Influence of agar content on the growth parameters of Bacillus cereus

Factors governing Bacillus cereus colony growth on agar media as modified by the agar content (1-7%, w/v) were studied. Agar had a significant effect on the radial growth rate which diminished as the agar content increased. Cell density in colonies (colony density) was found to decrease during the incubation time, with lower values occurring in the presence of 1% agar. Size and DNA content of the cells grown on 1 and 7% agar were similar. An increasing proportion of cell population growing on 7% agar produced spores during the 24-h incubation period. It was shown that 'water condition' on the agar surface could be associated with colony density, with 7% agar media presenting the thinnest nominal thickness of the liquid film. On the other hand, the partial drying phenomena of the agar media which occur during preparation and incubation, could not account for the observed differences in colony growth.