Taxonomy and ecology
The genus Fusobacterium belongs to the family Fusobacteriaceae and includes about 15 species. The type species is F. nucleatum. Species of major medical importance also include F. mortiferum and F. necrophorum.
Normally, fusobacteria colonize various human mucous membranes. Their preferred habitat in the oral cavity is the gingival crevices. In addition, they are consistently found in the microflora of the intestine and genital tract.
Morphological and cultural properties
Fusobacteria are pleomorphic bacilli. They got their name from their frequently encountered characteristic spindle-like shape (thickened in the center and tapered at the ends).
Key characteristics:
- Staining: Gram-negative (Gram negative).
- Motility and spore formation: the bacteria are strictly non-motile and incapable of forming spores.
- Respiration: strict (obligate) anaerobes.
When cultivated on blood agar, they form small colonies. A characteristic cultural feature is the formation of a zone of $\alpha$-hemolysis around the grown colonies. During metabolism, the microorganisms produce a large amount of indole and butyric acid. These exact substances determine the most important diagnostic feature of the culture — a pronounced putrid odor. The bacteria also exhibit lecithinase activity.
Pathogenicity factors and resistance
The antigenic structure of fusobacteria is represented by somatic O-antigen. Despite belonging to the normal microflora, they possess a powerful arsenal of pathogenicity factors that trigger the infectious process when immunity is weakened or mucous membranes are injured.
Key pathogenicity factors:
- Endotoxin — released when the bacterial cell wall is destroyed.
- Pili — structures ensuring reliable adhesion to host cells.
- Volatile fatty acids.
- Aggressive enzymes — neuraminidase, phospholipase, and leukocidin (destroy host tissues and defense cells).
Environmental stability: The resistance of fusobacteria is extremely low. Being strict anaerobes, they die instantly upon any contact with atmospheric oxygen. Furthermore, microorganisms are highly sensitive to standard disinfectants and antiseptic solutions.
Remember the spindle (fusus) hidden in the gum: it fears air (strict anaerobe), cannot move (non-motile), and smells bad (butyric acid + indole).
Class and exam questions
Why did fusobacteria get this name?
The name comes from the Latin word fusus (spindle). This is due to the characteristic morphology of the bacteria — they appear as pleomorphic spindle-shaped bacilli.
By what characteristic feature can the growth of fusobacteria on a nutrient medium be suspected?
The main diagnostic feature is the sharp putrid odor of the culture. It appears because the bacteria actively produce butyric acid and indole.
How do fusobacteria tolerate being in the external environment?
They are extremely unstable. Being obligate anaerobes, they die almost instantly when exposed to open air.
What else to study on this topic
- Features of the pathogenesis of infections caused by F. nucleatum
- The role of F. necrophorum in the development of severe necrotic processes
- Mechanism of action of aggressive enzymes (leukocidin and phospholipase)
- Specifics of isolation and cultivation of obligate anaerobes
- Interaction of fusobacteria with other members of the gingival crevice microflora
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