Molecular epidemiology of hybrid CTX-M type beta-lactamases among Escherichia coli isolates from human and animals

Abstract

A novel and effective mechanism of forming resistance to beta-lactam antibiotics by producing hybrid CTX-M-type beta-lactamases through genetic recombination was reported recently, albeit sporadically. Its primary introductory factor is the horizontal resistant gene transfer that occurs within the 876 bp 〖bla〗_(CTX-M) gene in Escherichia coli isolates, followed by the extensive application of beta-lactam antimicrobials. 〖bla〗_(CTX-M) originated from the nonpathogenic commensal Kluyvera spp., but it has become widely distributed in pathogenic organisms by plasmid-mediated gene transfer. Beta-lactams are effective against both gram-positive and gram-negative bacteria. However, misusing or overusing of beta-lactams has caused a globally corresponding emergence of antimicrobial resistant strains because of the selective pressure.

This study included a total of 1,441 E. coli isolates, including 1,101 from six categories of animals and 340 from humans of both sexes. In total 612 strains were detected as extended spectrum beta-lactamase (ESBL)-positive. By using molecular tests on 248 ESBL-producers, 14 non-duplicate strains harbouring hybrid CTX-M-type beta-lactamases were found in a subset of 170 animal strains.

A total of three alleles for hybrid CTX-M-type beta-lactamases (n = 14) were found, namely, CTX-M-64 (n = 6), CTX-M-123 (n = 6), and CTX-M-132 (n = 2). Six 〖bla〗_(CTX-M-64) and one 〖bla〗_(CTX-M-132) genes were located on approximately 70 kb IncI2 plasmids, and five 〖bla 〗_(CTX-M-123) genes were carried by 110 kb IncI1 (plasmid ST108) plasmids. All the 〖bla〗_(hybrid CTX-M) genes were placed between ISEcp1 followed by a 45 bp spacer and a 47 bp spacer followed by a 344 bp orf477delta gene. Among the six CTX-M-64-harbouring isolates detected, three of them were ST1011, phylogroup E, from chickens; the remaining three were ST224, phylogroup B1; ST93 CC168, phylogroup A; and ST117, phylogroup F. Every CTX-M-64-harbouring isolate had the co-resistance trait against both ciprofloxacin and tetracycline. The CTX-M-123-harbouring isolates were ST1485, phylogroup E; ST2732, phylogroup D; ST10, phylogroup A; ST457, phylogroup F; and ST 1196, phylogroup B1. All six CTX-M-123-harbouring strains were co-resistant to trimethoprim-sulphamethoxazole and tetracycline, and all of their transconjugants or transformants inherited the ability to resist fosfomycin. Both CTX-M-132-harbouring strains came from phylogroup A. One strain was ST3103, and the other one with a non-transferable plasmid was ST746.

This study unravels the molecular epidemiology of hybrid CTX-M-type beta-lactamase-harbouring E. coli isolates. It also compared the prevalence of these isolates among both animal and human strains. Although hybrid CTX-Mtype beta-lactamases were not found in 340 clinical urinary isolates, 14 hybrid CTX-M-type beta-lactamases were identified in 14 animals in just one year. Hence, this study sets off alarm bells for the public to become more concerned about the antimicrobial resistance situation among animals. Furthermore, the discovery of hybrid CTX-M-type beta-lactamase-harbouring isolates also indicates a novel mechanism of antimicrobial resistance, i.e., homologous recombination, and shows a new way to study antimicrobial resistance.