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 periodonticum 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.
Although not a component of oral microbiota, it is noteworthy that Fusobacterium necrophorum is the cause of Lemierre's disease characterized by acute jugular vein septic thrombophlebitis and subsequent systemic metastatic abscesses.
Fusobacteria
Fusobacterium nucleatum
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.
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 methyl mercaptan, 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:
■ acute (necrotizing) ulcerative gingivitis or trench mouth (see Chapter 33)
■ Vincent's angina, an ulcerative tonsillitis causing tissue necrosis, often due to extension of acute ulcerative gingivitis
■ cancrum oris or noma: a sequela of acute ulcerative gingivitis with resultant gross tissue loss of the facial region.
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. 31.2).
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.
Fig. 18.1 A photomicrograph of fusobacteria showing characteristic Gramnegative, 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.)
Fig. 18.3 Structure of a spirochaete (top) and the morphology of the three major genera of spirochaetes.
low proportions in dental plaque, is the sole representative of this genus.
Spirochaetes
Spirochaetes are a diverse group of spiral, motile organisms comprising five genera. Of these, three genera are human pathogens:
■ Treponema causes syphilis, bejel, yaws, pinta and, in the oral cavity, acute necrotizing ulcerative gingivitis (together with fusobacteria)
■ Borrelia causes relapsing fever and Lyme disease
■ Leptospira causes leptospirosis.
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 by flexion of cells. Because of their weak refractile nature, dark-ground microscopy is used to visualize these organisms in the laboratory, although immunofluorescence is more useful for identification purposes. All spirochaetes are strictly anaerobic or microaerophilic.
Leptotrichia
Leptotrichia spp. are oral commensals previously thought to belong to the genus Fusobacterium. They are Gramnegative, strictly anaerobic, slender, filamentous bacilli, usually with one pointed end. Leptotrichia buccalis, present in
Treponema
The coils of Treponema are regular, with a longer wavelength than that of Leptospira (Fig. 18.3). A number of species and subspecies are recognized, some of which are important systemic pathogens, whereas others are oral inhabitants implicated in periodontal disease.
Fig. 18.4 Treponema pallidum spirochetes in a tissue fluid sample from a secondary clinical lesion stained with a silver stain (Courtesy Professor Dr Willie Van Heerden, University of Pretoria, South Africa).
Fig. 18.5 Scanning electron micrograph of the radicular surface of a tooth affected by advanced periodontal disease showing the spirochaetes on the root surface.
Treponema pallidum
Habitat and transmission
Lesions of primary and secondary syphilis
Transmission is by direct contact with lesions, body secretions, blood, semen and saliva, usually during sexual contact, and from mother to foetus by placental transfer.
Characteristics
Slender, corkscrew-shaped cells with 6-12 evenly spaced coils, 6-14 x 0.2 μm; too slender to visualize by light microscopy but can be seen by silver impregnation (Fig. 18.4) or immunofluorescent techniques; strictly anaerobic and extremely sensitive to drying and heat, hence dies rapidly outside the body.
Culture and identification
Cannot be cultured in vitro, but can be propagated in the testes of rabbits; Treponema pallidum thus harvested can be used as antigens to detect specific antibody in the patient's serum.
Dark-ground microscopy of tissue fluid from primary and secondary clinical lesions helps identification, but serological tests are the mainstay of diagnosis.
Pathogenicity
Causes syphilis, a sexually transmitted disease with protean manifestations (see Chapter 27). The virulence factors of Treponema pallidum are not well characterized. Immunopathology plays a significant role in disease manifestations, especially in the late (tertiary and quaternary) stages of the disease.
Antibiotic sensitivity and control
Penicillin is the drug of choice; for allergic patients, tetracycline is an alternative. Prevention of syphilis is based on early detection, contact tracing and serological testing of pregnant women.
Treponema pallidum subsp. pertenue
The agent of yaws, characterized by chronic, ulcerative, granulomatous lesions of skin, mucosae and bone. The disease, widespread in the tropics, is spread by direct contact.
Treponema carateum
The agent of pinta, a non-venereal skin infection characterized by depigmented and hyperkeratotic skin. The disease affects mainly dark-skinned natives of Central and South America and the West Indies.
Oral treponemes
All oral spirochaetes are classified in the genus Treponema. Although many species have been described, only four have been cultivated and maintained reliably: Treponema denticola, Treponema vincentii, Treponema pectinovarum and Treponema socran- skii. In another classification, they are categorized according to cell size as small, medium and large spirochaetes.
Habitat and transmission
Predominantly the oral cavity of humans and primates, at the gingival margin and crevice in particular. Transmission routes are unknown. Infections are endogenous.
Characteristics
Motile, helical rods, 5-15 x 0.5 μm, with irregular (three to eight) spirals, which are less tightly coiled than, for instance, Treponema pallidum (Figs 18.3 and 18.5). Cell walls are Gram-negative but stain poorly. The size is variable and can be used as a basis for classification (large, medium or small).
