Sub-inhibitory membrane damage undermines Staphylococcus aureus virulence.

Abstract We investigated antibacterial properties of a recently described membrane-active lipopeptide, C 10 OOc 12 O (decanoyl-ornithyl-ornithyl-dodecanoyl-ornithyl-amide) against Gram-positive bacteria (GPB). Minimal inhibitory concentrations (MICs) and kinetics were compared in culture media and plasma. Chemo-sensitization to antibiotics was determined using the checkerboard assay. Membrane damages were estimated using diverse membrane potential sensitive dyes. ATP levels and relevant enzymes activities were measured using commercial bioassay kits. While relatively weakly active in simple culture media, sub-MIC levels (~ten-fold) of C 10 OOc 12 O have significantly improved the antibacterial function of Human plasma. Mechanistic studies indicated that C 10 OOc 12 O-treated bacteria have sustained mild membrane damage(s) in association with rapid (within 2 min) but low ( agr -regulated virulence factors (lipase and α-toxin) and for sensitizing MRSA USA300 to the antibiotic oxacillin to the point of reverting the bacteria status from oxacillin-resistant to oxacillin-sensitive (i.e., oxacillin MIC was reduced from 32 to 0.1 mg/l). These findings argue that by means of mild depolarization, C 10 OOc 12 O affects the quorum sensing regulator in a manner that transiently weakens bacterial defenses, thereby enforcing studies that support the potential usefulness of fighting S. aureus (and possibly other GPB) infections, by targeting its virulence.