Genetic characterization of extended-spectrum beta-lactamases in Escherichia coli isolates of pigs from a Portuguese intensive swine farm.

There is a great concern by the emergence and the wide dissemination of extended-spectrum beta-lactamases (ESBLs) among animal Escherichia coli isolates. We intended to determinate the carriage level and type of ESBLs in E. coli obtained from fecal samples from pigs raised on an intensive pig farm in Portugal; further to characterize other associated resistance genes and their plasmid content, the phylogenetic groups, and the clonal relationship of ESBL-positive isolates. Sixty-five fecal samples were seeded in Levine media supplemented with cefotaxime for E. coli recovery. Susceptibility to 16 antimicrobial agents was performed by disk diffusion agar. ESBL-phenotypic detection was carried out by double-disk test; and the presence of the genes encoding TEM, OXA, SHV, and CTX-M type beta-lactamases was studied by polymerase chain reaction and sequencing. Other mechanisms of antimicrobial resistance and phylogenetic groups were also determined. Clonal relationship was performed by pulsed-field gel electrophoresis. ESBL-producing E. coli isolates were detected in 16 fecal samples, and one isolate per sample was studied. The CTX-M-1 type ESBL was detected in the 16 isolates. The gene encoding TEM-1 was identified to be associated with eight CTX-M-1-positive isolates. The tet(A) gene was found in 12 of 14 tetracycline-resistant isolates, and the aadA or strA-strB genes were found in the streptomycin-resistant isolates. Fourteen and two ESBL-containing isolates belonged to A and B1 phylogenetic groups, respectively. Clonal relationship of ESBL-containing isolates identified seven unrelated patterns. Swine represent an important reservoir of ESBL-containing E. coli isolates, especially of the CTX-M-1 type.