Group I Clostridium botulinum Strains Show Significant Variation in Growth at Low and High Temperatures

The minimum and maximum growth temperatures of 23 group I Clostridium botulinum strains of the toxin types A, AB, B, and F were determined. Moreover, the maximum growth rates at 20, 37, and 42 degrees C of the same strains were recorded. The minimum growth temperatures varied from 12.8 to 16.5 degrees C, whereas the maximum growth temperatures showed even wider variation, from 40.9 to 48.0 degrees C. At 20 and 37 degrees C, a twofold difference in maximum growth rates between the slowest and the fastest growing strains was found; at 42 degrees C the difference was more than 30-fold. As expected, all strains grew significantly slower at 20 degrees C than at 37 degrees C. However, eight type B strains grew substantially faster at 42 degrees C than they did at 37 degrees C. These findings indicate that the optimum growth temperature for some group I C. botulinum type B strains is higher than the temperature of 37 degrees C that is generally accepted. A significant correlation between maximum growth rates at 42 degrees C and maximum growth temperatures was found for type B and F strains, whereas for type A strains no such correlation could be found. Strain variation was particularly high for the type B strains, reflecting the wide genetic diversity of this toxin type. The significant variation between strains of group I C. botulinum may have an impact on inoculation studies and predictive modeling when assessing the safety of foods.