Molecular characterisation of antibiotic resistance mechanisms in Salmonella enterica isolates in Ireland

Abstract number: p1253

Lee  D., Fanning  S., Coffey  A.

The primary objective of this study was to determine the level and range of antibiotic resistance among a broad collection of Salmonella enterica isolates from both human and animal sources in Ireland. The emergence of antibiotic resistance in Salmonella was highlighted over a decade ago. Many cases of Salmonella infection now involve multi-drug resistant strains. Quinolones, including Fluoroquinolones, are broad-spectrum antibiotics and are often the treatment of choice in cases of life-threatening Salmonellosis caused by multi-drug resistant strains. Resistance to the quinolones is mainly attributed to alterations of the target sites, caused by specific mutations within the chromosomal genes encoding DNA gyrase (gyr A, B) and topoisomerase IV (par C, E). These mutations are generally located within an area known as the Quinolone Resistance Determining Region (QRDR). The antibiotic resistance profile was determined for 167 Salmonella isolates, against a panel of 17 antibiotics representing those regularly used for clinical treatment of Salmonella infections. Isolates with reduced susceptibility to quinolones were selected for further study with the aim of determining the correlation between specific mutations and resistance. The QRDR's of gyr A, B and par C, E were amplified by the Polymerase Chain Reaction (PCR) and the products sequenced. The minimum inhibitory concentrations for Nalidixic acid (Nal), Ciprofloxacin (Cip), and Norfloxacin (Nx) was determined using the E-test. Within the gyrA gene, two different mutations were identified. 9 isolates harboured a nucleotide substitution at codon 87 (GAC-TAC), all of animal origin. One isolate had a nucleotide substitution at codon 83 (TCC-TTC), of human origin. Both mutations correlated with observed high level resistance of >256 mg/ml to Nal, a reduced susceptibility to Cip and Nx was also observed. Two isolates harboured a single mutation located within the QRDR of par C at codon 57 (ACC-AGC) which was linked to intermediate resistance to Nal. All mutations resulted in amino acid substitutions. While treatment failure with quinolones has been reported in Asia and the United States, to date resistance to the quinolones in Ireland is rare, however constant screening for quinolone resistance among Salmonella isolates is vital.