Reduced vancomycin susceptibility in Staphylococcus aureus clinical isolates: a spectrum of less investigated uncertainties.

BACKGROUND

Staphylococcus aureus clinical isolates with vancomycin MICs of 2 µg/ml have been associated with vancomycin therapeutic failure and the heterogenous vancomycin-intermediate S. aureus (hVISA) phenotype. While carriage of van genes has usually been associated with higher level of MIC and frank vancomycin resistance, the unrecognized risk of hetero-resistance is frequently underestimated. Methods used for assessing vancomycin susceptibility have also shown different concordance and variable performance and accessibility in routine clinical diagnostics posing a challenge to inform treatment selection in hospital settings.

METHODS

A total of 195 clinical samples were obtained among which 100 S. aureus isolates were identified. Ninety-six MRSA isolates have been identified using cefoxitin disc and mecA gene detection. The vanA and vanB genes have been screened for in the studied isolates using conventional PCR amplification. Examination of reduced vancomycin susceptibility has been performed using vancomycin screen agar, Broth Micro Dilution method (BMD), and VITEK2. Blood isolates were screened for hVISA using PAP-AUC method.

RESULTS

Vancomycin screening agar applied to 96 MRSA isolates revealed 16 isolates with reduced vancomycin susceptibility. Further MIC testing revealed that 7 isolates were VISA and only 1 isolate was identified as VRSA using both BMD MIC method and VITEK2. Among 24 tested blood isolates, 4 isolates (16.7%) revealed the hVISA phenotype as identified using PAP-AUC method. Using PCR, vanA gene was identified in 5 S. aureus isolates (5%). Three of them were VSSA while the other two isolates were VISA.

CONCLUSION

In this study, we report the very low prevalence of VRSA among the tested S. aureus clinical isolates (1%) and the existence of hVISA phenotype among studied S. aureus blood isolates at the rate of 16.7% in our setting. Fifty percent (8/16) of isolates that demonstrated reduced vancomycin susceptibility using vancomycin agar screen tested susceptible using both broth dilution method and VITEK2. These finding together with the concerning silent carriage of vanA gene among VSSA and VISA (5%) may underly hidden and uninvestigated factors contributing to vancomycin treatment failure that warrant cautious vancomycin prescription.