Effects of Glucose, pH, and Dissolved-Oxygen Tension on Bacillus cereus Growth and Permeability Factor Production in Batch Culture

Abstract

The production of a Bacillus cereus enterotoxin, measured as rabbit skin permeability factor (PF), in response to differences in glucose availability, pH, and dissolved oxygen tension was studied in a 1-liter batch fermentor system. Glucose had to be present for toxigenesis to occur. In uncontrolled fermentation an increasing inhibition of PF production and growth occurred as pH dropped occurred below 6.5. Optimum pH for toxigenesis was 7.0 to 7.5, and fermentations maintained at this level yielded 10- to 20-fold more PF than comparable uncontrolled fermentations. PF production was appreciably diminished at or below pH 6.0 and at or above pH 8.5. Peak PF titer was associated with a drop in acid output, and the titrant utilization profile could be used as an indication of this point. Productivity was greatest in the early exponential phase of growth and decreased to zero at the transition phase. Differences in dissolved oxygen tension affected both the maximum productivity early in the fermentation and the rate of its decrease as growth progressed. The optimum dissolved oxygen tension for toxigenesis was 0.002 atm, and the most rapid growth occurred at 0.10 atm. Productivity and growth were reduced under anerobic conditions, whereas a hyperoxic environment severely reduced productivity, but not growth. Postexponential-phase loss of toxic activity coincided with a rapid increase in cellular oxygen demand. Neither was inhibited by the presence of glucose. However, PF loss was completely prevented by stringent oxygen limitation. Extracellular proteolytic activity did not appear to be responsible for the loss of toxic activity.