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Using an agar dilution method we measured the minimum inhibitory concentration (MIC) of 12 antimicrobial agents against 11 strains of iota-toxigenic strains of Clostridium spiroforme. Each strain was isolated from a separate outbreak of toxic diarrhoea of rabbits. Vancomycin and bacitracin, both agents used to treat intestinal clostridioses of humans and other animals, had a relatively high MIC (8 micrograms/ml or more). Metronidazole was uniformly active against C. spiroforme. With MIC of 8 micrograms/ml or more, both lincomycin (11 strains) and erythromycin (9 strains) were relatively inactive against C. spiroforme, conversely, penicillin G was active (MIC for 8 strains was 0.5 micrograms/ml or less). Exposure to any one of these drugs has been implicated as a predisposing factor for C. spiroforme mediated diarrhoea of rabbits. The greatest variation in MIC was seen for erythromycin (8-fold), penicillin G (8-fold) and tetracycline (16-fold).

The helically-coiled bacterium, Clostridium spiroforme, has been shown to consist of an ordered aggregation of numerous individual semi-circular cells joined end to end.

The effects of divalent cations (Ca2+, Co2+ and Zn2+) on the production of iota toxin by Clostridium spiroforme were studied. Toxin production had an absolute requirement for one or more cations in the range 1-5 mM. Using bispecific antisera, we showed that production of both the components of the toxin (ia and ib) were enhanced by divalent cations added to brain-heart infusion supplemented with peptone and glucose.

Commercial rabbits showing clinical signs of enteritis-complex were examined for the presence of Clostridium spiroforme and its iota-like toxin. The bacterium was detected by Gram stain in 52.4% of 149 cecal samples and iota-like toxin in 7.4%. From 29 strains of C. spiroforme tested, 26 were toxigenic, originating from 24 of 29 rabbitries. In 13.4% of the samples, C. spiroforme was present as the only known disease agent. Gross and microscopic lesions were similar to those described in the literature. In the other samples, C. spiroforme was associated with attaching effacing Escherichia coli (29.5%), Bacillus piliformis (10.3%), rotaviruses (25.6%), coronavirus (2.6%), Eimeria spp. (44.9%) and cryptosporidia (6.4%). In 33.3% of C. spiroforme-containing samples, more than one of these agents was present. There was no significant difference between the presence of these organisms in C. spiroforme-positive and negative samples. On the basis of these results as well as that of previous data, we suggest that C. spiroforme-mediated diarrhea is favoured by maldigestion, initiated by infectious agents and/or nutritional factors.

Newly weaned rabbits had diarrhoea only if they were infected with Clostridium spiroforme. In adult rabbits exposure to both clindamycin and C. spiroforme was necessary to induce disease. All diseased animals harboured C. spiroforme and its toxin. Adult rabbits given a course of clindamycin survived when held in a protected environment as did those challenged with C. spiroforme alone. At necropsy none of these apparently healthy animals showed signs of diarrhoea or caecitis. These findings suggest that, in the development of enterotoxaemia, weaning or clindamycin treatment and infection with C. spiroforme are separate events and that disease follows infection with this organism from the environment, as opposed to overgrowth by undetectable levels of C. spiroforme resident in the gut. Our data indicate that C. spiroforme is not a normal component of the rabbit gut flora and that the normal bowel ecology of the adult must be disrupted before C. spiroforme will colonize.