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C olistin is recognized as the last resort for treatment of life-threatening infections caused by multidrug-resistant bacteria; however, the increasing prevalence of colistin-resistant bacteria harboring the mobile colistin resistance (mcr) gene poses a threat to the treatment (1). In this regard, it is considered that the abuse of antibiotic use in the agricultural field may be involved in the emergence and dissemination of antibiotic-resistant bacteria. The current study revealed that more than 70% of the residents in a rural community in Vietnam where colistin-based drugs and colistincontaining feeds were frequently used in livestock breeding (2) retained colistinresistant Escherichia coli (CR-E) harboring mcr in their fecal microbiota (3). Considering this high level of dissemination of CR-E in the residents, the involvement of backyard breeding livestock, which is common in the area, seems likely. Supporting this notion, we found that most of the backyard livestock in the community carried commensal CR-E harboring mcr. To the best of our knowledge, this is the first report of such an exceptionally high CR-E contamination in livestock. A total of 72 stool specimens were obtained from backyard livestock, including pigs and chickens, from 41 households in the village of Naguyen Xa, Thai Binh Province, Vietnam, where the CR-E prevalence of the residents was found to be extremely high (3) in August 2018. Rectal swab specimens were collected from the livestock and inoculated onto a selective agar medium (CHROMagar COL-APSE; CHROMagar, Paris, France) for the isolation of colistin-resistant Gram-negative bacteria. One representative E. coli-like colony was isolated and characterized further for bacterial identification, antibiotic susceptibility, colistin resistance genes, and relevance between isolates, as described previously (4-6). As shown in Table 1, most of the livestock tested showed the presence of CR-E harboring mcr; only 3 animals showed negative results. Differences in the prevalences of CR-E between chickens and pigs were not determined. The MICs of colistin for these isolates was Ն8 mg/liter. In the PCR analysis of colistin resistance genes (mcr-1 to-5) in CR-E isolates, mcr-1 was almost solely detected. There was one exception: an isolate from a pig that carried mcr-3. The susceptibility of CR-E isolates to other antibiotics, including ampicillin, cefoxitin, cefotaxime, ceftazidime, meropenem, streptomycin, kanamycin, gentamicin, ciprofloxacin, nalidixic acid, tetracycline, chloramphenicol, fosfomycin, and sulfamethoxazole-trimethoprim, showed that 95.6% were multidrug resistant, that is, resistant to at least one antibiotic in three or more antibiotic classes (7). The

The wide distribution of colistin-resistant bacteria in developing countries has become a common phenomenon. To understand the mechanisms underlying their distribution, we studied the mcr genetic background of colistin-resistant Escherichia coli isolates from the fecal microbiota of healthy human residents from a community in Vietnam with a high prevalence of colistin-resistant E. coli with mcr. Fifty-seven colistin-resistant isolates were obtained from 98 residents; one isolate was collected from each individual and analyzed for mcr. We found that 36.8% of the isolates carried chromosomal mcr-1. Further, 63.2% and 1.8% of the isolates carried mcr-1 on the plasmid and the plasmid/chromosome, respectively. Whole-genome sequencing of genetically unrelated isolates showed that the majority (6 of 7) of the isolates had the chromosomal mcr-1 in a complete ancestral mcr-1 transposon Tn6330, ISApl1-mcr-1-PAP2-ISApl1, which was inserted at various positions on the chromosomes. In addition, the majority (87.5%) of Tn6330 of mcr-1-carrying plasmids (n ϭ 8) lacked both upstream and downstream ISApl1 transposons. The results obtained in this study indicate that plasmid-to-chromosomal transfer of mcr-1 may have occurred recently in the fecal microbiota of the residents. Additionally, Tn6330 on the chromosome may lose ISApl1 from the transposon during multiplication to gain a more stable mcr-1 state on the chromosome. Stabilization of resistance by the chromosomal incorporation of mcr-1 would be an additional challenge in combating the dissemination of resistant bacteria. IMPORTANCE Elucidation of the mechanism of the wide dissemination of colistinresistant bacteria in communities of developing countries is an urgent public health issue. In this study, we investigated the genetic background of the colistin resistance gene mcr in E. coli isolates from the fecal microbiota of healthy human residents living in a community in Vietnam with a high prevalence of colistin-resistant E. coli. Our study revealed for the first time, a surprisingly high percentage (36.8%) of colistin-resistant E. coli carrying chromosomal mcr-1, the emergence of which may have occurred recently, in the fecal microbiota of the community residents. The mcr-1 transposon on the chromosome may develop into a more stable genotype by the loss of insertion sequences (ISs). Our results are valuable in understanding the mechanism underlying the increasing prevalence of colistin-resistant bacteria within a community.

Extended-spectrum, β-lactamase-producing Escherichia coli (ESBL-E) harboring the bla -encoding plasmid (ESBL-E55) has been reported to be associated with urinary tract infection (UTI). The aims of this study were to clarify the prevalence of ESBL-E55 in pork meats and workers from the same wholesale market, as well as patients with UTI from a nearby hospital in Vietnam; we also investigated the plasmids encoding bla. Sequencing analysis showed that 66.6% of the ESBL-E isolated from pork meats contained bla , whereas the gene was present in 25.0% of workers and 12.5% of patients with UTI. Plasmid analysis showed that several sizes of plasmid encoded bla in ESBL-E55 isolated from pork meats, whereas ESBL-E55 isolated from workers and patients with UTI contained only 104-139 kbp of bla -encoding plasmids. This indicates that the 104-139 kbp sizes of bla-encoding plasmids were commonly disseminated in pork meats, wholesale market workers, and patients with UTI.