Evidence for contribution of tripartite hemolysin BL, phosphatidylcholine-preferring phospholipase C, and collagenase to virulence of Bacillus cereusendophthalmitis

Bacillus cereus causes a highly fulminant endophthalmitis which usually res ults in blindness. We previously concluded that hemolysin BL (HBL), a tripa rtite necrotizing pore-forming toxin, is a probable endophthalmitis virulen ce factor because it is highly toxic to retinal tissue in vitro and in vivo . We also determined that B. cereus produces additional retinal toxins that might contribute to virulence. Here we fractionated crude B. cereus cultur e supernatant by anion-exchange chromatography and found that in vitro reti nal toxicity was also associated with phosphatidylcholine-preferring phosph olipase C (PC-PLC). The pure enzyme also caused retinal necrosis in vivo. W e showed that phosphatidylinositol-specific PLC and sphingomyelinase were n ontoxic and that two hemolysins, cereolysin O and a novel hemolysin designa ted hemolysin IV, were marginally toxic in vitro. The histopathology of exp erimental septic endophthalmitis in rabbits mimicked the pathology produced by pure HBL, and both HBL and PC-PLC were detected at toxic concentrations in infected vitreous fluid. Bacterial cells were first seen associated wit h the posterior margin of the lens and eventually mere located throughout t he lens cortex. Detection of collagenase in the vitreous humor suggested th at infiltration was facilitated by the break-down of the protective collage n lens capsule by that enzyme. This work supports our conclusion that HBL c ontributes to B. cereus virulence and implicates PC-PLC and collagenase as additional virulence factors.