Correlation of methicillin resistance and virulence genes of Staphylococcus aureus with infection types and mode of acquisition in Sofia, Bulgaria

Background: Infections due to methicillin resistant Staphylococcus aureus (MRSA) which is the most virulent species among the staphylococci have become a global health challenge. The aim of this study was to assess the correlation of genes encoding virulence and methicillin resistance in invasive and non-invasive isolates from inpatients/outpatients with staphylococcal infections in Sofia, Bulgaria. Materials and methods: Non-duplicate S. aureus isolates were recovered from clinical samples obtained from a total of 368 in-patients with healthcare-associated infections and outpatients with community acquired infections, following overnight cultures of samples on Columbia agar with 5% sheep blood at 35°C. The isolates were presumptively identified by colony and Gram stain morphology, positive catalase reaction and plasma-coagulase test. Isolates were screened for methicillin resistance by the cefoxitin disk method according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) protocol. The mec A and mec C, and 12 staphylococcal virulence genes were detected by a combination of monoplex and multiplex polymerase chain reaction (PCR) assays. Results: The prevalence of MRSA based on carriage of mec A gene was 12%; 7.7% for outpatients and 16.2% for inpatients ( p <0.05). The frequency of toxin genes detection in the staphylococcal isolates were as follows; sei (72.6%), seb (59.8%), seh (41.3%), sec (38.3%), seg (37.5%), sej (32.3%), sea (26.6%), sed (10.3%), tst (6.5%), and see (4.3%). The virulence genes, tst , sea , seb , sec , seg , seh and sei were more frequently associated with MRSA than methicillin sensitive (MSSA) strains ( p <0.05). About one-third of the clinical S. aureus isolates harbored seven virulence genes; sea, seb, sec, see, seg, seh and sei, that were detected significantly more among the invasive isolates ( p <0.05). Conclusions: This study shows the occurrence of highly virulent staphylococcal isolates in our geographical region. Keywords: Staphylococcus aureus , virulence, methicillin resistance