The autolytic phenotype of the Bacillus cereus group

Abstract

Aim: To determine the autolytic phenotype of five species in the Bacillus cereus group.

Methods and results: The autolytic rate of 96 strains belonging to five species in the B. cereus group was examined under starvation conditions at pH 6, 6.5 and 8.5 in different buffers. The autolytic rate was strain-dependent with a wide variability at pH 6, but higher and more uniform at pH 6.5. At pH 8.5, and respect to the extent of autolysis at pH 6.5, it was relatively low for most of the strains with the lowest values between 13 and 52% in Bacillus mycoides and Bacillus pseudomycoides. Peptidoglycan hydrolase patterns evaluated by renaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis using cells of Bacillus thuringiensis ssp. tolworthi HD125 as an indicator, revealed complex profiles with lytic bands of about 90, 63, 46, 41, 38, 32, 28 and 25 kDa in B. cereus, B. thuringiensis and Bacillus weihenstephanensis. Bacillus mycoides and B. pseudomycoides had simpler profiles with lytic bands of 63, 46 and 38 kDa. Changes in the autolytic pattern were observed for cells harvested at the stationary phase of growth (72 h) showing an increase in the intensity of the 25 kDa band in the case of B. cereus, B. thuringiensis and B. weihenstephanensis, while no changes were observed for B. mycoides. Using Micrococcus lysodeicticus and Listeria monocytogenes as indicators lytic activity was retained by proteins of 63, 46, 38, 32 and 25 kDa and a new one of about 20 kDa in B. mycoides. Growth in the different media did not affect the autolytic pattern. NaCl abolished the activity of all the peptidoglycan hydrolases except for those of B. mycoides and B. weihenstephanensis. Lytic activity was retained in the presence of MgCl(2), MnCl(2) and EDTA and increased at basic pH.

Conclusions: Bacillus cereus/B. thuringiensis/B. weihenstephanensis showed a high extent of autolysis around neutral pH, even though they presented relatively complex autolysin profiles at alkaline pH. Bacillus mycoides/B. pseudomycoides had a higher extent of autolysis at acidic pH and a simpler autolysin pattern.

Significance and impact of the study: Information on the autolytic phenotype expand the phenotypic characterization of the different species in the B. cereus group.