In vitro and in vivo models of Staphylococcus aureus endophthalmitis implicate specific nutrients in ocular infection.

Ama Sadaka,Michael S Gilmore,Kelli Palmer,Takashi Suzuki,José A Lemos

doi:10.1371/journal.pone.0110872

Abstract

PurposeTo define global transcriptional responses of Staphylococcus aureus and its codY mutant (CodY is a transcription regulator of virulence and metabolic genes in response to branched-chain amino acids) when growing in bovine aqueous (AH) and vitreous humor (VH) in vitro, and to investigate the impact of codY deletion on S. aureus virulence in a novel murine anterior chamber (AC) infection model.MethodsFor the in vitro model, differential transcriptomic gene expression of S. aureus and its codY mutant grown in chemically defined medium (CDM), AH, and VH was analyzed. Furthermore, the strains were inoculated into the AC of mice. Changes in bacterial growth, electroretinography and inflammation scores were monitored.ResultsBovine AH and VH provide sufficient nutrition for S. aureus growth in vitro. Transcriptome analysis identified 72 unique open reading frames differentially regulated ≥10-fold between CDM, AH, and VH. In the AC model, we found comparable growth of the codY mutant and wild type strains in vivo. Average inflammation scores and retinal function were significantly worse for codY mutant-infected eyes at 24 h post-infection.ConclusionOur in vitro bovine AH and VH models identified likely nutrient sources for S. aureus in the ocular milieu. The in vivo model suggests that control of branched-chain amino acid availability has therapeutic potential in limiting S. aureus endophthalmitis severity.

Highlights

Staphylococcus aureus is a commensal bacterium on the skin and mucosa, but is a leading cause of infections in humans
S. aureus SA564 in vitro growth in bovine AH and vitreous humor (VH) S. aureus SA564 is a clinical isolate that was previously used to evaluate a role for codY in S. aureus virulence regulation [29]
As a first step in understanding S. aureus physiology and metabolism during endophthalmitis, we evaluated S. aureus SA564 growth in pooled AH and VH harvested from commercially obtained bovine eyes

Summary

Introduction

Staphylococcus aureus is a commensal bacterium on the skin and mucosa, but is a leading cause of infections in humans. When opportunistic pathogens infect sterile sites, they adapt, proliferate in the host, and exhibit virulence. In the presence of GTP and/or BCAA, CodY shows a higher affinity for its DNA targets, while in the absence of nutrients, there is a decrease in the GTP and BCAA levels causing decreased affinity of CodY to the DNA and induction of its regulon. CodY-regulated genes trigger adaptation to starvation [2,3,4,5,6,7,8,9] as well as play a role in virulence of pathogenic bacteria [3,4,10]

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