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Plasmid-mediated resistance to thrombin-induced platelet microbicidal protein in staphylococci: role of the qacA
L I Kupferwasser1, R A Skurray, M H Brown
1Division of Adult Infectious Diseases, St. John's Cardiovascular Research Center, Harbor-UCLA Medical Center, Torrance, California 90509, USA. kupferwasser@hotmail.com
Abstract:
Thrombin-induced platelet microbicidal protein 1 (tPMP-1) is a small, cationic peptide released from rabbit platelets following thrombin stimulation. In vitro resistance to this peptide among strains of Staphylococcus aureus correlates with the survival advantage of such strains at sites of endothelial damage in humans as well as in experimental endovascular infections. The mechanisms involved in the phenotypic resistance of S. aureus to tPMP-1 are not fully delineated. The plasmid-encoded staphylococcal gene qacA mediates multidrug resistance to multiple organic cations via a proton motive force-dependent efflux pump. We studied whether the qacA gene might also confer resistance to cationic tPMP-1. Staphylococcal plasmids encoding qacA were found to confer resistance to tPMP-1 in an otherwise susceptible parental strain. Deletions which removed the region containing the qacA gene in the S. aureus multiresistance plasmid pSK1 abolished tPMP-1 resistance. Resistance to tPMP-1 in the qacA-bearing strains was inoculum independent but peptide concentration dependent, with the level of resistance decreasing at higher peptide concentrations for a given inoculum. There was no apparent cross-resistance in qacA-bearing strains to other endogenous cationic antimicrobial peptides which are structurally distinct from tPMP-1, including human neutrophil defensin 1, protamine, or the staphylococcal lantibiotics pep5 and nisin. These data demonstrate that the staphylococcal multidrug resistance gene qacA also mediates in vitro resistance to cationic tPMP-1.
Insights
The staphylococcal multidrug resistance gene qacA confers resistance to thrombin-induced platelet microbicidal protein 1 (tPMP-1). This finding reveals a novel mechanism for Staphylococcus aureus survival against this important antimicrobial peptide.
Area of Science:
- Microbiology
- Molecular Biology
- Antimicrobial Resistance
Background:
- Thrombin-induced platelet microbicidal protein 1 (tPMP-1) is a cationic peptide crucial for innate immunity.
- Staphylococcus aureus strains exhibiting in vitro resistance to tPMP-1 show enhanced survival in infections.
- The precise mechanisms underlying S. aureus resistance to tPMP-1 remain incompletely understood.
Purpose of the Study:
- To investigate whether the staphylococcal multidrug resistance gene qacA mediates resistance to the cationic antimicrobial peptide tPMP-1.
- To elucidate the role of the qacA efflux pump in conferring resistance to tPMP-1.
Main Methods:
- Introduction of staphylococcal plasmids encoding qacA into susceptible S. aureus strains.
- Deletion analysis of the qacA gene region on the S. aureus multiresistance plasmid pSK1.
- Assessment of tPMP-1 resistance levels in qacA-bearing and control strains.
- Evaluation of cross-resistance to other cationic antimicrobial peptides.
Main Results:
- Staphylococcal plasmids carrying the qacA gene conferred significant resistance to tPMP-1 in susceptible S. aureus.
- Abolition of tPMP-1 resistance was observed when the qacA gene region was deleted from the pSK1 plasmid.
- Resistance was peptide concentration-dependent and inoculum-independent.
- No cross-resistance was observed to other structurally distinct cationic antimicrobial peptides.
Conclusions:
- The staphylococcal multidrug resistance gene qacA is responsible for mediating in vitro resistance to the cationic antimicrobial peptide tPMP-1.
- The qacA efflux pump plays a key role in the survival of S. aureus against tPMP-1.
- This study identifies a novel function of the qacA gene in antimicrobial peptide resistance.