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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.

Antibiotic residues in aquatic products in Vietnam were investigated. A total of 511 fish and shrimp samples were collected from markets in Ho Chi Minh City (HCMC), Thai Binh (TB), and Nha Trang (NT) from July 2013 to October 2015. The samples were extracted with 2% formic acid in acetonitrile and washed with dispersive C18 sorbent. Thirty-two antibiotics were analyzed by LC-MS/MS. Of the 362 samples from HCMC, antibiotic residues were found in 53 samples. Enrofloxacin was commonly detected, at a rate of 10.8%. In contrast, samples from TB and NT were less contaminated: only 1 of 118 analyzed samples showed residues in TB and only 1 of 31 showed residues in NT. These differences were attributed to the local manufacturing/distribution systems. To understand the current status of antibiotic use and prevent adverse effects that may be caused by their overuse, continual monitoring is required.

Background: Widespread dissemination of extended-spectrum β-lactamase (ESBL)-producing bacteria is a threat to public health. Since livestock products are possible reservoirs of ESBL-producing bacteria, food-borne dissemination of ESBL-producing bacteria and the characteristics of such organisms in food products should be assessed to evaluate potential sources of contamination. Results: A total of 253 food samples from poultry, pork, shrimp, and fish were collected from local markets in a rural area of Vietnam from 2013 to 2014. ESBL-producing Escherichia coli were isolated from these samples, and their ESBL genotypes, phylogenetic groups, and antibiotic resistance profiles were assessed. Overall, a high percentage (68.4 %) of the food samples was contaminated with ESBL-producing E. coli, and samples from livestock and fishery products showed similar levels of contamination. The ESBL-producing E. coli isolated from the food samples harbored genes of the CTX-M-1, CTX-M-9, and TEM groups. Analysis of the antibiotic resistance profiles of the ESBL-producing E. coli isolates from the food samples showed a high degree of multidrug resistance. The prevalence of extensively multidrug-resistant ESBL-producing E. coli that were resistant to at least five antibiotic drug classes in poultry, pork, shrimp, and fish samples, was 92.1, 69, 56.5, and 62.5 %, respectively. Conclusions: The results of this study confirmed the widespread dissemination of ESBL-producing E. coli in both livestock and fishery products from a rural area in Vietnam. The high prevalence of extensively multidrug-resistant ESBL-producing E. coli in food products highlights the importance of continuous monitoring of food products for the presence of these bacteria, particularly in underdeveloped countries.