Bacillus cereus cell and spore properties as influenced by the micro-structure of the medium

Aim: To investigate the effect of different growth conditions on Bacillus cereus cell and spore properties. Methods and Results: Bacillus cereus was grown on agar plates with different surface water conditions (wet and dry) or viscosity. Cell populations displayed different types of behaviour, and heterogeneity was manifested in cell motility and dimension. Spore populations were heterogeneous regarding their properties, namely size and thermal resistance. The smallest spores were produced from flagellated cells, which also displayed jet-motility, growing on the wettest agar. Cytometric analysis also revealed within the smallest spores a sub-population labelled by propidium iodide (PI), indicating that spore populations were partly damaged. Nonmotile cells grown on diffusion-limiting media were elongated and produced the least thermal-resistant spores. Conclusions: The micro-structural properties of the media were found to influence cell and spore properties. Abundant surface water enabled flagellar motility and resulted in a heterogeneous cell and spore population, the latter including small and damaged spores. High viscosity gave rise to filamentous cells and more heat-sensitive spores. Significance and Impact of the Study: This study provides useful information on conditions resulting in heterogeneous populations of damaged and heatsensitive spores.