LETTER
The first report of plasmid-mediated colistin resistance among Enterobacteriaceae in China sounded alarms in the medical community worldwide (1). Since then, Escherichia coli isolates harboring colistin resistance encoded by the mcr-1 gene have been globally reported (2). To our knowledge, we hereby report the first detection of mcr-1 gene in a human E. coli isolate from the Indian subcontinent.
In February 2016, a total of 29 extended-spectrum-beta-lactamase (ESBL)-producing E. coli isolates recovered from patients with wound infections admitted to the Burn & Reconstructive Surgery Centre, Allied Hospital Faisalabad, Pakistan, were tested for plasmid-mediated colistin resistance. Phenotypic resistance to polymyxin B was determined using a Vitek-2 compact system (bioMérieux, Germany) followed by determination of the colistin MIC by broth microdilution in 96-well plate test panel (Micronaut S; Merlin, Bornheim-Hersel, Germany). One colistin-resistant E. coli isolate (X26) with a MIC of 4 μg/ml was collected from a 35-year-old male patient suffering from burn-associated wounds. The presence of the mcr-1 gene was confirmed by TaqMan-based real-time PCR (3). The isolate exhibited multidrug resistance but was susceptible to carbapenems (Table 1). The presence of the blaCTX-M-15 gene in E. coli X26 was confirmed by PCR and sequencing (4).
TABLE 1.
Antibiotic resistance profile of E. coli X26
| Antimicrobial agent | MIC (μg/ml)a |
|---|---|
| Amikacin | 4 |
| Aztreonam | >64 |
| Ciprofloxacin | >4 |
| Colistin | 4 |
| Cefepime | >64 |
| Ceftazidime | >64 |
| Cefotaxime | >64 |
| Fosfomycin | ≤16 |
| Gentamicin | >16 |
| Imipenem | ≤0.25 |
| Meropenem | ≤0.25 |
| Marbofloxacin | >4 |
| Piperacillin-tazobactam | >128 |
| Tetracycline | 4 |
| Tigecycline | ≤5 |
| Tobramycin | >16 |
| Trimethoprim-sulfamethoxazole | >320 |
MICs were determined using a Vitek-2 system (bioMérieux), except for colistin, whose MIC was determined using broth microdilution in a 96-well plate test panel (Micronaut S; Merlin, Bornheim-Hersel, Germany).
The plasmid carriage of E. coli isolate X26 was analyzed using a conventional plasmid profile analysis (5). The conjugative transfer ability of the colistin resistance was investigated by plasmid conjugation experiments using sodium azide-resistant E. coli K-12-J53 as a recipient strain. Transconjugants were selected on LB agar plates supplemented with colistin (4 μg/ml) and sodium azide (100 μg/ml) and subsequently confirmed by PCR for the mcr-1 gene. PCR-based plasmid replicon typing of transconjugants showed the IncI2 type. Pulsed-field gel electrophoresis (PFGE) was performed to determine the genetic relatedness of the E. coli X26 isolate to recently reported mcr-1-carrying ESBL-producing E. coli isolate PK-13 of sequence type 354 (ST354) recovered from a wild migratory bird in Pakistan (6). Multilocus sequence typing (MLST) was carried out using the method developed by Wirth et al. (7). MLST results revealed E. coli X26 to be a new ST.
Colistin has become the last line of defense for the treatment of infections caused by Gram-negative bacteria, in particular, carbapenem-resistant Enterobacteriaceae (CRE). The presence of plasmid-mediated colistin resistance in Indian subcontinent, where there is already burden of CRE, is worrisome, as it threatens the use of last-resort antibiotics. Recently, we reported the presence of the mcr-1 gene in an E. coli isolate from a wild migratory bird in Pakistan (6). The plasmid size (∼63 kb), as well as the replicon type of E. coli isolate X26, corresponds to that described by Liu and colleagues (1) and to our recent findings (6), thus indicating that IncI2 represents a major plasmid type involved in the spread of colistin resistance in our region. The PFGE pattern and sequence type showed no genetic relatedness between E. coli X26 and our recently reported strain (6), suggesting that different clonal types are involved in the dissemination of the mcr-1 gene. In conclusion, the presence of plasmid-mediated colistin resistance in a clinical isolate adds to the worry as this potent gene may spread to other susceptible bacteria, making pan-resistant pathogens.
ACKNOWLEDGMENTS
Mashkoor Mohsin was supported by a postdoctoral fellowship from the Alexander von Humboldt Foundation, Germany.
This study received no specific funding.
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