MICROBIOLOGY Textbook - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.5. FERMENTATION

9.5.5. Fermentation by Bifidobacteria

Bifidobacteria were first isolated in 1900 by the French scientist H. Tissier from the stool of breastfed infants. These Bacteria got their name due to their ability to fork at the ends (from the Latin bifidus — cleft). Bifidobacteria belong to the family Actinomycetaceae and the genus Bifidobacterium.

Bifidobacterium Cells appear as extremely pleomorphic rods, measuring (0.5–1.3) × (1.5–8) µm, slightly curved, club-shaped, and frequently branched. They occur singly, in pairs, in V- or Y-shaped arrangements, rosettes, palisades, or rarely in chains. Bifidobacteria are Gram-positive, non-motile, non-spore-forming, capable of forming a microcapsule, and acid-sensitive.

On solid nutrient media, they form colonies resembling "little Nails" or buckwheat grains. The optimal growth Temperature for bifidobacteria is 37–41 °C, the optimal pH ranges from 6.0 to 7.0, and they do not grow at pH below 4.5 or above 8.5.

Bifidobacteria are strict anaerobes, catalase-negative, and highly fastidious regarding nutritional requirements. The proliferation of these microorganisms depends on a vast array of growth factors. Certain bifidobacteria strains grow exclusively in the presence of specific growth factors found in human breast milk but absent in cow's milk. These factors are nitrogen-containing Oligosaccharides composed of N-acetylglucosamine, N-acetylgalactosamine, N-acetylmannosamine, and others, which are essential for the synthesis of bifidobacterial Cell walls. The best-known bifidogenic factor is lactulose (beta-galactoside-fructose). Lactulose is not hydrolyzed by the enzyme β-galactosidase synthesized by the small intestinal mucosa, and thus it reaches the Large Intestine. There, it is actively utilized by bifidobacteria, Lactobacillus acidophilus, and enterococci, which accumulate lactic and acetic acids, thereby lowering the pH of the fecal mass. Substances that stimulate the growth of bifidobacteria and lactobacilli in the intestine (in vivo) are termed prebiotics.

Bifidobacteria ferment CARBOHYDRATES via the so-called fructose-6-phosphate pathway, during which two moles of glucose are converted into lactic and acetic acids in a 2:3 ratio, yielding 2.5 ATP per mole of glucose (Fig. 31).

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Fig. 31. Scheme of carbohydrate Fermentation by bifidobacteria

Bifidobacteria play a vital role in the Human and Animal Organism. It is now well established that There is a close correlation between human health and the COMPOSITION OF THE intestinal microflora, referred to as the normal microflora.

The beneficial effects of bifidobacteria on The Human Body are attributed to their enzymatic, vitamin-synthesizing, and antagonistic activities.

During carbohydrate fermentation, as indicated in the pathway, bifidobacteria produce acetic acid and the L(+)-isomer of lactic acid. This leads to a decrease in the intra-intestinal pH, which inhibits the proliferation of pathogenic bacteria, reduces the absorption of ammonia and biogenic amines into the bloodstream, and promotes the assimilation of calcium, iron, and vitamin D. Acetic acid, produced in large quantities, exerts a stronger antagonistic effect against Gram-negative bacteria compared to lactic acid. Bifidobacteria synthesize a variety of Vitamins and Essential Amino Acids, lower Cholesterol levels, and exhibit anticarcinogenic and antimutagenic activities.

The normal intestinal microflora provides colonization resistance, which prevents the gut from being colonized by harmful microflora. Bifidobacteria adhere to the intestinal mucosal epithelium, creating a mechanical barrier against the invasion of enteric pathogens. Furthermore, bifidobacterial cells synthesize an exopolysaccharide, forming a biofilm on mucosal surfaces.