The effect of oxygen and redox potential on growth of Clostridium botulinum type E from a spore inoculum

Small volumes of oxygen introduced into vials of medium at pH 7 prepared under anaerobic conditions reacted with the medium during sterilization by heating, and also raised the redox potential. The partial pressure of oxygen (pO2) in the medium, and the redox potential (Eh) were measured and their effect on the number of spores of Clostridium botulinum type E required to produce growth, and hence on the probability of growth from single spore inocula, was determined. In the presence of a pO2 of up to 4·5 × 10−3 atm resulting in an Eh of c. + 217 mV, the probability of growth from single spores within 5 days at 20°C was equal to that in strictly anaerobic conditions at an Eh of − 400 mV. At pO2 values of between 5·3 × 10−3 atm and 8·4 × 10−3 atm corresponding to Eh values of between + 226 mV and + 254 mV an inoculum of between 10 and 1000 spores was required to produce growth and at pO2 values of between 1·12 × 10−2 atm,and 1·6 × 10−2 atm, corresponding to redox potentials of between + 271 mV and + 294 mV, the number of spores required to produce growth was between 2 × 104 and > 2 × 105. The relationship between pO2 and Eh depends on the chemical nature of a culture medium or food, and in order to assess the probable influence of these parameters on growth of C. botulinum in a medium or a food it is necessary to determine both the redox potential and the partial pressure of oxygen.