Evaluation of Reduced Susceptibility to Quaternary Ammonium Compounds and Bisbiguanides in Clinical Isolates and Laboratory-Generated Mutants of Staphylococcus aureus

Abstract

The MICs and minimum bactericidal concentrations (MBCs) for the biocides benzalkonium chloride and chlorhexidine were determined against 1,602 clinical isolates of Staphylococcus aureus. Both compounds showed unimodal MIC and MBC distributions (2 and 4 or 8 mg/liter, respectively) with no apparent subpopulation with reduced susceptibility. To investigate further, all isolates were screened for qac genes, and 39 of these also had the promoter region of the NorA multidrug-resistant (MDR) efflux pump sequenced. The presence of qacA, qacB, qacC, and qacG genes increased the mode MIC, but not MBC, to benzalkonium chloride, while only qacA and qacB increased the chlorhexidine mode MIC. Isolates with a wild-type norA promoter or mutations in the norA promoter had similar biocide MIC distributions; notably, not all clinical isolates with norA mutations were resistant to fluoroquinolones. In vitro efflux mutants could be readily selected with ethidium bromide and acriflavine. Multiple passages were necessary to select mutants with biocides, but these mutants showed phenotypes comparable to those of mutants selected by dyes. All mutants showed changes in the promoter region of norA, but these were distinct from this region of the clinical isolates. Still, none of the in vitro mutants displayed fitness defects in a killing assay in Galleria mellonella larvae. In conclusion, our data provide an in-depth comparative overview on efflux in S. aureus mutants and clinical isolates, showing also that plasmid-encoded efflux pumps did not affect bactericidal activity of biocides. In addition, current in vitro tests appear not to be suitable for predicting levels of resistance that are clinically relevant.

Chlorhexidine and mupirocin susceptibilities in methicillin-resistant Staphylococcus aureus isolates from bacteraemia and nasal colonisation

“…The only isolate with reduced susceptibility to CHX did not carry qac genes and, on the other hand, isolates carrying qac genes did not reveal high MICs and MBCs. These controversial results have also been observed in other works [15][16][17][18]. However, other studies have reported reduced susceptibility to CHX in strains carrying qacA/B genes [11] and also in strains carrying qacC genes [19].…”

Detection of Antiseptic Resistance Genes among Staphylococcus aureus Colonising Nurses and Coagulase-Negative Staphylococci Isolated from Clinical Specimens at Teaching Hospitals in Southwest of Iran

“…The QACs resistant genes such as qacA/B, qacC, and smr have been isolated in various environments from clinical CoNS isolates and different Staphylococcal species (13,16). Whereas the current antiseptic MICs related to qac gene positive Staphylococci will not allow survival at in-use concentrations, even moderately increased resistance may allow persistence when residual disinfectants are present (16,22).…”

“…Resistance to ethidium bromide and quaternary ammonium compounds can be acquired by the qacC gene. It is usually located on conjugative plasmids in S. aureus and other Staphylococci clinical isolates (13,14).…”

Microbicides Alter the Expression and Function of RND-Type Efflux Pump AdeABC in Biofilm-Associated Cells of Acinetobacter baumannii Clinical Isolates

“…There is also increasing interest in the incorporation of microbicides into medical devices such as catheters and surgical dressings, with the intention of inhibiting microbial colonization and biofilm formation (34,38,39). Despite the evident benefits of microbicides in the majority of these applications, concerns have been raised that their extensive use may select for bacteria with reduced susceptibility to them or crossresistance to clinically relevant antibiotics (36,40,41). For example, triclosan can select for mutants of A. baumannii that display reduced susceptibilities to multiple antibiotics from chemically distinct classes, in addition to triclosan resistance (42).…”