Lack of correlation of virulence gene profiles of Staphylococcus aureus bacteremia isolates with mortality.

Purpose: Whole genome sequencing (WGS) analysis of Staphylococcus aureus is increasingly used in clinical practice. Although bioinformatics tools used in WGS analysis readily define the S. aureus virulome, the clinical value of this type of analysis is unclear. Here, virulence genes in S. aureus bacteremia (SAB) isolates were evaluated by WGS, with superantigens (SAgs) further evaluated by conventional PCR and functional assays, and results correlated with mortality.

Methods: 152 SAB isolates collected throughout 2015 at a large Minnesota medical center were studied and associated clinical data analyzed. Virulence genes were identified from previously-reported WGS data (https://doi.org/10.1371/journal.pone.0179003). SAg genes sea, seb, sec, sed, see, seg, seh, sei, sej, and tst were also assessed by individual PCR assays. Mitogenicity of SAgs was assessed using an in vitro proliferation assay with splenocytes from HLA-DR3 transgenic mice.

Results: Of the 152 SAB isolates studied, 106 (69%) were methicillin-susceptible S. aureus (MSSA). The number of deaths attributed and not attributed to SAB, and 30-day survivors were 24 (16%), 2 (1%), and 128 (83%), respectively. From WGS data, both MSSA and MRSA had high proportions of adhesion (>80%) and immune-evasion (>70%) genes. There was no difference in virulomes between survivor- and non-survivor-associated isolates. Although over 60% of SAB isolates produced functional SAgs, there were no differences in the distribution or prevalence of SAg genes between survivor- and non-survivor-associated isolates.

Conclusion: In this study of one year of SAB isolates from a large medical center, the S. aureus virulome, as assessed by WGS, and also for SAgs using individual PCRs and phenotypic characterization, did not impact mortality.