13: Actinomycetes, clostridia and Bacillus species

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.

Actinomyces israelii

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.


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.

Table 13.1 Common Clostridium species associated with human disease

Clostridium spp. Disease
C. welchii Gas gangrene, food poisoning, bacteraemia, soft-tissue infections
C. tetani Tetanus
C. botulinum Botulism (foodborne, infant, wound)
C. difficile Pseudomembranous colitis, antibiotic-associated diarrhoea
Other species
(e.g. C. septicum, C. ramosum, C. novyi, C. bifermentans)
Bacteraemia, gas gangrene, soft-tissue infections

Jan 4, 2015 | Posted by in General Dentistry | Comments Off on 13: Actinomycetes, clostridia and Bacillus species
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