LETTER
The recent identification of members of the family Enterobacteriaceae harboring the plasmid-mediated transferable colistin resistance mcr-1 gene is of great concern to public health (1–4). Here, we report on the occurrence of mcr-1-harboring extended-spectrum β-lactamase (ESBL)-producing members of Enterobacteriaceae from river water in Switzerland and ready-to-eat imported vegetables.
For this study, 74 ESBL-producing members of Enterobacteriaceae isolated from 21 rivers and lakes sampled in 2012 in Switzerland (5) and 60 ESBL-producing members of Enterobacteriaceae isolated from 42 imported vegetable samples (11 from the Dominican Republic, 13 from India, 11 from Thailand, and 8 from Vietnam) (6) were screened by PCR for the presence of the mcr-1 gene.
The mcr-1 gene was detected in 1 out of 74 water strains (an isolate from the river Birs) and 2 out of 60 vegetable strains (products from Thailand and Vietnam), and sequencing of the amplicons showed a 100% identity with the published mcr-1 sequence (1). The colistin resistance was transferable by transformation experiments into Escherichia coli DH5-alpha. All strains were Escherichia coli and belonged to different multilocus sequence types (MLSTs), harbored different blaESBL genes, and showed a multiresistance phenotype (Table 1). The diversity of ESBL genes and MLSTs identified among mcr-1-positive isolates suggests that the mcr-1 gene might be carried on different plasmids.
TABLE 1.
Characteristics and resistance profiles of three ESBL-producing and mcr-1-positive strains of Enterobacteriaceaea
| Characteristic or antimicrobial agent | Strain OW3E1 | Strain H226B | Strain 2SK1 |
|---|---|---|---|
| Characteristic | |||
| Sample type | River water sampled in Switzerland | Cha-om imported from Thailand | Basil leaves imported from Vietnam |
| Yr | 2012 | 2014 | 2014 |
| Species | E. coli | E. coli | E. coli |
| Phylogroup | B1 | A | B1 |
| Sequence type | ST359 | ST167 | ST4683 |
| blaESBL type | SHV-12 | CTX-M55 | CTX-M-65 |
| MIC for colistin (μg/ml) | 6 | 6 | 6 |
| Antimicrobial agent | |||
| AM | R | R | R |
| AMC | S | S | R |
| CF | R | R | R |
| CTX | R | R | R |
| CIP | R | R | R |
| GM | S | R | R |
| TE | R | R | R |
| S | R | R | R |
| C | R | S | R |
| K | S | R | S |
| NA | R | R | R |
| SMZ | R | R | R |
| TMP | R | S | R |
The characteristics and resistance profiles of three strains are shown. Whether the strain was resistant (R) or susceptible (S) to the antimicrobial agents is shown. Antimicrobial agent abbreviations: AM, ampicillin; AMC, amoxicillin-clavulanic acid; CF, cephalothin; CTX, cefotaxime; CIP, ciprofloxacin; GM, gentamicin; TE, tetracycline; S, streptomycin; C, chloramphenicol; K, kanamycin; NA, nalidixic acid; SMZ, sulfamethoxazole; TMP, trimethoprim.
The spread of mcr-1-harboring, ESBL-producing members of the Enterobacteriaceae in surface water suggest environmental contamination. Appropriate measures urgently need to be enforced in order to reduce the anthropogenic burden of antibiotic resistance in the environment, such as the judicious use of antibiotics in human and veterinary medicine as well as in agriculture. In addition, improvement of water status is of major concern. New strategies for the treatment of wastewaters, e.g., the use of sand filters or more-stringent chlorine disinfection, need to be taken into consideration to prevent resistant bacteria from being released into the aquatic environment. Moreover, these data show that the international production and trade of fresh vegetables constitute a possible route for the spread of antibiotic-resistant, and particularly colistin-resistant, Enterobacteriaceae.
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