and are cytotoxic to human cells due to high expression of alpha-hemolysin Hla.

and are newly identified species of the -related complex. as occurring globally and showing significant prevalence and comparable infection and morbidity rates compared to , is becoming clinically important. Whole genome sequencing has revealed the presence of several virulence genes but the molecular mechanisms of infection and virulence are largely unknown. Here, we studied the effect of a previously characterized clinical isolate on human cells . The clinical isolate, together with the type strain MSHR1132T and the type strain FSA084T, had a cytotoxic effect on the cells, which showed necrotic cell death after a few hours of treatment. The protein causing the cytotoxic effect was purified and identified by mass spectrometry as alpha-hemolysin, Hla, which is awell-known pore-forming toxin in . The cytotoxic effect could be blocked with an antibody against Hla. showed 12-15 fold higher expression levels of at the RNA level and 4-6 fold higher expression levels at the protein level compared to . The higher expression levels of were supported by higher RNA levels of the regulatory factors and . Also, the RNAIII component of the accessory gene regulator (agr) quorum sensing system was 8,000-10,000 fold higher in the isolates compared to . This is the first study on the effect of on ahuman cell line and strengthens the idea of significant virulence of .