Comprehensive 'omic' analysis of Staphylococcus aureus strains isolated from gangrenous and subclinical ewe mastitis

Staphylococcus aureus is a major pathogen involved in intra mammary infections (or mastitis) in ruminants. Accuteness of the disease vary from subclinical mastitis to gangrenous mastitis. The origin of such a variability depends on both S. aureus virulence and host susceptibility and is not clearly established yet.. Objective: Identify differences found in S. aureus strains inducing mastites with diverse acuteness in order to identify staphylococcal virulence markers. Method: Genome sequences of two S. aureus strains isolated from gangrenous mastitis (O11) and subclinical mastitis (O46) were obtained (Illumina). Both strains were grown in vitro in conditions mimicking a mastitis context, and their gene expression profile and proteome were analyzed and compared using comparative genomic hybridization and 2D gel electrophoresis. Results: O11 and O46 are genotypically and genetically closely related. Proteomic analysis of the two strains nevertheless revealed clear differences, notably in the exoprotein patterns (higher production of toxins such as lukM/F’-PV, alpha hemolysine, and glycolytic enzymes like enolase or adhesion proteins by O11 strain compared to O46). The genome sequence of the 2 strains allowed the identification of an additional phage in O46 strain, SNPs (1189 synonymous in 487 ORFs and 1104 non synonymous in 680 ORFs in the core genome) and indels (~80) in both strains. SNPs are scattered on the chromosome and involve all the functional categories including virulence factors, some of which harbor several SNPs. Transcriptomic analysis allowed to link genome and proteome data and explained some of the differences observed between the two strains. Conclusions Altogether, these results show that differences in terms of pathogenicity in S. aureus mastitis isolates can be correlated to gene content and, mainly, to the strain capability to express and produce virulence factors. Some genes were found specifically present or overexpressed in one strain or the other and could be involved to the differences observed in the mastitis acuteness. They would thus be interesting markers to characterize virulence potential of S. aureus strains isolated from ewe mastitis.