Resurgence of Chloramphenicol Resistance in Methicillin-Resistant Staphylococcus aureus Due to the Acquisition of a Variant Florfenicol Exporter (fexAv)-Mediated Chloramphenicol Resistance in Kuwait Hospitals.

Following a surge in the prevalence of chloramphenicol-resistant methicillin-resistant Staphylococcus aureus (MRSA) in Kuwait hospitals, this study investigated the genotypes and antibiotic resistance of the chloramphenicol-resistant isolates to ascertain whether they represented new or a resurgence of sporadic endemic clones. Fifty-four chloramphenicol-resistant MRSA isolates obtained in 2014–2015 were investigated. Antibiotic resistance was tested by disk diffusion and MIC determination. Molecular typing was performed using spa typing, multilocus sequence typing, and DNA microarray. Curing and transfer experiments were used to determine the genetic location of resistance determinants. All 54 isolates were resistant to chloramphenicol (MIC: 32–56 mg/L) but susceptible to florfenicol. Two chloramphenicol-resistance determinants, florfenicol exporter (fexA) and chloramphenicol acetyl transferase (cat), were detected. The fexA-positive isolates belonged to CC5-ST627-VI-t688/t450/t954 (n = 45), CC5-ST5-V-t688 (n = 6), whereas the cat-positives isolates were CC8-ST239-III-t037/t860 (n = 3). While cat was carried on 3.5–4.4 kb plasmids, the location of fexA could not be established. DNA sequencing of fexA revealed 100% sequence similarity to a previously reported fexA variant that confers chloramphenicol but not florfenicol resistance. The resurgence of chloramphenicol resistance was due to the introduction and spread of closely related fexA-positive CC5-ST5-V and CC5-ST627-VI clones.