Investigation of fluoroquinolone resistance mechanisms in clinical Acinetobacter baumannii isolates

Acinetobacter baumannii is an opportunistic pathogen which is one of the most important agents of nosocomial infections. Being resistant to many antibiotics makes difficult to treat the infections it causes. Fluoroquinolones are used in the treatment of nosocomial and community-acquired infections and it is important to determine the mechanisms of resistance to these antibiotics. It is aimed to investigate ciprofloxacin (CIP) and levofloxacin (LEV) minimum inhibitor concentrations (MIC), clonal relationships, mutations that occur in DNA gyrase and topoisomerase IV genes and overexpression of efflux pumps in nosocomial A. baumannii isolates. 79 CIP-resistant and two CIP-susceptible A. baumannii strains isolated from different clinical samples (one per patient) in Ege University Department of Clinical Microbiology were included into the study. CIP and LEV MIC values were determined by broth microdilution method. Clonal relationship among the strains was investigated by ERIC (enterobacterial repetitive intergenic consensus) –PCR. Mutations that occur in gyr-A and par-C genes were detected by DNA sequence analysis in 16 strains representing each clone and subtype. Overexpression of efflux pumps was evaluated by the broth microdilution and fluorometric accumulation experiments carried out in the presence of pump inhibitors, in the representative strains. MIC50 and MIC90 values for CIP were 256 and ≥256, same values for LEV were 32 and 128 respectively. Overall, CIP MIC values were found to be higher than that of LEV in the investigated A. baumannii strains. Isolates were grouped into 10 different clusters, but the vast majority of them were observed to be clonally related. The most common and important mutations detected in the gyr-A and par-C genes of the representative isolates were, Ser83Leu, Asp87Glu and Ser80Leu, respectively. While the overexpression of efflux pumps was observed in seven of the representative strains by microdilution method and in 11 strains by fluorometric assay, the results were positive in five strains by both methods. CIP was found to be better than LEV in detecting the overexpression of pumps by microdilution method. It was concluded that, high fluoroquinolone MIC values of the investigated nosocomial and clonally related A. baumannii strains were derived from target mutations and overexpression of efflux pumps. Keywords: Acinetobacter baumannii; Fluoroquinolone; QRDR; efflux; ERIC-PCR