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

Abstract

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 mecA and mecC, 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 mecA 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