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Abstract
Integrons are mobile genetic elements that incorporate an open reading frame or gene
cassettes. They have an important role in the acquisition and dissemination of antimicrobial
resistance genes. Yet the occurrence of integrons carrying antimicrobial resistance
genes in bacterial pathogens of pet animals is seldom addressed. The purpose of this
study was to describe the incidence of class 1 and 2 integrons in clinical isolates
of Escherichia coli (n=82) from cats and dogs provided by diagnostic laboratories
in five States of the USA. An association between resistance genes in the integrons
and the isolates' phenotypes was found. Integrons were detected using PCR and then
further characterized by restriction fragment-length polymorphism analysis and amplicon
sequencing. Class 1 integrons were detected in 27% of the isolates, while only 2%
(n=2) of the isolates were positive for the presence of class 2 integrons. Seventy-two
percent (n=59) of the isolates did not carry integrons. Eleven gene cassettes were
found either alone or in combination with other gene cassettes, which encoded resistance
to aminoglycosides (aadA1, aadA2, aadA5, aacA4, and aadB), trimethoprim (dfrA1, dhfrA17,
and dfrA12), chloramphenicol (catB3 and cmlA6), and streptothricin (sat1), respectively.
All integron-positive isolates were characterized by resistance to least two drug
classes and 35% produced extended-spectrum beta-lactamases. The association of integrons
carried on plasmids and antimicrobial resistance was confirmed by curing experiments
for three isolates. Resistance was resolved once large plasmids (size range 97-169
kb) carrying the class 1 integron were lost. Therefore, integrons appear to have an
essential role in facilitating the dissemination of the resistance genes and contributing
to the creation of multi-drug resistant phenotypes.
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