Comparative Studies of Mutations in Animal Isolates and Experimental In Vitro- and In Vivo-Selected Mutants of <i>Salmonella</i> spp. Suggest a Counterselection of Highly Fluoroquinolone-Resistant Strains in the Field

TLDR

That's the purpose. Mechanism: "An assay combining allele-specific PCR and restriction fragment length polymorphism was developed to rapidly screen mutations at codons 81, 83, and 87 of gyrA." So mechanism: They developed an allele-specific PCR and RFLP assay to rapidly screen gyrA codons 81, 83, 87. Summarize: Sequencing revealed gyrA mutations at codons Gly-81 or Ser-83, sometimes with Asp-87; no mutations in gyrB, parC, parE.

Abstract

ABSTRACT The occurrence of mutations in the genes coding for gyrase ( gyrA and gyrB ) and topoisomerase IV ( parE and parC ) of Salmonella typhimurium experimental mutants selected in vitro and in vivo and of 138 nalidixic acid-resistant Salmonella field isolates was investigated. The sequencing of the quinolone resistance-determining region of these genes in highly fluoroquinolone-resistant mutants (MICs of 4 to 16 μg/ml) revealed the presence of gyrA mutations at codons corresponding to Gly-81 or Ser-83, some of which were associated with a mutation at Asp-87. No mutations were found in the gyrB , parC , and parE genes. An assay combining allele-specific PCR and restriction fragment length polymorphism was developed to rapidly screen mutations at codons 81, 83, and 87 of gyrA . The MICs of ciprofloxacin for the field isolates reached only 2 μg/ml, versus 16 μg/ml for some in vitro-selected mutants. The field isolates, like the mutants selected in vivo, had only a single gyrA mutation at codon 83 or 87. Single gyrA mutations were also found in highly resistant in vitro-selected mutants (MIC of ciprofloxacin, 8 μg/ml), which indicates that mechanisms other than the unique modification of the intracellular targets could participate in fluoroquinolone resistance in Salmonella spp. A comparison of experimental mutants selected in vitro, field strains, and mutants selected in vivo suggests that highly fluoroquinolone-resistant strains are counterselected in field conditions in the absence of selective pressure.