Ethidium bromide exposure unmasks an antibiotic efflux system in Rhodococcus equi

Background This study introduces a newly created strain (Rhodococcus equiEtBr25) by exposing R. equi ATCC 33701 to ethidium bromide (EtBr), a substrate for MDR transporters. Such an approach allowed us to investigate the resulting phenotype and genetic mechanisms underlying the efflux-mediated resistance in R. equi. Methods R. equi ATCC 33701 was stimulated with increasing concentrations of EtBr. The antimicrobial susceptibility of the parental strain and R. equiEtBr25 was investigated in the presence/absence of efflux pump inhibitors (EPIs). EtBr efflux was evaluated by EtBr-agar method and flow cytometry. The presence of efflux pump genes was determined by conventional PCR before to quantify the expression of 30 genes coding for membrane transporters by qPCR. The presence of erm(46) and mutations in 23S rRNA, and gyrA/gyrB was assessed by PCR and DNA sequencing to exclude the occurrence of resistance mechanisms other than efflux. Results R. equiEtBr25 showed an increased EtBr efflux. Against this strain, the activity of EtBr, azithromycin and ciprofloxacin was more affected than that of rifampicin and azithromycin/rifampicin combinations. Resistances were reversed by combining the antimicrobials with EPIs. Gene expression analysis detected a marked up-regulation of REQ_RS13460 encoding for a Major Facilitator Superfamily (MFS) transporter. G→A transition occurred in the transcriptional repressor tetR/acrR adjacent to REQ_RS13460. Conclusions Exposure of R. equi to EtBr unmasked an efflux-mediated defence against azithromycin and ciprofloxacin, which seemingly correlates with the overexpression of a specific MFS transporter. This genotype may mirror an insidious low-level resistance of clinically important isolates that could be countered by EPI-based therapies.