Characterization of multidrug-resistant Escherichia coli isolates from animals presenting at a university veterinary hospital

Abstract

In this study, we examined molecular mechanisms associated with multidrug resistance (MDR) in a collection of Escherichia coli isolates recovered from hospitalized animals in Ireland. PCR and DNA sequencing were used to identify genes associated with resistance. Class 1 integrons were prevalent (94.6%) and contained gene cassettes recognized previously and implicated mainly in resistance to aminoglycosides, β-lactams, and trimethoprim (aadA1, dfrA1-aadA1, dfrA17-aadA5, dfrA12-orfF-aadA2, bla(OXA-30)-aadA1, aacC1-orf1-orf2-aadA1, dfr7). Class 2 integrons (13.5%) contained the dfrA1-sat1-aadA1 gene array. The most frequently occurring phenotypes included resistance to ampicillin (97.3%), chloramphenicol (75.4%), florfenicol (40.5%), gentamicin (54%), neomycin (43.2%), streptomycin (97.3%), sulfonamide (98.6%), and tetracycline (100%). The associated resistance determinants detected included bla(TEM), cat, floR, aadB, aphA1, strA-strB, sul2, and tet(B), respectively. The bla(CTX-M-2) gene, encoding an extended-spectrum β-lactamase (ESβL), and bla(CMY-2), encoding an AmpC-like enzyme, were identified in 8 and 18 isolates, respectively. The mobility of the resistance genes was demonstrated using conjugation assays with a representative selection of isolates. High-molecular-weight plasmids were found to be responsible for resistance to multiple antimicrobial compounds. The study demonstrated that animal-associated commensal E. coli isolates possess a diverse repertoire of transferable genetic determinants. Emergence of ESβLs and AmpC-like enzymes is particularly significant. To our knowledge, the bla(CTX-M-2) gene has not previously been reported in Ireland.

Fig. 1.

(a) Representation of the sequenced gene cassettes amplified with primers targeting conserved segments of the class 1 integrons after electrophoretical separation in 1.7% agarose gel. Lane 1, 1-kb DNA size marker (New England BioLabs); lane 2, E. coli 18; lane 3, E. coli 29; lane 4, E. coli 28; lane 5, E. coli 21; lane 6, empty; lane 7, E. coli 16; lane 8, E. coli 2; lane 9, E. coli CL45; lane 10, 100-bp DNA size marker. (b) Schematic representation of the organization of the sequenced gene cassettes of the class 1 integrons. Arrowheads represent the direction of the transcription. Integron-associated conserved features are indicated.

Fig. 2.

Agarose gel image showing plasmid profiles of E. coli 31 and its transconjugants. Chr, chromosomal DNA. Lane 1, E. coli 39R861; lane 2, E. coli V517; lane 3, E. coli 31 (AmcAmpCCipFfcNaSSuTeTmp resistance profile); lanes 4, 5, and 6, E. coli 31-derived transconjugants selected on chloramphenicol, streptomycin, and tetracycline, respectively, all with the CFfcSSuTe resistance profile.