Chapter 18 Fusobacteria, Leptotrichia and spirochaetes
Fusobacteria are non-sporing, anaerobic, non-motile, non- or weakly fermentative, spindle-shaped bacilli (with fused ends: hence the name). They are normal inhabitants of the oral cavity, colon and female genital tract and are sometimes isolated from pulmonary and pelvic abscesses. Fusospirochaetal infections, which they cause in combination with spirochaetes, are noteworthy. Fusobacterium nucleatum (the type species), Fusobacterium periodontium and Fusobacterium simiae are isolated mainly from periodontal disease sites, and others such as Fusobacterium alocis and Fusobacterium sulci are sometimes found in the healthy gingival sulcus. Non-oral species include Fusobacterium gonidiaformans, Fusobacterium russii and Fusobacterium ulcerans.
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.
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.
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.
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:
Leptotrichia spp. are oral commensals previously thought to belong to the genus Fusobacterium. They are Gram-negative, strictly anaerobic, slender, filamentous bacilli, usually with one pointed end. Leptotrichia buccalis, present in low proportions in dental plaque, is the sole representative of this genus.
Spirochaetes are helical organisms with a central protoplasmic cylinder surrounded by a cytoplasmic membrane (Fig. 18.3). The cell wall is similar to Gram-negative bacteria but stains poorly with the Gram stain. Underneath the cell wall run three to five axial filaments that are fixed to the extremities of the organism. Contractions of these filaments distort the bacterial cell body to give it its helical shape. The organism moves either by rotation along the long axis or b/>