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Role of Hemolysin BL in the Pathogenesis of Extraintestinal Bacillus cereus Infection Assessed in an Endophthalmitis Model

ABSTRACT Bacillus cereus is a rare cause of serious human infection but, paradoxically, causes one of the most severe posttraumatic or endogenous infections of the eye, endophthalmitis, which frequently results in blindness. The virulence of B. cereus endophthalmitis historically has been attributed to toxin production. We therefore sought to examine the contribution of the dermonecrotic toxin, hemolysin BL, to the pathogenesis of B. cereus infection in an endophthalmitis system that is highly amenable to study. The pathogenesis of infection resulting from intravitreal injection of 102 CFU of either a clinical ocular isolate of B. cereus producing hemolysin BL (HBL+) or an isogenic mutant in this trait (HBL−) was assessed bacteriologically and by slit lamp biomicroscopy, electroretinography, histology, and inflammatory cell enumeration. Both HBL+ and HBL− strains evoked severe intraocular inflammatory responses as early as 12 h postinfection, with complete loss of retinal responsiveness by 12 h. The infections caused by both strains spread of the infection to adjacent tissues by 18 h. No significant differences in intraocular bacterial growth (P ≥ 0.21) or inflammatory changes (P ≥ 0.21) were observed in eyes infected with either HBL+ or HBL− strains during the course of infection. The level of retinal responsiveness was greater in HBL− infected eyes than in HBL+-infected eyes at 6 h only (P = 0.01). These results indicate that hemolysin BL makes no essential contribution to the severe and rapid course of infection in the endophthalmitis model.