Morphology and Physiology
Bifidobacterium species are Gram-positive, non-acid-fast rods. They lack flagella (non-motile) and do not form spores.
These bacteria exhibit marked polymorphism. Under the microscope, they may appear as short, regular rods, thin cells with club-like swollen ends, or branched structures. They can be found singly, in pairs, arranged in V-shapes, "Chinese character" formations, rosettes, or chains.
Bifidobacteria are strict (obligate) anaerobes and capnophiles. Their growth requires the absence of oxygen (they are rapidly killed by exposure to air) and an elevated concentration of carbon dioxide. They grow on complex media at 37–40 °C within 1–2 days. On solid media, they form small, white or cream-colored colonies, whereas in semi-solid media, they exhibit characteristic "comet" or "nail-head" growth patterns.
Role in the Human Body
Bifidobacteria are primary constituents of the normal large intestinal microbiota. Their concentration in feces can reach $10^{10}$ CFU/g. Certain species, such as B. dentinum, inhabit the oral cavity, vagina, and skin.
Key Functions:
- Protective: Suppress the growth of pathogenic and opportunistic microbes via the production of lactic and acetic acids.
- Digestive: Participate in substrate degradation, activate membrane-bound digestion, and enhance the absorption of calcium, iron, and vitamin D (helping prevent anemia and rickets).
- Synthetic: Synthesize amino acids, vitamin K, and B-complex vitamins (including folic acid and $B_{12}$).
- Immune: Stimulate macrophages, protect secretory immunoglobulin A (IgA) from degradation, and modulate immune responses.
- Anti-allergic: Reduce histamine production by blocking the decarboxylation of dietary histidine.
Diagnostics and Application
Bifidobacteria are classic commensals and do not possess pathogenic properties. A deficiency of these microorganisms leads to protein-vitamin-mineral malnutrition. Probiotics—preparations containing live bacteria—are utilized to restore the microbiota.
In laboratory diagnostics, traditional biochemical methods (such as detecting fructose-6-phosphate phosphoketolase) are rarely used today due to the difficulty of maintaining strict anaerobic conditions and the large number of species sharing similar phenotypic traits. Polymerase chain reaction (PCR) serves as the primary modern identification method.