Comparative analysis of genetic characterization of β-lactam-resistant Escherichia coli from bulk tank milk in Korea

Background: This study was conducted to analyze the genetic characteristics of 41 beta-lactam-resistant Escherichia coli isolates, which are one of the common causes of environmental mastitis, isolated from the bulk tank milk of 290 dairy farms in five factories operated by three dairy companies in Korea. Results: Analysis of the phenotypic and genotypic characteristics of beta-lactam-resistant E. coli isolates revealed differences between factories even within the same company. Isolates from factory A1 and C1 showed high resistance to cephalothin (76.9 and 100%, respectively), which is a first-generation cephalosporins, whereas resistance to tetracycline was showed by only the isolates from factories B1 (60.0%), C2 (66.7%), and C3 (100%). Although all the 41 beta-lactam-resistant E. coli isolates were positive for bla(OXA-1), bla(TEM- 1) was highly prevalent in isolates from factories C2 (100%) and C3 (100%). Among 17 isolates resistant to both beta-lactams and aminoglycosides, the most common multilocus sequence type was ST399 (13isolates, 76.5%). Furthermore, 2 (11.8%) and 12 (70.6%) isolates belonged to the phylogenetic groups B2 and D, respectively, which are invasive strains that cause intestinal infections, respectively. The predominant serogroup was O15 (70.6%), which is a globally distributed extraintestinal pathogen. Interestingly, one isolate from factory A1 belonged to O157 and carried six virulence genes, simultaneously. Conclusions: Although E. coli isolates were isolated from bulk tank milk, and not the clinical mastitis samples, the presence of the phylogenetic groups B2 and D, and the serogroups O15 and O157, which harbor antimicrobial resistance genes and virulence factors, can pose a threat to public health.