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 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.
Bacillus cereus is recognized as a causative agent of gastrointestinal syndromes, but can also cause a devastating form of intraocular infection known as endophthalmitis. We have previously reported that the PlcR/PapR master virulence factor regulator system regulates intraocular virulence, and that the S-layer protein (SlpA) contributes to the severity of B. cereus endophthalmitis. To better understand the role of other B. cereus virulence genes in endophthalmitis, expression of a subset of factors was measured at the midpoint of disease progression in a murine model of endophthalmitis by RNA-Seq. Several cytolytic toxins were expressed at significantly higher levels in vivo than in BHI. The virulence regulators codY, gntR, and nprR were also expressed in vivo. However, at this timepoint, plcR/papR was not detectable, although we previously reported that a B. cereus mutant deficient in PlcR was attenuated in the eye. The motility-related genes fla, fliF, and motB, and the chemotaxis-related gene cheA were detected during infection. We have shown previously that motility and chemotaxis phenotypes are important in B. cereus endophthalmitis. The sodA2 variant of manganese superoxide dismutase was the most highly expressed gene in vivo. Expression of the surface layer protein gene, slpA, an activator of Toll-like receptors (TLR)−2 and −4, was also detected during infection, albeit at low levels. Genes expressed in a mouse model of Bacillus endophthalmitis might play crucial roles in the unique virulence of B. cereus endophthalmitis, and serve as candidates for novel therapies designed to attenuate the severity of this often blinding infection.
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