The 95ΔG mutation in the 5'untranslated region of the norA gene increases efflux activity in Staphylococcus epidermidis isolates

Active antimicrobial efflux in Staphylococcus epidermidis: building up of resistance to fluoroquinolones and biocides in a major opportunistic pathogen

Objectives

To analyse the efflux-mediated response of Staphylococcus epidermidis to ethidium bromide (EtBr), a substrate of multidrug efflux pumps (EPs).

Methods

The susceptible reference strain S. epidermidis ATCC 12228 was exposed to a step-wise adaptation to EtBr. The resulting EtBr-adapted strains were characterized regarding their antibiotic and biocide susceptibility by MIC determination and evaluation of efflux activity by re-determination of MICs in the presence of known efflux inhibitors and real-time fluorometry. Mutations in the QRDR of grlA and gyrA were screened by sequencing. The expression levels of S. epidermidis homologues of the main Staphylococcus aureus EP genes were quantified by RT-qPCR.

Results

Exposure to EtBr led to a gradual increase in resistance to antimicrobials, with the final EtBr-adapted strain, ATCC 12228_EtBr, displaying phenotypic resistance to fluoroquinolones and reduced susceptibility to several antiseptics and disinfectants, although no mutations were detected in the QRDR of the grlA/gyrA genes. A reduction in the MICs of fluoroquinolones and selected biocides promoted by efflux inhibitors suggested an efflux-mediated response to EtBr exposure. Detailed analysis of the EtBr-adapted strains detected a gradual increase in efflux activity. Gene expression assays revealed a temporal activation of S. epidermidis EPs, with an early response involving norA, SE2010 and SE1103 followed by a late response mediated by norA, which coincided with the occurrence of the mutation -1A→T in the norA promoter region.

Conclusions

This study demonstrated that S. epidermidis has the potential to develop a multiple resistance phenotype mediated by efflux when exposed to a non-antibiotic substrate of multidrug EPs.

Intrinsic role of coagulase negative staphylococci norA-like efflux system in fluoroquinolones resistance

NorA is a Staphylococcus aureus multidrug transporter that exports structurally distinct compounds including fluoroquinolones. In this study norA-like genes of Staphylococcus epidermidis (norA _SEP) and Staphylococcus haemolyticus (norA _SHAE) were identified and sequenced. The nucleotide identity of norA _SEP and norA _SHAE with norA was 75.3 and 74.1%, respectively, and the amino acid identity 87.7 and 86%, respectively. Inactivation of norA _SEP increased the ciprofloxacin susceptibility of E. coli DH5α carrying the pB SK 198 norA _SEP EZ cat norA _SEP plasmid.