Antimicrobial susceptibility revealed that EC1002 had a high-level resistance to carbapenems, cephalosporins, and aminoglycosides, but was susceptible to quinolones and tigecycline. Meanwhile, EC2474 was resistant to all antibiotics tested, except tigecycline (Table 1). Clonal diversity was assessed by pulsed-field gel electrophoresis (PFGE), which demonstrated that both isolates were clonally unrelated (Supplementary Data). Multilocus sequence typing indicated that EC1002 belonged to sequence type (ST) 2373 and EC2474 was ST131. Polymerase chain reaction and sequencing identified both isolates encoding the MCR-1, blaNDM-1, and blaCTX-M-15 genes. EC1002 also positive for aac(6′)-Ib and blaTEM-1, and EC2474 was positive for blaCTX-M-55 and rmtB (Table 1).
Characteristics of Colistin-Resistant and Carbapenem-Resistant Escherichia coli Strains and Their Transconjugants
| Species . | EC1002 . | Transconjugants . | EC2474 . | Transconjugants . | J53 . | ||
|---|---|---|---|---|---|---|---|
| MCR-1 . | NDM-1 . | MCR-1 . | NDM-1 . | ||||
| Resistance genes | MCR-1 blaNDM-1 blaCTX-M-14 aac(6′)-Ib blaTEM-1 | MCR-1 | blaNDM-1 aac(6′)-Ib blaCTX-M-15 blaTEM-1 | MCR-1 blaNDM-1 blaCTX-M-15 blaCTX-M-55 rmtB | MCR-1 blaCTX-M-15 | blaNDM-1 blaCTX-M-55 | NA |
| Sequence type | ST2373 | NA | NA | ST131 | NA | NA | NA |
| Plasmid(s) size | ∼80 kb ∼105 kb ∼125 kb ∼210 kb | ∼80 kb | ∼125 kb | ∼110 kb ∼130 kb ∼250 kb | ∼250 kb | ∼110 kb | NA |
| MIC of antimicrobial drug, μg/mLa | |||||||
| Ampicillin | >128 | 8 | >128 | >128 | >128 | >128 | 2 |
| Piperacillin-tazobactam | 64 | 4 | 64 | >128 | 64 | 64 | 1 |
| Cefazolin | >128 | 2 | >128 | >128 | >128 | >128 | 1 |
| Cefoxitin | >128 | 2 | >128 | >128 | >128 | >128 | 1 |
| Ceftriaxone | >128 | 1 | >128 | >128 | >128 | >128 | 0.064 |
| Cefepime | >128 | 1 | 32 | >128 | >128 | 32 | 0.064 |
| Aztreonam | 64 | 1 | 16 | >128 | 16 | 16 | 0.125 |
| Amikacin | 64 | 1 | 64 | 64 | 2 | 2 | 0.25 |
| Gentamicin | 16 | 1 | 16 | 16 | 16 | 1 | 0.5 |
| Tobramycin | 16 | 1 | 16 | 16 | 16 | 1 | 0.5 |
| Ciprofloxacin | 0.5 | 0.25 | 0.25 | 8 | 0.25 | 0.25 | 0.032 |
| Levofloxacin | 1 | 0.25 | 0.25 | 8 | 1 | 0.25 | 0.032 |
| Ertapenem | 16 | 0.5 | 8 | 16 | 4 | 8 | 0.064 |
| Imipenem | 16 | 1 | 16 | 16 | 4 | 16 | 0.125 |
| Meropenem | 16 | 0.5 | 16 | 8 | 4 | 8 | 0.064 |
| Tigecycline | 1 | 0.5 | 0.5 | 1 | 0.25 | 0.25 | 0.125 |
| Colistin | 4 | 4 | 0.5 | 4 | 4 | 0.5 | 0.5 |
| Polymyxin B | 4 | 2 | 0.5 | 4 | 4 | 0.5 | 0.25 |
| Species . | EC1002 . | Transconjugants . | EC2474 . | Transconjugants . | J53 . | ||
|---|---|---|---|---|---|---|---|
| MCR-1 . | NDM-1 . | MCR-1 . | NDM-1 . | ||||
| Resistance genes | MCR-1 blaNDM-1 blaCTX-M-14 aac(6′)-Ib blaTEM-1 | MCR-1 | blaNDM-1 aac(6′)-Ib blaCTX-M-15 blaTEM-1 | MCR-1 blaNDM-1 blaCTX-M-15 blaCTX-M-55 rmtB | MCR-1 blaCTX-M-15 | blaNDM-1 blaCTX-M-55 | NA |
