Inoculum size of Clostridium botulinum 56A spores influences time-to-detection and percent growth-positive samples

The influence of inoculum size on the growth kinetics of Clostridium botulinum 56A and percentage of growth-positive samples was studied in a complete factorial design with factors of inoculum size (1, 100, or 10,000 spores), pH, and sodium-chloride concentration. Growth was followed hourly as change in A₆₂₀. Polynomial regression was used to analyze the data. The time-to-detection and percent growth-positive samples were significantly affected by inoculum size and its quadratic term. When inoculum size increased from 1 to 100 spores/sample, the percent growth-positive samples increased, and the time-to-detection decreased. When the inoculum was 1000 spores/sample or higher, there was little additional effect. Inoculum size might influence results through simple probability or quorum sensing. The maximum growth rate was independent of inoculum levels.