Molecular characterization of staphylococcal cassette chromosome mec and virulence encoding genes in methicillin-resistant staphylococci at a medical center in Lebanon

Background: Methicillin-resistant staphylococci (MRS) are major human pathogens accounting for most hospital-acquired (HA) and community acquired (CA) infections worldwide. The recent increase in MRS in a medical center in Lebanon elicited the determination of SCCmec types, genotypes, and prevalence of Panton-Valentine leucociden (PVL) and toxic shock syndrome toxin-1 (TSST-1) among the MRS isolates.   Methods: Thirty-six MRS isolates collected between October 2010 and September 2011 at a medical center, Lebanon were typed using phenotypic and genotypic methods. Antimicrobial susceptibility was determined using the disk diffusion agar method. SCCmec typing was performed by multiplex PCR and sequence analysis. The prevalence of the genes encoding PVL and TSST-1 virulence factors and their transcription levels, were determined respectively by PCR and semi-quantitative real-time PCR. The genomic relatedness of the isolates was assessed by random amplified polymorphic DNA (RAPD) analysis.Results: Antimicrobial susceptibility revealed three distinct antibiotypes. The predominant SCCmec type found among the MRS isolates was type IVa (51%). Twenty-nine percent harbored SCCmec type III and 14% harbored SCCmec type II. One isolate harbored SCCmec type IVc, and another  harbored SCCmec type I. All methicillin-resistant Staphylococcus aureus (MRSA) isolates were negative for the gene encoding for PVL, and two were positive for the gene encoding for TSST-1. RAPD analysis demonstrated high genomic diversity among the MRS isolates.Conclusion: This study demonstrated the SCCmec types and the clonality of the MRS strains, allowing the differentiation between HA and CA-MRS strains. CA-MRS have  increased  in the hospital environment and rendered highly resistant to erythromycin and clindamycin