Chapter 13 Actinomycetes, clostridia and Bacillus species
Actinomycetes, which were formerly thought to be fungi, are true bacteria with long, branching filaments analogous to fungal hyphae. The two important genera of this group are Actinomyces and Nocardia. The chemical structure of the cell wall of these organisms is similar to that of corynebacteria and mycobacteria, and some are acid-fast. Actinomyces spp. are microaerophilic or anaerobic; Nocardia spp. are aerobic organisms.
Although most Actinomyces are soil organisms, the potentially pathogenic species are commensals of the mouth in humans and animals. They are a major component of dental plaque, particularly at approximal sites of teeth, and are known to increase in numbers in gingivitis. An association between root surface caries of teeth and Actinomyces has been described. Other sites colonized are the female genital tract and the tonsillar crypts.
A number of Actinomyces species are isolated from the oral cavity. These include Actinomyces israelii, Actinomyces gerencseriae, Actinomyces odontolyticus, Actinomyces naeslundii (genospecies 1 and 2), Actinomyces myeri and Actinomyces georgiae. A close relationship between Actinomyces odontolyticus and the earliest stages of enamel demineralization, and the progression of small caries lesions have been reported. The most important human pathogen is A. israelii.
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.)
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.
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.
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.
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.
Nocardia species are soil saprophytes and cause nocardiosis in humans, especially in immunocompromised patients. These organisms are aerobic, Gram-positive rods, which form thin, branching filaments. Nocardia asteroides causes the most common form of human nocardiosis, which is essentially a pulmonary infection that progresses to form abscesses and sinus tracts.
Clostridia comprise many species of Gram-positive, anaerobic spore-forming bacilli (but spores are not found in infected tissues); a few are aerotolerant. They are an important group of pathogens widely distributed in soil and in the gut of humans and animals. There are four medically important species (Clostridium tetani, Clostridium botulinum, Clostridium welchii and Clostridium difficile) that cause significant morbidity and mortality, especially in developing countries. The major diseases caused by these organisms are listed in Table 13.1.
|C. welchii||Gas gangrene, food poisoning, bacteraemia, soft-tissue infections|
|C. botulinum||Botulism (foodborne, infant, wound)|
|C. difficile||Pseudomembranous colitis, antibiotic-associated diarrhoea|
(e.g. C. septicum, C. ramosum, C. novyi, C. bifermentans)
|Bacteraemia, gas gangrene, soft-tissue infections|
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.