Analysis of methicillin-resistant Staphylococcus aureus isolates from catheter infections isolated in a large Italian hospital.

Objective: Twelve methicillin-resistant Staphylococcus aureus (MRSA) isolates from nosocomial catheter infections were studied. We determined their antibiotic susceptibility pattern (penicillin, oxacillin, erythromycin, tetracycline, clindamycin, telithromycin, gentamycin, ciprofloxacin, quinupristin-dalfopristin, rifampin, vancomycin and linezolid), and the presence of the genetic determinants of antibiotic resistance. To evaluate their genetic variability, isolates were subjected to multilocus sequence typing (MLST), SmaI macrorestriction/PFGE analysis and to staphylococcal chromosomal cassette/cassette chromosome recombinase (SCCmec/ccr) complex type determination. Methods: The determination of the MICs was performed following the Clinical Laboratory Standards Institute guidelines. Genetic determinants of antibiotic resistance were screened by PCR, using specific primer set described in the literature, while a standard multiplex PCR protocol was used to assess the SCCmec and ccrAB complex type. MLST, SmaI macrorestriction of chromosomal DNA and PFGE analysis were performed according to standard procedures. Results: An almost complete correlation was found between phenotypes and genotypic traits of antibiotic resistance. A major group of ten strains bore the SCCmec type I structure and showed the common sequence type st228. Among this group, only seven strains had a SmaI macrorestriction profile matching that of the archaic pandemic clone. One strain, resistant to all antibiotics but vancomycin, was st239 and SCCmec type IIIA. However, its SmaI macrorestriction profile was not related to that of the pandemic clones having a type IIIA SCCmec. It was not possible to assign the SCCmec type of the remaining two strains. They presented very rare multilocus sequence types, one being st83 and the other st395, and possessed a SmaI macrorestriction profile that was not related to that any of the known pandemic clones. Conclusions: i) An extremely resistant strain belonging to st239 was isolated and characterized; ii) in the st228 group, SmaI macrorestriction analysis was able to identify different subclones, showing an higher discriminatory power than MLST; iii) the analysis of the SCCmec/ccrAB complex of two strains with rare sequence types was not possible with the multiplex PCR methodology used, indicating the possible presence of new variants of the SCCmec