Morphology and physico-chemical properties of Bacillus spores surrounded or not with an exosporium: consequences on their ability to adhere to stainless steel

This study was designed to elucidate the influence of spore properties such as the presence of an exosporium, on their ability to adhere to materials. This analysis was performed on 17 strains belonging to the B. cereus group and to less related Bacillus species. We first demonstrated that spores of the B. cereus group, surrounded by an exosporium, differed in their morphological features such as exosporium size, number of appendages or hair-like nap length. We also found that the saccharidic composition of exosporium differed among strains, e.g. concerning a newly identified rhamnose derivative: the 2,4-O-dimethyl-rhamnose. Conversely, spores of distant Bacillus species shared morphological and physico-chemical properties with B. cereus spores. Some external features were also observed on these spores, such as a thin loose-fitting layer, whose nature is still to be determined, or a thick saccharidic layer (mainly composed of rhamnose and quinovose). The ability of spores to adhere to stainless steel varied among strains, those belonging to the B. cereus group generally being the most adherent. However, the presence of an exosporium is not sufficient to explain the ability of spores to adhere to inanimate surfaces. Indeed, when the 17 strains were compared, hydrophobicity and the number of appendages were the only significant adhesion parameters. Furthermore, the differences in spore adhesion observed within the B. cereus group were related to differences in the number of appendages, the exosporium length and to a lesser extent, the zeta potential.