Hemolytic Escherichia coli are important pathogens in neonatal and weaned pigs. In this study, we analyzed 95 hemolytic E. coli isolated from intestinal contents or fecal samples of diarrheic piglets in 15 states of the United States between November 2013 and December 2014. Phenotypic antimicrobial susceptibility was determined through Sensititre BOFO6F plates for all the strains. They were all resistant to clindamycin, penicillin, tiamulin, tilmicosin, and highly resistant to oxytetracycline (91.6%), chlortetracycline (78.9%), ampicillin (75.8%), and sulfadimethoxine (68.4%). 86.2% of them were multidrug resistant. Whole genome sequencing (WGS) showed that 55 strains were enterotoxigenic E. coli (ETEC) and 40 strains were non-ETEC, and the strains belonged to 22 known and 2 novel sequence types (STs). ST100 and ST10 were the main and predominant STs in ETEC strains, whereas the non-ETEC strains were diverse with ST23 and ST761 as the main STs. Antibiotic resistance gene/mutation profiling of the genomes confirmed the results of antimicrobial susceptibility test. Notably, significant differences were found in the susceptibility to enrofloxacin between ETEC and non-ETEC (58.2% vs. 5.0%) and gentamicin (32.7% vs. 7.5%). ampH, ampC2, and ampC1 were the most common beta-lactamase genes in all E. coli strains, and extended-spectrum beta-lactamase (ESBL) genes were rare in these isolates. This study provides new insights into antibiotic resistance and genotypes of intestinal pathogenic E. coli associated with swine disease in the United States, and support the utility of WGS in accurate prediction of resistance to most antibiotics.
Keywords: antibiotic resistance genes; antimicrobial resistance; hemolytic; molecular typing; whole genome sequence.