Taxonomy and Ecology
The genus Fusobacterium belongs to the family Fusobacteriaceae and comprises about 15 species. The type species is F. nucleatum. Other species of major medical importance include F. mortiferum and F. necrophorum.
Normally, fusobacteria colonize various human mucous membranes. Their preferred habitat in the oral cavity is the gingival crevice. In addition, they are consistently found as part of the normal microbiota of the gut and genital tract.
Morphological and Cultural Properties
Fusobacteria are pleomorphic bacilli. They got their name from their frequently observed characteristic spindle shape (thickened in the center and tapered at the ends).
Key characteristics:
- Gram stain: Gram-negative (G-).
- Motility and spore formation: The bacteria are strictly non-motile and do not form spores.
- Respiration: They are strict (obligate) anaerobes.
When cultured on blood agar, they form small colonies. A characteristic cultural feature is the formation of a zone of $\alpha$-hemolysis around the growing colonies. During metabolism, these microorganisms produce large amounts of indole and butyric acid. These substances are responsible for the key diagnostic feature of the culture — a pronounced putrid odor. The bacteria also exhibit lecithinase activity.
Pathogenicity Factors and Resistance
The antigenic structure of fusobacteria includes the somatic O-antigen. Despite being part of the normal microbiota, they possess a potent arsenal of virulence factors that trigger infectious processes when immunity is compromised or mucous membranes are traumatized.
Key Virulence Factors:
- Endotoxin — released upon the breakdown of the bacterial cell wall.
- Pili — structures providing strong adhesion to host cells.
- Volatile fatty acids.
- Aggressive enzymes — neuraminidase, phospholipase, and leukocidin (destroy host tissues and defense cells).
Environmental Resistance: Fusobacteria have extremely low environmental resistance. Being strict anaerobes, they die instantly upon any contact with atmospheric oxygen. Furthermore, the microorganisms are highly susceptible to standard disinfectants and antiseptic solutions.