Low-level resistance of staphylococcus aureus to thrombin-induced platelet microbicidal protein 1 in vitro associated with qacA gene carriage is independent of multidrug efflux pump activity
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Date
2006Author
Bayer, Arnold S
Kupferwasser, L I
Skurray, Ronald Anthony
Grkovic, Steve
Jones, T
Mukhopadhay, K
Yeaman, M R
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Thrombin-induced platelet microbial protein 1 (tPMP-1), a cationic antimicrobial polypeptide released
from thrombin-stimulated rabbit platelets, targets the Staphylococcus aureus cytoplasmic membrane to
initiate its microbicidal effects. In vitro resistance to tPMP-1 correlates with survival advantages in vivo.
In S. aureus, the plasmid-carried qacA gene encodes a multi-drug transporter, conferring resistance to
organic cations (e.g., ethidium [Et]) via proton motive force (PMF)-energized export. We previously
showed that qacA also confers a tPMP-1-resistant (tPMP-1r) phenotype in vitro. The current study
evaluated whether (i) transporters encoded by the qacB and qacC multidrug resistance genes also confer
tPMP-1r and (ii) tPMP-1r mediated by qacA is dependent on efflux pump activity. In contrast to tPMP-1r
qacA-bearing strains, the parental strain and its isogenic qacB- and qacC-containing strains were tPMP-1
susceptible (tPMP-1s). Efflux pump inhibition by cyanide m-chlorophenylhydrazone abrogated Etr, but not
tPMP-1r, in the qacA-bearing strain. In synergy assays, exposure of the qacA-bearing strain to tPMP-1 did
not affect the susceptibility of Et (ruling out Et–tPMP-1 cotransport). The following cytoplasmic membrane
parameters did not differ significantly between the qacA-bearing and parental strains: contents of
the major phospholipids; asymmetric distributions of the positively charged species, lysyl-phosphotidylglycerol;
fatty acid composition; and relative surface charge. Of note, the qacA-bearing strain exhibited
greater membrane fluidity than that of the parental, qacB-, or qacC-bearing strain. In conclusion, among
these families of efflux pumps, only the multi-drug transporter encoded by qacA conferred a tPMP-1r
phenotype. These data suggest that qacA-encoded tPMP-1r results from the impact of a specific transporter upon
membrane structure or function unrelated to PMF-dependent peptide efflux.