gms | German Medical Science

Objectives: S. aureus possesses several multidrug resistance (MDR) efflux pumps that confer resistance to fluoroquinolones, biocides and dyes. Apart from contributing to resistance, there is increasing evidence for a role of efflux pumps in host colonisation or invasion. We compared 31 S. aureus bacteremia (SAB) isolates of catheter-related uncomplicated SAB (CATH) with 27 isolates of SAB with complicated, community-acquired SAB (CA-SAB) to identify strains with an effluxing phenotype.

Methods: We performed MIC assays according to NCCLS/CLSI guidelines with the common efflux pump substrates norfloxacin, ciprofloxacin, moxifloxacin, minocycline and chlorhexidine with and without efflux pump inhibition using reserpine (20µg/ml) and INF 392 (5µM). A significant inhibition of efflux was defined as an at least fourfold reduction in MIC values. Strains with such reductions for at least three compounds were considered efflux capable.

Results: The number of isolates with an at least fourfold reduction in MIC values screened by reserpine (INF 392) was for norfloxacin 52(53)/58, ciprofloxacin 6(12)/58 and for chlorhexidine 58(54)/58. No significant inhibition of efflux was observed with moxifloxacin and minocycline. Comparing CA-SAB and CATH isolates revealed no differences between the two groups: fourfold MIC reduction by reserpine (INF 392) was observed for norfloxacin in 25(25)/27 isolates in CA-SAB and 27(28)/31 isolates in CATH, for ciprofloxacin in 3(6)/27 and 3(6)/31 and for chlorhexidine in 27(26)/27 and 31(28)/31 isolates, respectively.

The number of identified efflux capable strains was 10% (6/58, 3/27 of which in CA-SAB and 3/31 in CATH) for the reserpine-based and 20% (12/58, 6/27 of which in CA-SAB and 6/31 in CATH) for the INF 392-based screening. Modification of the definition of efflux capable isolates to a fourfold MIC reduction for at least two of the investigated compounds yielded 90% (52/58, 25/27 of CA-SAB and 27/31 of CATH isolates) for the reserpine-based and 84% (49/58, 24/27 of CA-SAB and 25/31 of CATH isolates) for the INF 392-based screening.

Conclusion: INF 392-based screening did not appear to be superior in discriminating between effluxing and non-effluxing S. aureus isolates. Overall, no significant differences concerning efflux capability between the groups of CA-SAB and CATH were detected using reserpine- or INF 392-based (phenotypic) screening. These findings should be further verified/complemented by gene expression analyses.

Keywords: S. aureus, MDR efflux pumps, resistance mechanisms, INSTINCT