Morphology and Physiology
Bifidobacteria (Bifidobacterium) are gram-positive, acid-sensitive rods. They lack flagella (non-motile) and do not form spores.
These bacteria exhibit pronounced polymorphism. In preparations, they may appear as short regular rods, thin cells with club-shaped swellings at the ends, or branched structures. The bacteria are arranged singly, in pairs, in the shape of the Latin letter V, "Chinese characters", rosettes, or chains.
Bifidobacteria are strict (obligate) anaerobes and capnophiles. Their growth requires an absence of oxygen (they die instantly in air) and an elevated carbon dioxide content. They grow on complex nutrient media at 37–40 °C within 1–2 days. On solid media, they form small white or cream-colored colonies, while in semi-solid media, they show characteristic growth resembling "comets" or "nails".
Role in the Human Body
Bifidobacteria are the main representatives of the normal microflora of the large intestine. Their concentration in feces reaches $10^{10}$ CFU/g. Some species, such as B. dentinum, inhabit the oral cavity, vagina, and skin.
Main functions:
- Protective: suppress the growth of pathogenic and opportunistic microbes through the production of lactic and acetic acids.
- Digestive: participate in the digestion of substrates, activate membrane digestion, and improve the absorption of calcium, iron, and vitamin D (preventing the development of anemia and rickets).
- Synthetic: produce amino acids, vitamin K, and B vitamins (including folic acid and $B_{12}$).
- Immune: stimulate macrophages, protect secretory immunoglobulin A (IgA) from degradation, and regulate immune responses.
- Antiallergic: reduce histamine production by blocking the decarboxylation of dietary histidine.
Diagnostics and Application
Bifidobacteria do not possess pathogenic properties; they are classic commensals. A deficiency of these microorganisms leads to protein-vitamin-mineral insufficiency. To restore microflora, probiotics are used—preparations containing live bacteria (Bifidumbacterin, Linex, etc.).
In laboratory diagnostics, traditional biochemical methods (determining the enzyme fructose-6-phosphate phosphoketolase) are rarely used today to identify bifidobacterial species. This is due to the difficulty of maintaining strict anaerobic conditions and the large number of species with similar properties. The main modern identification method is PCR (polymerase chain reaction).
Briefly: answers to topical questions
Which species of bifidobacteria are part of the normal human microflora?
The normal human microflora includes several key species of bifidobacteria. The main representatives are:
- Bifidobacterium bifidum (B. bifidum) — the main inhabitant of the gastrointestinal tract.
- Bifidobacterium breve (B. breve) — a representative of the normal intestinal microflora.
- Bifidobacterium longum (B. longum) — a representative of the normal intestinal microflora.
- Bifidobacterium infantis (B. infantis) — a representative of the normal intestinal microflora.
- Bifidobacterium adolescentis (B. adolescentis) — a representative of the normal intestinal microflora.
- Bifidobacterium dentinum (B. dentinum) — inhabits the oral cavity.
Which elective nutrient media are used to cultivate bifidobacteria?
Blaurock medium is used as an elective nutrient medium for the cultivation of bifidobacteria. The following media are utilized:
- Blaurock medium — an elective semi-solid medium providing characteristic growth in the form of "nails" or "comets".
- Thioglycollate medium — a semi-solid medium on which growth in the form of "nails" or uniform turbidity is also possible.
Overall, bifidobacteria require complex nutrient media and strict anaerobic conditions with an elevated carbon dioxide content.
Which probiotic preparations based on bifidobacteria are used to correct microflora?
Various groups of probiotics based on bifidobacteria are used to correct microflora. The main preparations include:
- Bifidumbacterin (B. bifidum) — a monocomponent preparation.
- Bificol (B. bifidum, E. coli) — an associated preparation.
- Bifiform (B. bifidum, E. faecium) — an associated preparation.
- Bifilong (B. bifidum, B. longum) — an associated preparation.
- Bifilact (B. bifidum, L. acidophilus) — an associated preparation.
- Linex (L. acidophilus, B. infantis, E. faecium) — an associated preparation.
- Bifiliz (B. bifidum) — a combined preparation with added lysozyme.
Bifidumbacterin forte and Probifor are also used.
Class and exam questions
Why are bifidobacteria named that way?
The name is related to their tendency to form forked, branching shapes (from Latin *bifidus* — cleft or divided).
Can bifidobacteria be grown in a standard laboratory?
This is difficult because they are strict anaerobes and die instantly in the presence of oxygen. Their cultivation requires special gas mixtures ($85\%\ N_2, 10\%\ CO_2, 5\%\ H_2$) and anaerobic conditions.
Which method is best suited for determining the species of bifidobacteria?
In modern practice, the "gold standard" for determining the species affiliation of bifidobacteria is PCR (polymerase chain reaction).
What else to study on this topic
- Key species of the genus Bifidobacterium
- Blaurock medium and thioglycollate medium: growth characteristics
- Mechanisms of antagonistic activity of bifidobacteria
- Biochemical pathway involving fructose-6-phosphate phosphoketolase
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