Prevalence and transmission of antimicrobial resistance in Escherichia coli isolated from food-producing animals

Abstract

Escherichia coli is a significant reservoir of antimicrobial resistance determinants which can spread pathogenic bacteria to human and animals. E. coli strains are able to efficiently exchange genetic mobile material such as integrons, transposomes and plasmid of pathogens. Acquired resistance mechanism mediated by these determinants play on important role in acquisition and dissemination of resistance mechanism. Thus in order to investigate and analyze the prevalence and transferability of antimicrobial resistance in farm animals, E. coli strains isolated from the pathogenic lesions and fecal samples during the year 2009-2015 from beef cattle, pigs and chickens were included in this study. The first study on the prevalence and characterization of E. coli isolated from beef cattle farms showed the diverse patterns of phenotype and genotype in antimicrobial resistance and pathogenicity. The most dominant virulence gene was f17. The 152 isolates showed multidrug-resistance. Antimicrobial susceptibility test determined that the most frequent resistance phenotype was streptomycin (63.1%), followed by tetracycline 54.5%), cephalothin (32.8%), and sulphamethoxazole/trimethoprim (16.6%). PCR and sequencing showed the prevalence of associated resistance determinants as follows: strA-strB (39.0%, 113/290), tet(A) (27.6%, 80/290), blaTEM (23.8%, 69/290), and sul2 (22.1%, 97/290). PFGE and O serotyping identified that E. coli isolates in this study showed the high degree of clonal diversity in genetic relation. Second study was focused on ampicillin-resistant bovine E. coli strains harboring β-lactamases which have possibility to evolve into Extended-spectrum β-lactamase (ESBL) or plasmid-mediated AmpC β-lactamase. In this study, 78 E. coli isolates from beef cattle were included in this study. In the disc diffusion test with β-lactams, 38.5% of the isolates showed resistance to ampicillin, amoxicillin, and cephalothin, together. However, none of the isolates had determined to produce ESBL or AmpC β-lactamases by double disc synergy method. All isolates encoded genes for TEM-1-type β-lactamase. In plasmid replicon typing, IncFIB and IncFIA were identified in 71.4% and 41.0% of plasmids, respectively. Of transferable replicon, IncFIB and IncFIA were the most frequent type detected (61.5% and 41.0%, respectively). Based on these results, we might suggest that the transferable plasmids could provide significant effect on the acquisition and dissemination of β-lactam resistance as well as selection pressure although the level of antimicrobial usage in beef cattle is relatively low compared to those in other livestock animals in Korea. In third study, the prevalence and transferability of resistance in tetracycline-resistant E. coli isolates from beef cattle in South Korea were carried out. Among 155 E. coli isolates, 146 were confirmed to be resistant to tetracycline. The tetracycline resistance gene tet(A) (46.5%) was the most prevalent. Ninety-one (62.3%) isolates were determined to be multidrug-resistant by the disc diffusion method. MIC testing using the principal tetracyclines, revealed that isolates carrying tet(B) had higher MIC values than isolates carrying tet(A). Conjugation assays showed that 121/155 (82.9%) isolates could transfer a tetracycline resistance gene to a recipient via the IncFIB replicon (65.1%, 95/155). This study suggests that the high prevalence of tetracycline-resistant E. coli isolates in beef cattle might be due to the transferability of tetracycline resistance genes between E. coli populations which have survived the selective pressure caused by the use of antimicrobial agents. In final study, a total of 281 E. coli strains isolated from pigs and chickens were investigated for ESBL-production. Fourteen E. coli isolates were identified to produce ESBL. The most common CTX-M- and CMY-types were CTX-M-15 (8/14) and CMY-2 (3/14). All ESBL-producing isolates showed resistance to the extent of the fourth-generation cephalosporins, along with multi-drug resistance. A conjugation assay demonstrated that blaCTX-M and blaCMY genes have the potential to be transferred to non-resistant E. coli. The horizontal dissemination of blaCTX-M and blaCMY genes was mediated mainly by Frep and IncI1 plasmids. PFGE revealed that isolates tested in this study were very diverse, clonally. To our knowledge, this is the first report of E. coli isolate possessing blaCMY-6 from chickens in South Korea. Distribution of resistance determinants in transferable plasmid of E. coli investigated in these studies could be critical in the public health. In addition future use of antimicrobial agents for human and veterinary purpose should be limited because of the increase in antimicrobial resistance for E. coli in human and farm animals. Thus reasonable use and long-term surveillance are needed for minimizing the emergence and spread of antimicrobial resistance in E. coli.