic. B. longum may be considered as the most common species of bifidobacteria, being found both in infant and adult faeces. This species is closely related to B. infantis, which often leads to identification problems.