Genetic characterization of Panton-Valentine Leukocidin-positive, Methicillin-susceptible Staphylococcus aureus in skin and soft tissue infections, Italy

Objectives Community-acquired methicillin-resistant S. aureus (CA-MRSA) as well as methicillin-susceptible S. aureus (MSSA) harboring the Panton-Valentine leukocidin (PVL) frequently cause skin and soft-tissue infections (SSTIs) and severe infections, such as necrotizing pneumonia. It has been suggested that the overall genetic background of the S. aureus strain determines clinical severity and the epidemiologic features of infections. The aim of this study was to investigate the genetic profile of PVL-positive MSSA strains collected in Italy. Methods Twelve PVL-positive MSSA strains were isolated in Italy between 2005 and 2009 from SSTIs, sepsis or nasal swabs. Species identification, methicillin susceptibility and detection of PVL toxin were obtained by PCR. The isolates were characterized by spa typing and multilocus sequence typing (MLST). All isolates were also genotyped by a DNA-microarray that detects approximately 180 distinct genes and their different alleles. Results and conclusions DNA array-based analysis assigned the isolates to the four agr groups and to capsule type 5 or type 8. All isolates harbored genes encoding biofilm production and several genes encoding MSCRAMMs (microbial surface components recognizing adhesive matrix molecules). Gamma-hemolysin, hld, beta-hemolisyn genes and beta-hemolysin integrating phages were always detected, as well as leukocidin (lukF-PV and lukS-PV), lukX and lukY genes. All isolates harbored different protease, set and ssl genes while bap, LukM, lukF-PV(83), entC, entD, entE, entJ, entL, entR, exfoliative toxin and edinC genes were absent. Carriage of other virulence genes was variable and reflected grouping of the strains. The beta-lactamase operon was always detected. Methicillin resistance genes were never detected and carriage of other resistance genes was infrequent and variable. DNA microarray-based analysis clustered the isolates according to clonal complexes as defined by ST and spa type, as follows: CC1/ t127 , CC5/ t1154, CC22/ t005 (2 strains), CC30/ t318, CC88/ t2310, CC121-ST1210/ t284 and t645 (3 isolates), CC152/ t1172, ST1209/ t1445 (2 isolates). Within each cluster, strains showed very similar DNA microarray profiles. Isolates belonging to different clusters have quite distinct hybridization patterns. In some cases, strains clustering in different MLTS/spa types were assigned to the same agr group, confirming the lack of correlation among agr group affiliation, ST and genetic profile. PVL-positive MSSA isolates are characterized by a high frequency of virulence genes and results confirm that these strains belong to diverse genetic backgrounds. The DNA microarray used facilitated a comprehensive genomic profiling of S. aureus isolates. The definition of strains based on the presence of genetic elements encoding pathogenic potential or antibiotic resistance is important for a clinical microbiologist as these properties are of direct consequence for patient care.