Prevalence and Conjugal Transfer of Vancomycin Resistance among Clinical Isolates of Staphylococcus aureus

Aims: Emergence of vancomycin resistant Staphylococcus aureus (VRSA) has been of great concern in clinical settings. Our study aimed to detect vancomycin resistance and the possibility of its conjugal transfer among clinical isolates of S. aureus.

Study Design: Medical microbiology.

Place and Duration of Study: This study was carried out in the department of microbiology, Faculty of pharmacy, between October 2011 and January 2013.

Methodology: Two hundreds and seven samples were collected from different types of infections and examined for Staphylococcus aureus using standard bacteriological procedures, Antimicrobial susceptibility testing against vancomycin and some antimicrobials was done by agar dilution method, resistance to methicillin was determined by disc diffusion test using cefoxitin (30µg), Vancomycin resistance gene transfer was tested by broth matting procedure and confirmed by plasmid and DNA product analysis.

Results: Sixty-three samples were positive for Staphylococcus aureus. All isolates were resistant to penicillin while the lowest resistance was to amikacin and vancomycin. One isolate was VRSA (MIC ≥ 16 µg/ml and positive for vanA gene 1032bp) while 5 isolates were VISA. All VRSA and VISA isolates were MRSA and showed a high resistance to other examined antibiotics. Vancomycin resistance was plasmid mediated and successfully transferred from VRSA donor to a vancomycin sensitive recipient isolate (MIC 2 µg/ml). The MIC of vancomycin for the transconjugant strain was 32 µg/ml. PCR product analysis for the transconjugate was positive for vanA gene.

Conclusion: Low incidence of vancomycin resistance, All VRSA and VISA were methicillin resistant, and mostly isolated from skin infections. Also, vancomycin resistance was plasmid mediated and successfully transferred to susceptible strains which is alarming. Skin colonization by VRSA strains harboring plasmid carrying resistance genes may result in widespread of these strains that could lead to outbreaks of staphylococcal diseases in both hospital and community.