Habitat and transmission

This organism is a commensal of the mouth and possibly of the female genital tract. It is a major agent of human actinomycosis.

Characteristics

Gram-positive filamentous branching rods. Non-motile, non-sporing and non-acid-fast. Clumps of the organisms can be seen as yellowish ‘sulphur granules’ in pus discharging from sinus tracts, or the granules can be squeezed out of the lesions. (Strains belonging to A. israelii serotype II are now in a separate species, A. gerencseriae, a common but minor component of healthy gingival flora.)

Culture and identification

Grows slowly under anaerobic conditions, on blood or serum glucose agar at 37°C. After about a week, it appears as small, creamy-white, adherent colonies on blood agar. The colonies resemble breadcrumbs or the surface of ‘molar’ teeth (Fig. 13.1). Because of the exacting growth requirements and the relatively slow growth, isolating this organism from clinical specimens is difficult, particularly because the other, faster-growing bacteria in pus specimens tend to obscure the slow-growing actinomycetes. ‘Sulphur granules’ in lesions are a clue to their presence. When possible, these granules should be crushed, Gram-stained and observed for Gram-positive, branching filaments, and also cultured in preference to pus.

Fig. 13.1 Molar tooth-shaped colonies of Actinomyces israelii on blood agar.

Pathogenicity

Most (70–80%) actinomycotic infections are chronic, granulomatous, endogenous infections of the orofacial region (Fig. 13.2). Typically, the lesions present as a chronic abscess, commonly at the angle of the lower jaw, with multiple external sinuses. There is usually a history of trauma such as a tooth extraction or a blow to the jaw. Actinomycetes are also isolated from infections associated with intrauterine devices, but their pathogenic role is unclear.

Fig. 13.2 A histopathological section from an actinomycetic lesion of the mandible showing a branching filamentous mass of Actinomyces spp. infiltrating the bony cortex.

While the majority of the lesions (60–65%) are in the cervicofacial region, some 10–20% are abdominal (usually ileocaecal) and others are in the lung (thoracic) or skin. Although most infections are monomicrobial in nature (i.e. with Actinomyces alone causing the disease), a significant proportion of infections could be polymicrobial, with other bacteria such as Aggregatibacter actinomycetemcomitans, Haemophilus spp. and anaerobes acting as co-infecting agents.

Treatment and prevention

Sensitive to penicillin, but prolonged courses up to 6 weeks are necessary for chronic infections. Oral penicillins such as amoxicillin are now popular. Recalcitrant lesions respond well to tetracycline because of its good bone penetration. Surgical intervention may be necessary in chronic jaw lesions.

Prevention of these infections is difficult because of their endogenous nature.

Habitat

Soil, water, decaying animal and plant matter, and human and animal intestines.

Characteristics

Gram-positive rods, but older cultures may stain irregularly. All species form characteristic endospores, which create a bulge in the bacterial body, for instance, the drumstick-shaped C. tetani (this shape is useful in laboratory identification of the organisms). Some species are motile with peritrichous flagella (e.g. C. tetani), while others (e.g. C. welchii) have a capsule.