This is a Gram-positive bacillus that varies considerably in
its appearance. It is generally about 4-6 μm by 0.6 μm, but shorter forms, long
forms, and very long filaments also occur. Spores are readily formed and, as
they develop, various shapes arise ranging from swollen Gram-positive 'citron-bodies'
to obviously sporing forms in which the oval spores may be central or
subterminal and are clearly bulging. Older Gram-negative cells may be seen. The
organism is actively motile with numerous peritrichous flagella. It is one of
the less exacting anaerobes and grows well at 37°C on ordinary media. It is
saccharolytic and glucose promotes growth. Surface colonies are irregular,
transparent, droplet-like, later becoming greyish and opaque with projecting
radiations that are coarser than those of Clostridium tetani. On horse blood agar,
haemolysis is observed. If the surface of the medium is not relatively dry, the
culture spreads across it.
ANIMAL PATHOGENICITY. Intramuscular injection of cultures in laboratory animals produces a spreading inflammatory oedema, with slight gas formation in the tissues. The organisms invade the blood and the animal dies within a day or two. Smears from the liver show long filamentous forms and also citron bodies.
EPIDEMIOLOGY The primary habitat of Clostridium septicum is either the soil or the animal intestine. Areas with large numbers of spores appear to be associated with a higher incidence of Clostridium septicum infections in man and animals than less heavily infected areas. It has been assumed that the spore is the primary agent of infection, but recent evidence indicates that vegetative forms of the organism may be abundant in the soil, and this finding merits further investigation. Clostridium septicum is responsible for braxy in sheep and for malignant oedema following wound infection in cattle and sheep.