The tip of the iceberg: quinolone-resistance conferred by mutations in gyrA gene in non-typhoidal Salmonella strains

Food-borne infections due to Salmonella spp. seldom require antimicrobial therapy, but this is compulsory in systemic salmonellosis. Salmonella resistance to a large panel of antibiotics has been described worldwide. Since the introduction of nalidixic acid in therapy, Salmonella spp. have steadily developed resistance, especially over the last three decades. The source of quinolone resistance is thought to be the selective pressure determined by the use of quinolones in both human and veterinary practices. Resistance acquisition of Salmonella strains is a stepwise process. Several mechanisms are described, which can lead to the development of quinolone resistance. The main mechanism is considered to be linked with mutations in the quinolone-resistance determining region (QRDR) of the target genes (gyrA and gyrB encoding DNA gyrase, and parC and parE encoding topoisomerase IV). This first step in mutational resistance usually determines a rise in the nalidixic acid minimal inhibitory concentration (MIC). The most common amino acid substitutions in the GyrA subunit, resulting in varied degrees of quinolone resistance, occur at codons Ser83 and Asp87. Higher levels of resistance may occur by further mutational steps, with amino acid changes in the same or a different target enzyme. Other mechanisms are as well involved, like increased efflux or plasmid-mediated resistance. Acknowledgement of the epidemiology and the onset mechanisms of quinolone resistance in Salmonella spp. is compulsory, and surveillance for resistant bacteria among human, animal and food sources remains critical.