Affiliation:
1. Department of Bacterial Diseases, Veterinary Laboratories Agency-Weybridge, Addlestone, Surrey KT15 3NB
2. Laboratory of Enteric Pathogens, Central Public Health Laboratory, London NW9 5HT, United Kingdom
Abstract
ABSTRACT
This study is focused on real-time detection of
gyrA
mutations and of the presence of class I integrons in a panel of 100 veterinary isolates of
Salmonella enterica
from farm animals. The isolates were selected on the basis of resistance to nalidixic acid, representing a variety of the most prevalent serotypes in England and Wales. In addition, organic solvent (cyclohexane) resistance in these isolates was investigated in an attempt to elucidate the presence of efflux pump mechanisms. The most prevalent mutation among the isolates studied was Asp87-Asn (
n
= 42), followed by Ser83-Phe (
n
= 38), Ser83-Tyr (
n
= 12), Asp87-Tyr (
n
= 4), and Asp87-Gly (
n
= 3). Two distinct subpopulations were identified, separated at the 1-mg/liter breakpoint for ciprofloxacin: 86% of isolates with mutations in codon 83 showed MICs of ≥1 mg/liter, while 89.8% of isolates with mutations in codon 87 presented MICs of ≤0.5 mg/liter. Cyclohexane resistance was more prevalent among Ser83 mutants than among Asp87 mutants (34.7 and 4%, respectively), and in 79% of isolates that presented both
gyrA
mutations and cyclohexane resistance, the level of ciprofloxacin resistance was ≥2.0 mg/liter. Thirty-four isolates contained class I integrons, with 71% of the
S. enterica
serovar Typhimurium isolates and 6.9% of isolates belonging to other serotypes containing such elements. The methods used represent sensitive ways of investigating the presence of
gyrA
mutations and of detecting class-I integrons in
Salmonella
isolates. The results can be obtained in less than 1 h from single colonies without the need for purifying DNA.
Publisher
American Society for Microbiology
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