Antimicrobial susceptibility and adaptative changes in MRSA lineages exposed to increasing concentrations of fluoroquinolones and chlorhexidine

Selective pressure from antimicrobial use can drive the emergence of methicillin-resistant Staphylococcus aureus (MRSA) isolates. We evaluated resistance rates to fluoroquinolones and chlorhexidine digluconate (CD) and the effect of their selective pressure on lineages of MRSA. Minimum inhibitory concentrations (MICs) for fluoroquinolones and CD were determined for 75 isolates from 11 lineages. Phenotypic expression of efflux pumps was assessed by the Cartwheel method and associated genes were detected by PCR. Cross-resistance between antibiotics and CD and point gene mutations were investigated after exposure to increasing concentrations of fluoroquinolones and CD. The MIC90 values for ciprofloxacin, moxifloxacin, and CD in 75 MRSA isolates were 128, 8 and 1 mg/L, respectively. ST5-SCCmecII, ST239-III and ST1-IV lineages showed the highest resistance rates to fluoroquinolones, and ST5-II showed the highest MIC90 for CD. Among 17 (22.6%) MRSA isolates with detectable efflux pump activity, seven showed a higher efflux potential, including four ST5-II isolates. The qacA/B genes were found in 14 (18.6%) isolates, and 10 were ST5-II, while the smr gene was frequent among isolates with SCCmecIV (71.4%). Eight of 10 selected strains showed increased resistance to fluoroquinolones and/or tetracycline and increased phenotypic expression of efflux pumps. Point mutations in the gyrA, parC, norA, and/or norB genes emerged in three strains after exposure to higher doses of ciprofloxacin or CD. Resistance to fluoroquinolones was high among MRSA isolates, especially those from ST5-II. MRSA isolates subjected to in vitro selective pressure of antimicrobials showed increased MICs and gene mutations, reinforcing the importance of the rational use of antimicrobial agents in clinical practice.