Molecular Characterization of CTX-M β-Lactamase and Associated Addiction Systems in Escherichia coli Circulating among Cattle, Farm Workers, and the Farm Environment

A total of 84 extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli isolates from cattle, farm workers, and the farm environment isolated from February to September 2008 in the Republic of Korea were investigated. All 84 ESBL-producing isolates carried bla(CTX-M) genes that belonged to the CTX-M-1 (n = 35) or CTX-M-9 (n = 49) family. The most predominant CTX-M type identified was CTX-M-14 (n = 49), followed by CTX-M-32 (n = 26). The bla(CTX-M) genes were identified most commonly in E. coli isolates from feces (n = 29), teats (n = 25), and milk (n = 14). A bla(CTX-M-14) gene was also detected in an E. coli isolate from a farmer's hand. Transfer of the bla(CTX-M) gene from 60 bla(CTX-M)-positive E. coli isolates to the recipient E. coli J53 strain by conjugation was demonstrated. Plasmid isolation from bla(CTX-M)-positive transconjugants revealed a large (95- to 140-kb) conjugative plasmid. Almost all (82/84) bla(CTX-M) genes possessed an insertion sequence, ISEcp1, upstream of the bla(CTX-M) gene. Only in the case of the CTX-M-14 genes was IS903 downstream of the gene. The bla(CTX-M) genes were associated with seven kinds of addiction systems. Among them, pndAC, hok-sok, and srnBC were the most frequently identified addiction systems in both wild strains and transconjugants. The spread of bla(CTX-M) genes was attributed to both clonal expansion and horizontal dissemination. Our data suggest that a combination of multiple addiction systems in plasmids carrying bla(CTX-M) genes could contribute to their maintenance in the host cells. To our knowledge, the bla(CTX-M-32) gene has not previously been reported in animal isolates from the Republic of Korea.