Spores ofClostridiumbotulinum were found to initiate growth and to produce toxin in aqueous suspensions of soya protein at pH values as low as 4‐2 and in skimmed milk at pH 4.4. Most of the experiments were done with mixed cultures ofCI. botulinumtypes A and B in the presence of two strains ofBacillus subtilis.The role of the latter organism was concluded to be to lower the oxygen content and the Ehof the suspensions. Toxin was produced at pH 4‐4 after 4 weeks of incubation at 30oC when either hydrochloric or citric acids were used as the acidulant and after 12 and 14 weeks when, respectively, lactic and acetic acids were used. Thus, amongst other factors the nature of the acid and not solely the pH value is an important factor in controlling the growth ofCl. botulinumat low pH. Pure cultures ofCl botulinumtype A grew at 30oC under strictly anaerobic conditions and produced toxin at pH 4‐3 in the presence of hydrochloric
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