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

In the Staphylococcus aureus ATCC25923 strain, the flqB mutation in the 5′untranslated region (5′UTR) of the norA gene causes increased norA mRNA expression and high efflux activity (HEA). The involvement of the norA gene 5′UTR in HEA has not been explored in S. epidermidis; therefore, we examined the function of this region in S. epidermidis clinical isolates. The selection of isolates with HEA was performed based on ethidium bromide (EtBr) MIC values and efflux efficiency (EF) using the semi-automated fluorometric method. The function of the 5′UTR was studied by quantifying the levels of norA expression (RT-qPCR) and by identifying 5′UTR mutations by sequence analysis. Only 10 isolates from a total of 165 (6.1%) had HEA (EtBr MIC = 300 μg/ml and EF ranged from 48.4 to 97.2%). Eight of 10 isolates with HEA had the 5′UTR ^95ΔG mutation. Isolates carrying the ^95ΔG mutation had higher levels of norA expression compared with those that did not. To corroborate that the ^95ΔG mutation is involved in HEA, a strain adapted to EtBr was obtained in vitro. This strain also presented the ^95ΔG mutation and had a high level of norA expression and EF, indicating that the ^95ΔG mutation is important for the HEA phenotype. The ^95ΔG mutation produces a different structure in the Shine-Dalgarno region, which may promote better translation of norA mRNA. To our knowledge, this is the first report to demonstrate the participation of the 5′UTR ^95ΔG mutation of the norA gene in the HEA phenotype of S. epidermidis isolates. Here, we propose that the efflux of EtBr is caused by an increment in the transcription and/or translation of the norA gene.