Some rare strains of Clostridium butyricum carry the gene encoding the botulinal type E neurotoxin and must be considered as possible hazards in certain types of food. The limiting growth conditions for C. butyricum were determined in peptone yeast glucose starch (PYGS) broth incubated anaerobically at 30°C for up to 42 days. The minimum pH values permitting growth depended on the acidulant and strain. Organic acids were more effective at inhibiting growth than HCl as expected. The lowest pH values at which growth of toxigenic and nontoxigenic strains of C. butyricum was observed in broth acidified with HCl were 4.1 and 4.2, respectively. In organic acids, however, the minimum pH varied between 4.4 and 5.1 depending on acid type and concentration. The minimum water activity for growth of toxigenic strains of C. butyricum was 0.96. The minimum growth temperatures of the toxigenic strains of C. butyricum (ca 10-11°C) were somewhat higher than for non-toxigenic ones (8°C). It was concluded that control of toxigenic C. butyricum in the food industry needs to allow for the greater pH tolerance of this species compared with proteolytic C. botulinum. 1. Introduction The main species of Clostridium associated with foodborne illness are Clostridium botulinum and C. perfringens and much effort has been directed towards defining the thermal treatments needed to inactivate their spores and the environmental conditions that will prevent their growth in food [1, 2]. Other Clostridia occur in certain types of food and; whilst they may cause serious spoilage problems, are not normally regarded as dangerous. Examples include Clostridium tyrobutyricum and other butyric Clostridia in cheese [3–7], C. pasteurianum and C. butyricum in canned fruit [8, 9], and the Clostridia that cause soft rot of vegetables [10]. Most strains of C. butyricum are harmless to humans and culture filtrates are not lethal to mice; in fact some strains have been investigated as probiotics for preventing diarrhoea [11–13], Helicobacter pylori infection [14], and inflammatory bowel disease [15]. However, some strains have acquired the type E botulinal neurotoxin gene (BoNT/E) and have caused both infant and classical foodborne botulism. The first recorded incident of C. butyricum type E botulism was a case of infant botulism in Italy [16]. Further cases of intestinal colonization by toxigenic C. butyricum strains were recorded in Italy but in older patients [17]. In 1994, an outbreak of botulism associated with C. butyricum occurred in the town of Guanyun, China, associated with consumption of a
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