Expression of Bacillus cereus Virulence-Related Genes in an Ocular Infection-Related Environment

Bacillus cereus produces many factors linked to pathogenesis and is recognized for causing gastrointestinal toxemia and infections. B. cereus also causes a fulminant and often blinding intraocular infection called endophthalmitis. We reported that the PlcR/PapR system regulates intraocular virulence, but the specific factors that contribute to B. cereus virulence in the eye remain elusive. Here, we compared gene expression in ex vivo vitreous humor with expression in Luria Bertani (LB) and Brain Heart Infusion (BHI) broth by RNA-Seq. The expression of several cytolytic toxins in vitreous was less than or similar to levels observed in BHI or LB. Regulators of virulence genes, including PlcR/PapR, were expressed in vitreous. PlcR/PapR was expressed at low levels, though we had reported that PlcR-deficient B. cereus was attenuated in the eye. Chemotaxis and motility genes were expressed at similar levels in LB and BHI, but at low to undetectable levels in vitreous, although motility is an important phenotype for B. cereus in the eye. Superoxide dismutase, a potential inhibitor of neutrophil activity in the eye during infection, was the most highly expressed gene in vitreous. Genes previously reported to be important to intraocular virulence were expressed at low levels in vitreous under these conditions, possibly because in vivo cues are required for higher level expression. Genes expressed in vitreous may contribute to the unique virulence of B. cereus endophthalmitis, and future analysis of the B. cereus virulome in the eye will identify those expressed in vivo, which could potentially be targeted to arrest virulence. Impact statementB. cereus is the causative agent of gastrointestinal infections, but can also cause a serious infection of the eye that often results in blindness or enucleation. Current therapeutic measures often fail to mitigate these poor outcomes. This necessitates the development of new treatment modalities based on new targets. To begin to better define those B. cereus factors with roles in intraocular infection, we analyzed the expression of genes related to gastrointestinal infections, as well as those with both known and hypothesized roles in intraocular infections, after growth in an ex vivo vitreous. Potentially targetable candidate genes were demonstrated to be expressed in vitreous, which suggests that these genes might contribute to the unique virulence of B. cereus endophthalmitis. Importantly, our results lay the groundwork for assessing the expression of these genes in vivo and defining the virulome of B. cereus in intraocular infections.