The combined effect of sub-optimal temperature and sub-optimal pH on growth and toxin formation from spores ofClostridium botulinum

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Abstract
Low-acid foods (pH .gtoreq. 4.5) are not sufficiently acidic to prevent growth of Clostridium botulinum in otherwise optimal conditions. The combination of sub-optimal pH and sub-optimal temperature may, however, result in a very significant reduction in the risk of growth of this bacterium compared with the risk in optimal conditions. The combined effect of incubation temperatures of 12 and 16 C and pH values between 5.2 and 5.5 on growth and toxin production from spores of Cl. botulinum during incubation for 28 d has been investigated. Growth and formation of toxin (type B) were detected only in medium at pH 5.5 and incubated at 16.degree. C. corresponding to a probability of growth from a single spore within 14 d of 1.6 .times. 10-5. The probability of growth in 28 d in the remaining conditions was < 9 .times. 10-6. After transfer of inoculated media from 12 to 30 C growth occurred at pH 5.2-5.5 within 19 d. After transfer of inoculated media from 12 to 20 C growth occurred at pH 5.5 and 5.4 but not at pH 5.3 or 5.2 in 40 d. Growth at pH 5.2-5.5 was accompanied by formation of toxin, in most cases of types A or B. In addition to the effect of sub-optimal temperature and pH, chelation of divalent metal ions by citrate may have contributed to inhibition.