Staphylococcal Cassette Chromosome Typing and Multilocus Variable Number Tandem Repeat Analysis of Methicillin Resistant Clinical Isolates with Vancomycin Creep Phenomenon.

Background: The association of treatment failure and mortality with vancomycin minimum inhibitory concentration creep (MIC) is a matter of serious concern in patients with severe methicillin resistant (MRSA) infections. The purpose of the study was to identify and characterize staphylococcal cassette chromosome (SCC) and clonal types of MRSA strains, exhibiting the vancomycin MIC creep phenomenon.

Methods: A total of 3305 strains were isolated from various clinical samples of Lahore General Hospital, Lahore, Pakistan. MRSA strains were identified by cefoxitin resistant (≤21mm) followed by and gene genotyping. Vancomycin MIC creep was determined by E-test. Isolates having MIC values >1.5 µg/mL were further subjected for SCC typing (I-V and XI) and multiple-locus variable number tandem repeat analysis (MLVA) by amplification of , and genes. A dendrogram was created based on the similarity index using bioneumerics software.

Results: About 13.3% (440/3305) isolates were MRSA with 99.3% (437/440) and 0.7% (3/440) carried and genes, respectively. In 120 MRSA isolates, the MIC of vancomycin was >1.5µg/mL. In MRSA isolates with high vancomycin MIC (>1.5µg/mL), the most common SCC type was SCC III (38.3%), followed by SCC IVa (15.8%), SCC IIIa (13.3%,), SCC IVc (7.5%), SCC IVe (5.8%), SCC IVd (5.8%), SCC IVb (4.2%), SCC II (2.5%), SCC V (1.7%), SCC I (1.7%) and SCC XI (1.7%). MLVA revealed 60 genotypic groups of MRSA isolates having a 92% similarity index.

Conclusion: SCC III was the most common type in genetically related MRSA isolates showing vancomycin MIC creep. The presence of SCC XI may further add burden to infection control measures.