Isolation and molecular characterization of nalidixic acid-resistant extraintestinal pathogenic Escherichia coli from retail chicken products

Abstract

Fluoroquinolone use in poultry production may select for resistant Escherichia coli that can be transmitted to humans. To define the prevalence and virulence potential of poultry-associated, quinolone-resistant E. coli in the United States, 169 retail chicken products from the Minneapolis-St. Paul area (1999 to 2000) were screened for nalidixic acid (Nal)-resistant E. coli. Sixty-two (37%) products yielded Nal-resistant E. coli. From 55 products that yielded both Nal-resistant and susceptible E. coli, two isolates (one resistant, one susceptible) per sample were further characterized. Twenty-three (21%) of the 110 E. coli isolates (13 resistant, 10 susceptible) satisfied criteria for extraintestinal pathogenic E. coli (ExPEC), i.e., exhibited >or=2 of pap (P fimbriae), sfa/foc (S/F1C fimbriae), afa/dra (Dr binding adhesins), iutA (aerobactin receptor), and kpsMT II (group 2 capsule synthesis). Compared with other isolates, ExPEC isolates more often derived from virulence-associated E. coli phylogenetic groups B2 or D (74% versus 32%; P < 0.001) and exhibited more ExPEC-associated virulence markers (median, 10.0 versus 4.0; P < 0.001). In contrast, the Nal-resistant and -susceptible populations were indistinguishable according to all characteristics analyzed, including pulsed-field gel electrophoresis profiles. These findings indicate that Nal-resistant E. coli is prevalent in retail poultry products and that a substantial minority of such strains represent potential human pathogens. The similarity of the Nal-resistant and -susceptible populations suggests that they derive from the same source population, presumably the avian fecal flora, with Nal resistance emerging by spontaneous mutation as a result of fluoroquinolone exposure.

FIG. 1.

PFGE profiles of E. coli isolates from retail chicken products. Lane numbers are shown below image. XbaI PFGE profiles reveal identity between Nal-susceptible (S) isolates 412, 436, and 458 (lanes 2, 4, and 6) and Nal-resistant (R) isolate 427, 453, and 461 (lanes 3, 5, and 7). Lanes 1 and 8, reference E. coli O157:H7 strain G5244.

FIG. 2.

RAPD profiles of poultry-source E. coli isolates refractory to macrorestriction analysis. Lane numbers are shown below image. According to analysis with RAPD primers 1254 and 1281 (as listed above the gel lanes), Nal-resistant (R) isolates (isolates 407, lanes 1 and 4; and 467, lanes 3 and 6) are indistinguishable from Nal-susceptible (S) isolate 372 (lanes 2 and 5), within the reproducibility limits of RAPD analysis.