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Antimicrobial Agents and Chemotherapy, September 1999, p. 2131-2137, Vol. 43, No. 9
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

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

Etienne Giraud,1 Anne Brisabois,2 Jean-Louis Martel,3 and Elisabeth Chaslus-Dancla1,*

Station de Pathologie Aviaire et de Parasitologie, Institut National de la Recherche Agronomique, Centre de Recherche de Tours-Nouzilly, 37380 Monnaie,1 Centre National d'Etudes Vétérinaires et Alimentaires, Laboratoire Central d'Hygiène Alimentaire, 75015 Paris,2 and Centre National d'Etudes Vétérinaires et Alimentaires, Laboratoire de Pathologie Bovine, 69341 Lyon cedex 07,3 France

Received 1 February 1999/Returned for modification 20 May 1999/Accepted 21 June 1999

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.

* Corresponding author. Mailing address: Station de Pathologie Aviaire et de Parasitologie, Institut National de la Recherche Agronomique, Centre de Recherche de Tours-Nouzilly, 37380 Monnaie, France. Phone: 33-2-47-42-77-65. Fax: 33-2-47-42-77-74. E-mail: chaslus{at}tours.inra.fr.

Antimicrobial Agents and Chemotherapy, September 1999, p. 2131-2137, Vol. 43, No. 9
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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