Most toxigenic strains of produce two toxins: An enterotoxin (toxin A) and a cytotoxin (toxin B). Only one strain (strain 8864) has been reported to produce toxin B but no toxin A. Serogroup F strains (44) of often isolated from asymptomatic infants, have been examined for toxin production. These strains, which were from distinct geographical and clinical sources, did not produce any detectable toxin A when examined in three distinct immunoassays. Nevertheless, all the strains produced a cytotoxin. Immunological differences between the cytotoxin of the serogroup F strains and that produced by strain VPI 10463 (serogroup G) were demonstrated with monoclonal antibodies specific for either the toxin B produced by strain VPI 10463 or lethal toxin (LT). Polymerase chain reaction amplification with primers derived from strain VPI 10463 toxin A and B genes showed that serogroup F strains seem to possess a toxin B gene homologous with that of strain VPI 10463 and at least fragments of the toxin A gene. When axenic mice were inoculated with serogroup F strains, the animals survived; they did not develop diarrhoea and no toxin A could be detected in their faeces. However, cytotoxin was detected. Furthermore, these mice were protected against subsequent challenge with the otherwise lethally toxigenic strain VPI 10463. The serogroup F strains appeared to be homogeneous and distinct from other strains with regard to toxin production.


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