Immune Inhibitor A Metalloproteases Contribute to Virulence in Bacillus Endophthalmitis

Abstract

Bacterial endophthalmitis is a devastating infection that can cause blindness following the introduction of organisms into the posterior segment of the eye. Over half of Bacillus endophthalmitis cases result in significant loss of useful vision. Often, these eyes have to be enucleated. Bacillus produces many virulence factors in the eye that may contribute to retinal damage and robust inflammation. This study analyzed Bacillus immune inhibitor A (InhA) metalloproteases, which digest extracellular matrix, tight junction proteins, and antimicrobial proteins. We hypothesized that InhAs contribute to Bacillus intraocular virulence and inflammation. We analyzed phenotypes and infectivity of wild type (WT), InhA1-deficient (ΔinhA1), InhA2-deficient (ΔinhA2), or InhA1, A2, and A3-deficient (ΔinhA1-3) Bacillus thuringiensis. In vitro analysis of growth, proteolysis, and cytotoxicity were compared between B. thuringiensis strains. WT and InhA mutants were similarly cytotoxic to retinal cells. Mutant ΔinhA1 and ΔinhA2 entered log phase growth earlier than WT. Proteolysis of the ΔinhA1-3 mutant was decreased, but this strain grew similar to WT in vitro. Experimental endophthalmitis was initiated by intravitreally infecting C57BL/6J mice with 200 CFU of B. thuringiensis WT or InhA mutants. Intraocular Bacillus and retinal function loss were quantified. Intraocular myeloperoxidase concentrations were quantified and histology was analyzed. Eyes infected with ΔinhA1 or ΔinhA2 strains contained greater numbers of bacteria than eyes infected with WT throughout the course of infection. Eyes infected with single mutants had inflammation and retinal function loss similar to eyes infected with WT strain. Eyes infected with ΔinhA1-3 cleared the infection, with less retinal function loss and inflammation compared to eyes infected with the WT strain. RT-PCR results suggested that single InhA mutant results may be explained by compensatory expression of the other InhAs in these mutants. These results indicate that together, the InhA metalloproteases contribute to the severity of infection and inflammation in Bacillus endophthalmitis.Author summary Bacterial endophthalmitis is an infection of the eye, which can follow accidental contamination of the posterior segment following ocular surgery (postoperative), a penetrating wound (post-traumatic), or during spread of bacteria into the eye from the bloodstream (endogenous). During bacterial endophthalmitis, virulent pathogens such as Bacillus cause ocular damage via the activities of an array of virulence factors, including proteases. A class of proteases that are expressed by Bacillus during ocular infection are the immune inhibitor A metalloproteases. Here, we used a mouse model of endophthalmitis to test mutant Bacillus that lack single or multiple InhAs to determine if these metalloproteases contributed to the virulence during the disease. In the absence of the production of all InhAs, Bacillus could not cause severe infection. Our study provides new insights into the virulence of Bacillus in the eye, and the contribution of its InhA metalloproteases to establishing infection.Competing Interest StatementThe authors have declared no competing interest.View Full Text