Reduced vancomycin susceptibility of methicillin-susceptible Staphylococcus aureus: no significant impact on mortality but increase in complicated infection [PublishAheadOfPrint]

Methicillin-susceptible Staphylococcus aureus (MSSA) bloodstream infections (BSIs) often lead to severe complications despite the availability of effective antibiotics. It remains unclear whether elevated vancomycin MICs are associated with worse outcomes.

We conducted a 2-year retrospective cohort study (n=252) of patients with MSSA BSIs at a tertiary care hospital. We defined reduced vancomycin susceptibility (RVS) as a Microscan MIC of 2 mg/L. All strains were genotyped (spa) and assessed for agr functionality. Multivariable logistic regression models were used to examine the impact of RVS phenotype and strain genotype on 30-day all-cause mortality and complicated bacteremia (metastatic spread, endovascular infection, or duration ≥3 days).

One third of patients (84/252) were infected with RVS isolates. RVS Infections were more frequently associated with metastatic or embolic sites of infection (36% vs. 17%, p<0.001), and endovascular infection (26% vs. 12%, p=0.004). They occurred more often in patients with fewer underlying comorbidities (CCI≥3: 73% vs. 88%, p=0.002). Genotyping identified 127 spa-types and 14 Spa-clonal complexes (Spa-CCs). Spa-CC002 and Spa-CC008 were more likely to exhibit the RVS phenotype versus other Spa-CCs (OR=2.2, p<0.01). The RVS phenotype was not significantly associated with 30-day mortality, however was associated with complicated bacteremia (aOR 2.35 [1.26, 4.37], p = 0.007) in adjusted analyses.

The association of RVS strains with complicated infection and fewer underlying comorbidities suggests the phenotype as a potential marker of strain virulence in MSSA BSIs. The RVS phenotype itself was not a significant predictor of mortality in this patient cohort. Further studies are necessary to explore this host-pathogen relationship.

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