Growth and toxin formation by Clostridium botulinum at low pH values

Spores of Clostridium botulinum 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 of CI. botulinum types A and B in the presence of two strains of Bacillus subtilis. The role of the latter organism was concluded to be to lower the oxygen content and the E_h of the suspensions. Toxin was produced at pH 4-4 after 4 weeks of incubation at 30^oC 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 of Cl. botulinum at low pH. Pure cultures of Cl botulinum type A grew at 30^oC under strictly anaerobic conditions and produced toxin at pH 4-3 in the presence of hydrochloric acid.