Heterogeneous vancomycin intermediate resistance within methicillin-resistant staphylococcus aureus clinical isolates in Alexandria province

ABSTRACT

Glycopeptides such as vancomycin are frequently the antibiotics of choice for the treatment of infections caused by methicillin resistant Staphylococcus aureus [MRSA]. Since vancomycin-intermediate Staphylococcus aureus [VISA] was first reported in Japan in 1997, there has been great concern that heterogeneous vancomycin-intermediate Staphylococcus aureus [hVISA] is the putative precursor of VISA. The aim of this study was to explore the prevalence of VISA and hVISA among MRSA strains isolated from hospitalized patients in Alexandria University Hospital over a 2 years period, and to investigate their clinical significance and mechanism of vancomycin resistance. Sixty two MRSA isolates were screened by using brain heart infusion agar supplemented with 4 micro g/ml vancomycin [BHI-V4] and macro E test. Minimum inhibitory concentrations [MICs] of vancomycin were determined by broth microdilution and standard E test. Population analysis profile [PAP] was performed for detecting the frequency of heterogeneous resistance for isolates grown on BHI-V4. Vancomycin intermediate resistant subpopulations of hVISA were viewed with scanning electron microscopy, and tested for the presence of van A gene by PCR. Twenty one [33.87 %] MRSA isolates grew on BHI-V4, 7 [11.29 %] isolates were suspected of having reduced susceptibility to vancomycin by macro E test. The PAP confirmed 6 [9.68 %] isolates as hVISA since they produced subpopulations with MIC of vancomycin of > 4 micro g/ml at frequency of 1 in 10[6] CFU/ml or higher. Vancomycin MIC values for all isolates were infection was variable as the hemodialysis patient improved, while death occurred in the two patients from the ICU, and the three diabetic patients underwent variable degrees of amputations. Scanning electron micrographs of vancomycin intermediate resistant subpopulations of hVISA showed enhanced cell wall thickness with evidence of increased extra-cellular material and irregular shape compared to vancomycin susceptible cells. All hVISA isolates were van A gene negative by PCR. It was concluded that this study is an early warning that MRSA strains with full resistance to vancomycin might emerge in Egypt in the future. Due to the increased use of vancomycin for treatment of MRSA infections, screening for hVISA in MRSA strains should be considered as a necessary part of infection control practice emphasizing the importance of a laboratory capability of its identification