A Mutation in the PP2C Phosphatase Gene in a Staphylococcus aureus USA300 Clinical Isolate with Reduced Susceptibility to Vancomycin and Daptomycin

摘要

Methicillin-resistant Staphylococcus aureus (MRSA) strains with reduced susceptibility to vancomycin (MIC of 4 to 8 μg/ml) are referred to as vancomycin-intermediate S. aureus (VISA). In this study, we characterized two isogenic USA300 S. aureus isolates collected sequentially from a single patient with endocarditis where the S. aureus isolate changed from being susceptible to vancomycin (VSSA) (1 μg/ml) to VISA (8 μg/ml). In addition, the VISA isolate lost beta-lactamase activity and showed increased resistance to daptomycin and linezolid. The two strains did not differ in growth rate, but the VISA isolate had a thickened cell wall and was less autolytic. Transcriptome sequencing (RNA-seq) analysis comparing the two isolates grown to late exponential phase showed significant differences in transcription of cell surface protein genes (spa, SBI [second immunoglobulin-binding protein of S. aureus], and fibrinogen-binding proteins), regulatory genes (agrBCA, RNAIII, sarT, and saeRS), and others. Using whole-genome shotgun resequencing, we identified 6 insertion/deletion mutations between the VSSA and VISA isolates. A protein phosphatase 2C (PP2C) family phosphatase had a 6-bp (nonframeshift) insertion mutation in a highly conserved metal binding domain. Complementation of the clinical VISA isolate with a wild-type copy of the PP2C gene reduced the vancomycin and daptomycin MICs and increased autolytic activity, suggesting that this gene contributed to the reduced vancomycin susceptibility phenotype acquired in vivo. Creation of de novo mutants from the VSSA strain resulted in different mutations, demonstrating that reduced susceptibility to vancomycin in USA300 strains can occur via multiple routes, highlighting the complex nature of the VISA phenotype.