Association of Vancomycin MIC and Molecular Characteristics With Clinical Outcomes in Methicillin-Susceptible Staphylococcus aureus Acute Hematogenous Osteoarticular Infections in Children [PublishAheadOfPrint]

Background: Methicillin-resistant Staphylococcus aureus (MRSA), particularly those belonging to the USA300 pulsotype, have been well described to cause severe osteoarticular infections (OAI). Vancomycin MIC ≥ 1.5 μg/ml has been demonstrated to contribute to disease severity in adults with MRSA and even MSSA bacteremia. Little data exists describing the outcomes of MSSA OAI in terms of molecular characteristics and vancomycin MIC.

Methods: All patients/isolates were chosen from a surveillance study at Texas Children's Hospital (TCH). S. aureus OAI isolates were identified from 2011-2016 and subjected to vancomycin E-tests, PFGE and PCR for PVL and agr group.

Results: 252 cases of S. aureus OAI were identified; 183 were MSSA (72.6%). During the study period, a decrease in the proportion of cases secondary to MRSA was observed, declining from 37.8% to 15.9% (p=0.02). 26.2% and 23.5% of MSSA isolates were USA300 and PVL-positive, respectively. An increase in the proportion of MSSA isolates with a vancomycin MIC ≥ 1.5 μg/ml occurred in the study period (p=0.004). In MSSA, elevated vancomycin MIC was associated with multiple surgical procedures and and venous thromboses even when adjusting for empiric β-lactam use. An increase in vancomycin MIC was noted among isolates belonging to agr group 4 during the study period.

Conclusion: Methicillin-resistance is declining among S. aureus OAI isolates at TCH. Simultaneously, vancomycin E-test MICs are increasing among MSSA isolates. Vancomycin MIC ≥ 2 μg/ml are associated with adverse clinical outcomes in MSSA irrespective of antibiotic choice suggesting that this may be a surrogate for organism virulence.

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