Demonstration of a cholesterol-dependent cytolysin in a noninsecticidal Bacillus sphaericus strain and evidence for widespread distribution of the toxin within the species

Abstract

During the course of screening Bacillus species from food and water in Norway, we isolated a strain of Bacillus sphaericus of DNA homology group V, not previously recognized to contain entomopathogenic strains, that was cytotoxic to Vero cell epithelia. Peptide mass fingerprinting of a protein purified from the culture supernatant of B. sphaericus B354 identified a cholesterol-dependent cytolysin (CDC) with high amino acid sequence identity with sphaericolysin, a CDC identified recently in B. sphaericus DNA homology group IIA. The toxin was haemolytic against erythrocytes from a range of species. Haemolysis was potentiated by dithiothreitol and inhibited by preincubation with cholesterol. The toxin induced lactate dehydrogenase release from Vero cells and formed pores in planar lipid bilayers. The distribution of CDC genes in B. sphaericus was examined, with CDC gene products obtained in 13 out of 17 strains representing four of the six DNA homology groups. Thus, we demonstrate the presence of a CDC in a nonentomopathogenic DNA homology group of B. sphaericus (group V) with typical CDC characteristics. CDCs appear to be present in a high proportion of B. sphaericus strains and are not restricted to group IIA insecticidal strains. © 2008 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.