LETTER
The gene mcr-1 was reported as the first plasmid-mediated colistin resistance gene in Escherichia coli isolates from food animals, food, and patients in China (1). Since then, detection of mcr-1-positive strains has been reported in Enterobacteriaceae worldwide (2–4). The emergence of mcr-1 has the potential to pose a major therapeutic challenge in the treatment of infections caused by Enterobacteriaceae. Here, we report the identification of mcr-1 in colistin-resistant Enterobacter aerogenes and Enterobacter cloacae isolates.
E. aerogenes strain GB68 was recovered from the vaginal secretion of a 37-year-old pregnant female in Guangzhou, China, in August 2014. She was admitted to a hospital for cervical cerclage. Ten days after the surgery, she was diagnosed with E. coli vaginal infection by vaginal culture. In addition, she had mycoplasma coinfection in the reproductive tract. The vaginal infection resolved after treatment with azithromycin and amoxicillin-clavulanic acid. E. cloacae strain GB38 was isolated from the urine of a 70-year-old male with urinary tract infection, who was admitted to the same hospital in September 2014.
The species were identified using the API 20E system. Susceptibility to various antimicrobial agents was tested by the agar dilution method (5). E. aerogenes GB68 was resistant to polymyxins, including colistin and polymyxin B with a MIC of 16 μg/ml each, cephalosporin, and ciprofloxacin. E. cloacae GB38 was resistant to all agents tested, including colistin and polymyxin B with a MIC of >32 μg/ml each, carbapenems, and tigecycline (Table 1). PCR analyses were performed to identify various resistance genes (6). The mcr-1 gene was detected, as well as genes blaCTX-M-15, blaTEM-1, qnrS, and aac(6′)-Ib-cr in both isolates and armA in E. cloacae GB38 (Table 1).
TABLE 1.
Characteristics of mcr-1-harboring Enterobacter aerogenes and Enterobacter cloacae isolates
| Characteristic | Enterobacter aerogenes GB68 | E. coli C600 (transconjugant of E. aerogenes GB68) | Enterobacter cloacae GB38 | E. coli C600 (transconjugant of E. cloacae GB38) | E. coli C600 |
|---|---|---|---|---|---|
| Isolation date | August 2014 | September 2014 | |||
| Inpatient or outpatient | Inpatient | Inpatient | |||
| Isolation site | Vaginal secretion | Urine | |||
| Resistance gene(s) | mcr-1, blaCTX-M-15, blaTEM-1, qnrS, aac(6′)-Ib-cr | mcr-1, blaCTX-M-15, blaTEM-1 | mcr-1, blaCTX-M-15, blaTEM-1, armA, qnrS, aac(6′)-Ib-cr | mcr-1 | |
| MIC (μg/ml) | |||||
| Colistin | 16 | 16 | >32 | 16 | <0.25 |
| Polymyxin B | 16 | 16 | >32 | 16 | <0.25 |
| Tigecycline | 2 | 0.25 | 4 | 0.5 | 0.5 |
| Ampicillin | >256 | >256 | >256 | 16 | 16 |
| Amoxicillin-clavulanic acid | 64 | 4 | 256 | <2 | <2 |
| Cefotaxime | 256 | >256 | >256 | <1 | <1 |
| Ceftazidime | 32 | 32 | >256 | 2 | <1 |
| Cefepime | 16 | 16 | >256 | <0.5 | <0.5 |
| Gentamicin | 64 | 4 | >256 | 4 | <1 |
| Amikacin | 4 | 4 | >256 | 4 | <2 |
| Ertapenem | <0.25 | <0.25 | >16 | <0.25 | <0.25 |
| Imipenem | <0.25 | <0.25 | >16 | <0.25 | <0.25 |
| Meropenem | <0.25 | <0.25 | >16 | <0.25 | <0.25 |
| Fosfomycin | <16 | <16 | 128 | <16 | <16 |
| Nitrofurantoin | 64 | 32 | 64 | 32 | <16 |
| Ciprofloxacin | 16 | 0.016 | 64 | 0.032 | <0.03 |
These two isolates were subjected to conjugation assays to assess the transferability of the mcr-1-harboring plasmid. The plasmid replicon type was determined by PCR (7). Colistin resistance in both isolates was successfully transferred to recipient strain E. coli C600. Additional PCR and sequencing analysis for resistance genes showed that transconjugants of E. aerogenes GB68 were positive for blaCTX-M-15 and blaTEM-1 as well. The results of S1 nuclease digestion followed by pulsed-field gel electrophoresis (S1-PFGE) and hybridizations for the transconjugants indicated that mcr-1 was located on an ∼65-kDa IncFI plasmid together with the two β-lactamase genes in E. aerogenes GB68 and on an ∼70-kDa IncFI plasmid in E. cloacae GB38.
Our study indicates that the mcr-1 gene can spread in Enterobacteriaceae species other than those initially reported, which include E. coli, Klebsiella pneumoniae, and Salmonella enterica. Broader surveillance of Enterobacteriaceae is warranted in areas with high prevalence of mcr-1 in these species.
The patients gave written consent for publication. This study was approved by the ethics committee at Zhongshan School of Medicine, Sun Yat-sen University, Guangzhou, China.
ACKNOWLEDGMENTS
We sincerely thank the patients for giving written consent for publication.
We declare no competing interests.
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