Influence of water activity on the growth of Clostridium perfringens

Each of four strains of Clostridium perfringens was grown in modified fluid thioglycolate medium which was adjusted to yield selected water activity (a(w)) levels. The adjustments to secure the desired a(w) levels were made with NaCl, KCl, or glucose. At each a(w) level, further modification was effected to produce four pH values. Cultures were incubated at either 37 or 46 C. The solute used to achieve the reduced a(w) levels appeared to have a definite effect on the magnitude of growth achieved, the rate of growth, and the limiting a(w) at which growth would occur. Use of glucose as the controlling solute permitted growth at the lowest a(w) level tested, 0.960, and yielded the greatest magnitude of growth as measured by turbidity values, at all of the a(w) levels investigated. Cultures grown in the medium with added KCl generally demonstrated the longest lag times and the least amount of growth. Regardless of specific solute used, as the a(w) level was lowered and the pH value decreased within each a(w) level, the rate and amount of growth were lessened. It appeared, however, that low pH values had less effect on inhibiting growth at low a(w) levels than at higher a(w) levels. Those cultures incubated at 46 C generally exhibited shorter lag periods than those at 37 C, although the maximal growth attained was somewhat less than that achieved at 37 C. The response to all of the investigated conditions was similar for each of the four strains tested.