Restoration of sensitivity of a diverse set of drug-resistant Staphylococcus clinical strains by bactericidal protein P128

Purpose. P128, a phage-derived lysin, exerts antibacterial activity on staphylococci by cleaving the pentaglycine-bridge of peptidoglycan. We sought to determine whether the presence of P128 could re-sensitize drug-resistant bacteria to antibiotics by virtue of its cell wall degrading property.

Methodology. P128 was tested in combination with standard-of-care (SoC) drugs by chequerboard assays on planktonic cells and biofilms of strains individually resistant to these drugs. The bactericidal effect of P128 and drug combinations on planktonic cells and biofilms was measured by c.f.u. reduction assays. A mouse model of MRSA bacteraemia was used to test the efficacy of P128 and oxacillin in combination.

Results. A combination of sub-MIC P128 (0.025–0.20 µg ml−1) and 0.5 µg ml−1 of oxacillin resulted in inhibition of bacterial growth in four MRSA strains. Similar results were seen with all the other drugs tested, wherein sub-MIC of P128 re-sensitized S. aureus and CoNS strains to SoC drugs. The chequerboard assays on strains of S. aureus and CoNS showed that combinations of P128 and antibiotics consistently inhibited bacterial growth on biofilms. Data from scanning electron microscopy and c.f.u. reduction assays on drug-resistant S. aureus and CoNS demonstrated that sub-MICs of P128 and SoC antibiotics could kill biofilm-embedded bacteria. In vivo, a combination of sub-therapeutic doses of P128 and oxacillin could help protect animals from fatal bacteraemia.

Conclusion. The ability of P128 to re-sensitize bacteria to SoC drugs suggests that combinations of P128 and SoC antibiotics can potentially be developed to treat infections caused by drug-resistant strains of staphylococci.