Staphylococcus aureus endovascular infections retain a high morbidity and mortality despite antibiotics and supportive care. The destruction of endothelial cells (ECs) is a critical step in the pathogenesis of S. aureus endovascular infections. In order to better understand S. aureus-induced EC damage, we systematically screened a collection of two-component regulatory system mutants of methicillin-resistant S. aureus (MRSA) USA300 strain JE2 for damage induction in human umbilical vein ECs (HUVECs). This screen revealed that the two-component regulatory system ArlRS is required for maximum damage: arlRS inactivation leads to a > 70% reduction in damage. In a different genetic S. aureus background (RN6390, MSSA strain) arlRS inactivation had a smaller but also significant effect on EC damage. In both strains, the reduction in EC damage was accompanied by a significant reduction in internalization. In conclusion, we determined a novel role of ArlRS in S. aureus-induced EC damage, which will help to better understand the pathogenesis of S. aureus endovascular infection.

There has been controversy about the intrinsic virulence of methicillin-resistant Staphylococcus aureus (MRSA) as compared to methicillin-susceptible S. aureus (MSSA). To address this discrepancy, the intrinsic virulence of 42 MRSA and 40 MSSA clinical isolates was assessed by testing endothelial cell (EC) damage, a surrogate marker for virulence in blood stream infections. Since these clinical isolates represent a heterogeneous group, well characterized S. aureus laboratory strains with SCCmec loss- and gain-of-function mutations were used in addition. The clinical MRSA isolates carrying typical hospital acquired SCCmec types (I, II or III) induced significantly less damage (47.8%) as compared to isolates with other SCCmec types (62.3%, p=0.03) and MSSA isolates (64.2%, p<0.01). There was a strong inverse correlation between high-level oxacillin resistance and low EC damage induction (R=0.4464, p<0.001). High-level oxacillin resistant strains (MIC >32μ/ml) grew significantly slower as compared to isolates with low-level resistance (p=0.047). The level of EC damage positively correlated with α- and δ-toxin production (p<0.0001 and p<0.05, respectively) but not with β-toxin production. Invasive MRSA isolates (n=21, 56.3%) were significantly less cytotoxic as compared to invasive MSSA isolates (n=20, 68.0%, p<0.05). There was no difference between EC damage induced by superficial versus invasive isolates in either MRSA or MSSA strains. Our data suggest that the intrinsic virulence of MRSA is similar or even reduced as compared to MSSA strains but is linked to the level of methicillin resistance.