Culture and identification
In contrast to Treponema pallidum, oral spirochaetes can be grown in vitro. They are strict anaerobes, slow-growing in oral treponema isolation (OTI) medium. Subspecies can be differentiated by fermentation reactions and serology (agglutination).
Suspect lesions of acute necrotizing ulcerative gingivitis or advanced periodontitis can be examined by obtaining a Gram-stained smear of deep gingival plaque and visualizing the characteristic fusospirochaetal complex under light microscopy (see Fig. 18.2); alternatively, dark-ground microscopy may be used.
Pathogenicity
These organisms are a component of the fusospirochaetal complex of acute necrotizing ulcerative gingivitis and Vincent's angina, and are a coagent of advanced periodontal disease. The ability to travel through viscous environments enables oral spirochaetes to migrate within the gingival crevicular fluid and to penetrate sulcular epithelial linings as well as gingival connective tissue. Virulence factors are little known; endotoxin is possibly contributory to disease. Treponema denticola is more proteolytic than other species and degrades collagen and dentine.
Antibiotic sensitivity and control
Sensitive to penicillin and metronidazole. Prevention of infection is achieved by good oral hygiene practices.
Borrelia
Borrelia burgdorferi
Habitat and transmission
Found in ticks and small mammals, particularly deer. Transmission is by a tick vector.
Characteristics
This species is a helical spirochaete, 0.18-0.25 x 4.3 μm. Gram-negative, it grows under microaerophilic conditions at 34°C. Identification is by serology and immunofluorescence or enzyme-linked immunosorbent assay (ELISA).
Pathogenicity
The agent of Lyme disease, a generalized infection with neurological and cardiac manifestations and arthritis. One of the earliest and most common neurological manifestations is unilateral facial palsy.
Antibiotic sensitivity
Sensitive to tetracycline and amoxicillin.
Other Borrelia species
These include Borrelia recurrentis and Borrelia duttonii, agents of louse-borne and tick-borne relapsing fever, respectively, seen in parts of Africa, Asia and South America.
Leptospira
Leptospira biflexa and Leptospira interrogans are the recognized species, each of which comprises a number of serogroups.
These organisms are found in damp environments such as stagnant water and wet soil. The kidneys of some rodents and domestic animals act as a reservoir for Leptospira interrogans. The urine of these animals serves as a vehicle of transmission of human leptospirosis, the symptoms of which vary from mild febrile illness to fatal attacks of jaundice and renal failure.
Key facts
• Fusobacteria are non-sporing, anaerobic, spindle-shaped bacilli inhabiting the oral cavity, colon and female genital tract.
• The type species Fusobacterium nucleatum and Fusobacterium periodonticum are isolated mainly from periodontal disease sites and hence considered to be periodontopathic bacteria.
• Fusospirochaetal infections caused by fusobacteria in combination with spirochaetes are acute ulcerative gingivitis, Vincent’s angina and cancrum oris (or noma).
• Spirochaetes are long, slender, coiled and highly mobile bacteria that do not take up the Gram stain.
Spirochaetes comprise three genera: Treponema, Borrelia and Leptospira.
Treponema pallidum, the agent of syphilis, cannot be cultivated in vitro and is uniformly sensitive to penicillin.
All oral spirochaetes are classified in the genus Treponema (type strain Treponema denticola).
Treponema denticola is a coagent of fusospirochaetal infection and advanced periodontal disease.
Treponema denticola, Tannerella forsythia and Porphyromonas gingivalis are considered the three agents of red complex bacteria almost always associated with periodontal disease.
Review questions (answers on p. 365)
Please indicate which answers are true, and which are false.
18.1 Which of the following statements on acute (necrotizing) ulcerative gingivitis (ANUG) are true?
A. ANUG is a polymicrobial infection
B. ANUG is a complication of advanced periodontal disease
C. a sequela of ANUG may be gross facial tissue loss
D. metronidazole is the antimicrobial of choice for ANUG
E. ANUG is often preventable with good oral hygiene
18.2 Spirochaetes:
A. possess cell walls similar to that of mycobacteria
B. are best viewed using dark-ground or fluorescence microscopy
C. are implicated in Vincent's angina
D. are found in the oral cavity and can be grown in vitro
E. are generally sensitive to penicillin
18.3 Spirochaetal infections:
A. are a cause of human Lyme disease
B. if systemic, are traditionally diagnosed using serology
C. can lead to liver failure and renal failure
D. may cause facial palsy
E. induce lifelong immunity
Further reading
Duerden, B. I., & Drasar, B. S. (Eds.), (1991). Anaerobes in human disease. London: Edward Arnold.
Greenwood, D., Slack, R., & Peutherer, J. (Eds.), (2003). Medical microbiology (16th ed.). Edinburgh: Churchill Livingstone. Chapters 37 and 38.
Holt, S. C., & Ebersole, J. L. (2005). Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia: the "red complex", a prototype polybacterial pathogenic consortium in periodontitis. Periodontology 2000, 38, 72-122.