Heterogeneity observed in the components of hemolysin BL, an enterotoxin produced by Bacillus cereus

Abstract

Hemolysin BL (HBL), a diarrheal enterotoxin originally isolated from Bacillus cereus strain F837/76, is composed of three antigenically distinct proteins designated B, L 1, and L 2. All three components are required for biological activity. Here, we report antigenic and physical variations in HBL components produced by other B. cereus isolates. Reactions of partial identity were observed in double gel immunodiffusion assays using antibodies to highly purified B, L 1, and L 2 components of F837/76 and culture supernatants of strains F837/76 and S1C. Western blot analysis showed that F837/76 produced one 38-kDa B protein, one 38-kDa L 1, and one 43-kDa L 2 protein. In strain S1C, two B (38 and 42 kDa), two L 1 (38 and 41 kDa), and one L 2 (43 kDa) proteins were detected. Further Western blot analysis of 127 B. cereus isolates showed that 90 produced one or more of the three HBL components. Approximately half of these 90 isolates (43/90; 48%) produced protein profiles which differed from that of F837/76. A total of four B, two L 1, and three L 2 component profiles with proteins of different sizes were observed. Individual strains produced various combinations of single or multiple bands of each component. In addition, some strains produced only one or two of the three HBL components. The public health significance of these strains is unknown, as all three components are required for biological activity. The data presented here demonstrates a high degree of heterogeneity in HBL and provide the basis for further studies to characterize the variations in HBL and to determine the role of the variant components in pathogenicity.