Growth Kinetics of a Native Toxigenic Isolate of Bacillus Cereus under the Influence of Incubation Temperature, pH and Sodium Chloride in Broth System

Aims: The growth behavior of vegetative cells and spores of a native toxigenic food isolate of Bacillus cereus CFR 1534 was studied under the influence of incubation temperature, pH and sodium chloride.

Place and Duration of Study: The study was undertaken in the Department of Human Resource Development, CSIR-Central Food Technological Research Institute, Mysore 570020, India. The duration of study was during the period October 2009 to February 2010.

Methodology: The experimental design was a central composite design (CCD) based on 3 factors and 5 levels. The factors for vegetative cells were incubation temperature (12-48ºC), pH level (5.5-7.5) and sodium chloride (2-6%). With spores, the ranges of pH and NaCl levels were the same, while incubation temperature range was 22 to 42ºC.

Results: Multiple regression analysis of experimental data relating to lag phase duration (LPD) and growth rate (GR) of B. cereus across the influencing factors in broth system revealed that LPD of vegetative cells and spores was primarily influenced by incubation temperature. In vegetative cells of B. cereus, the LPD was in the range of 3.1 to 31.5 h with the highest being observed at 48ºC and pH 6.5. The GR had the lowest of 0.2/h to highest of 2.2/h at 30ºC and pH 7.5. With spores, the lowest LPD of 5.8 h was at 42ºC and highest of 20.5 h at 22ºC and that of GR was in the range of 0.2/h at 22ºC to 0.7/h at 32ºC. Experimental tubes of vegetative cells with GR estimates of 1.3/h and above revealed positive reactions for toxigenic traits of haemolytic and lecithinase activities associated with B. cereus.

Conclusion: In the case of vegetative cells, incubation temperature in the range of 12-20ºC resulted in higher LPD, while in spores, higher LPD was observed with incubation temperatures of 22-26ºC. This could provide a basis to design protocols for a safe food in the food chain