Habitat and transmission

Several subspecies of F. nucleatum have been identified in different habitats. These include F. nucleatum subsp. polymorphum, found in the healthy gingival crevice, and F. nucleatum subsp. nucleatum, recovered mainly from periodontal pockets. A third subspecies is F. nucleatum subsp. vincentii. Infections are almost invariably endogenous.

Characteristics

Gram-negative, strictly anaerobic, cigar-shaped bacilli with pointed ends (Fig. 18.1). Cells often have a central swelling. A Gram-stained smear of deep gingival debris obtained from a lesion of acute ulcerative gingivitis is a simple method of demonstrating the characteristic fusobacteria, together with spirochaetes and polymorphonuclear leukocytes (Fig. 18.2). These, together with the clinical picture, confirm a clinical diagnosis of acute ulcerative gingivitis.

Fig. 18.1 A photomicrograph of fusobacteria showing characteristic Gram-negative, cigar-shaped cells with pointed ends.

Fig. 18.2 A Gram-stained smear obtained from deep gingival plaque of a patient with acute ulcerative gingivitis (see also Fig. 33.6) showing the fusospirochaetal complex. Note: the large cells are polymorphs.

Culture and identification

Grows on blood agar as dull, granular colonies with an irregular, rhizoid edge. Biochemical reactions and the acidic end products of carbohydrate metabolism help identification. As fusobacteria can remove sulphur from cysteine and methionine to produce odoriferous hydrogen sulphide and methylmercaptan, they are thought to be associated with halitosis.

Pathogenicity

The endotoxin of the organism appears to be involved in the pathogenesis of periodontal disease. It possesses remarkable adherence properties and the fusobacterium adhesin A (FadA), which confers this property has recently been isolated. F. nucleatum is usually isolated from polymicrobial infections; it is rarely the sole pathogen. Thus, in combination with oral spirochaetes (Treponema vincentii and others), it causes the classic fusospirochaetal infections. These are:

As fusobacteria coaggregate with most other oral bacteria, they are believed to be important bridging organisms between early and late colonizers during plaque formation (see Fig. 31.3).

Antibiotic sensitivity and prevention

Fusobacteria are uniformly sensitive to penicillin and, being strict anaerobes, are sensitive to metronidazole. Regular oral hygiene and antiseptic mouthwashes are the key to prevention of oral fusobacterial infections in susceptible individuals.