Role of swarming migration in the pathogenesis of Bacillus endophthalmitis

PURPOSE. Bacillus cereus causes one of the most rapidly blinding forms of bacterial endophthalmitis. Migration of B. cereus throughout the eye during endophthalmitis is a unique aspect of this disease that may contribute to intraocular virulence. This study was conducted to analyze the contribution of swarming and intraocular migration to the pathogenesis of experimental endophthalmitis. METHODS. Eyes were injected intravitreally with 100 colony-forming units (CFU) of either wild-type, nonswarming, or swarming-complemented strains of B. cereus. Pathogenicity was compared throughout the course of infection by biomicroscopy, histology, electroretinography, and bacterial and inflammatory cell quantitation. RESULTS. Wild-type, nonswarming, and swarming-complemented B. cereus strains grew to a similar number in the vitreous throughout the course of infection. Unlike the wildtype and swarming-complemented strains, the nonswarming mutant did not migrate to the anterior segment during infection. The rate of decrease in retinal responses of eyes infected with the all strains was similar, resulting in near complete elimination of retinal function by 12 hours. All Bacillus strains caused similar degrees of posterior segment inflammation and retinal destruction. However, the accumulation of inflammatory cells in the anterior chamber, hyphemae, and corneal ring abscesses did not occur in eyes infected with the nonswarming mutant. CONCLUSIONS. The deficiency in swarming had little effect on retinal function loss or the overall course or severity of experimental B. cereus endophthalmitis. However, a deficiency in swarming prevented Bacillus from migrating to the anterior segment, leading to less severe anterior segment disease.