Exotoxin-profiling and typing of clinical Panton-Valentine Leukocidin positive MSSA versus MRSA

Purpose: Staphylococcus aureus (SA) is one of the most important human pathogens worldwide. SA is causing infections ranging from mild to severe, especially Panton-Valentine leukocidin (PVL) came into focus for the severity of clinical symptoms. Within the last 20 years community acquired PVL positive MRSA stains spread globally and proved to be a public health burden. PVL seems to be highly associated with CA-MRSA but its role in pathogenesis is still controversial and in MSSA neglected. Methods & Materials: We investigated 79 PVL positive SA (18 MSSA and 61 MRSA) from Austria for SCCmec type, spa-type and 30 exotoxin-genes (sea-see, tst-1, seg-ser, seu, eta, etb, ahl, bhl, ghlabc, ghlvar, dhl, lukDE). Results: The most common agr-type was agr III (51%) followed by agr I (24%). The MRSA strains carried mainly SCCmec IV or V (93%). Analysis of spa-types revealed a diverse genetic background for MSSA and a narrower one for MRSA. The exotoxin-genes seb, seg, sei, sem, sen, seo, seu, ahl, bhl, and ghlabc were detected significantly more often (p<0,05) in MSSA compared to MRSA. 50% of MSSA carried 11 or more (min. 5 - max. 15; average 10) exotoxin-genes whereas 50% of MRSA carried only 6 or more (min. 1 - max. 12; average 7) exotoxin-genes. Conclusion: The PVL positive MRSA carried significantly less toxin-genes than PVL positive MSSA isolates. This finding supports the theory that the methicillin resistance together with to many toxin-genes would be too much of a cost of fitness or because of the benefit of methicillin resistance less toxin-genes are more sufficient.