Michael Yeaman

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

ABSTRACT 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 multidrug 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-1rqacA-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 multidrug 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.

CHAPTER 40 – Antimicrobial Host Defense

Platelets have long been recognized for their role in maintaining hemostasis and contributing to wound healing. Yet platelets have historically been underappreciated for their multiple and integral functions in antimicrobial host defense. Structural and functional features continue to reinforce the concept that platelets have explicit functions in antimicrobial host defense. Platelets express a diverse array of constitutive and inducible membrane receptors. Thus, platelets have key attributes of sentinel cells that are highly sensitive and responsive to a broad spectrum of agonists associated with microbial contamination, infection, or tissue inflammation. Upon activation, platelets transform from quiescent discoid forms to amoeboid cells that recognize and adhere to microbial pathogens or ligands displayed by tissues injured by trauma or infection.