| Sequence type | ST2373 | NA | NA | ST131 | NA | NA | NA |
| Plasmid(s) size | ∼80 kb ∼105 kb ∼125 kb ∼210 kb | ∼80 kb | ∼125 kb | ∼110 kb ∼130 kb ∼250 kb | ∼250 kb | ∼110 kb | NA |
| MIC of antimicrobial drug, μg/mLa | |||||||
| Ampicillin | >128 | 8 | >128 | >128 | >128 | >128 | 2 |
| Piperacillin-tazobactam | 64 | 4 | 64 | >128 | 64 | 64 | 1 |
| Cefazolin | >128 | 2 | >128 | >128 | >128 | >128 | 1 |
| Cefoxitin | >128 | 2 | >128 | >128 | >128 | >128 | 1 |
| Ceftriaxone | >128 | 1 | >128 | >128 | >128 | >128 | 0.064 |
| Cefepime | >128 | 1 | 32 | >128 | >128 | 32 | 0.064 |
| Aztreonam | 64 | 1 | 16 | >128 | 16 | 16 | 0.125 |
| Amikacin | 64 | 1 | 64 | 64 | 2 | 2 | 0.25 |
| Gentamicin | 16 | 1 | 16 | 16 | 16 | 1 | 0.5 |
| Tobramycin | 16 | 1 | 16 | 16 | 16 | 1 | 0.5 |
| Ciprofloxacin | 0.5 | 0.25 | 0.25 | 8 | 0.25 | 0.25 | 0.032 |
| Levofloxacin | 1 | 0.25 | 0.25 | 8 | 1 | 0.25 | 0.032 |
| Ertapenem | 16 | 0.5 | 8 | 16 | 4 | 8 | 0.064 |
| Imipenem | 16 | 1 | 16 | 16 | 4 | 16 | 0.125 |
| Meropenem | 16 | 0.5 | 16 | 8 | 4 | 8 | 0.064 |
| Tigecycline | 1 | 0.5 | 0.5 | 1 | 0.25 | 0.25 | 0.125 |
| Colistin | 4 | 4 | 0.5 | 4 | 4 | 0.5 | 0.5 |
| Polymyxin B | 4 | 2 | 0.5 | 4 | 4 | 0.5 | 0.25 |
Abbreviations: MCR-1, plasmid-mediated colistin resistance; MIC, minimum inhibitory concentration; NA, not applicable; NDM-1, New Delhi metallo-β lactamase 1.
a In vitro antimicrobial susceptibility was performed by broth microdilution method, and the MICs were interpreted according to Clinical and Laboratory Standards Institute criteria, except for tigecycline, colistin, and polymyxin B, for which interpretations were performed according to the European Committee on Antimicrobial Susceptibility Testing guidelines.
Characteristics of Colistin-Resistant and Carbapenem-Resistant Escherichia coli Strains and Their Transconjugants
| Species . | EC1002 . | Transconjugants . | EC2474 . | Transconjugants . | J53 . | ||
|---|---|---|---|---|---|---|---|
| MCR-1 . | NDM-1 . | MCR-1 . | NDM-1 . | ||||
| Resistance genes | MCR-1 blaNDM-1 blaCTX-M-14 aac(6′)-Ib blaTEM-1 | MCR-1 | blaNDM-1 aac(6′)-Ib blaCTX-M-15 blaTEM-1 | MCR-1 blaNDM-1 blaCTX-M-15 blaCTX-M-55 rmtB | MCR-1 blaCTX-M-15 | blaNDM-1 blaCTX-M-55 | NA |
| Sequence type | ST2373 | NA | NA | ST131 | NA | NA | NA |
| Plasmid(s) size | ∼80 kb ∼105 kb ∼125 kb ∼210 kb | ∼80 kb | ∼125 kb | ∼110 kb ∼130 kb ∼250 kb | ∼250 kb | ∼110 kb | NA |
| MIC of antimicrobial drug, μg/mLa | |||||||
| Ampicillin | >128 | 8 | >128 | >128 | >128 | >128 | 2 |
| Piperacillin-tazobactam | 64 | 4 | 64 | >128 | 64 | 64 | 1 |
| Cefazolin | >128 | 2 | >128 | >128 | >128 | >128 | 1 |
| Cefoxitin | >128 | 2 | >128 | >128 | >128 | >128 | 1 |
| Ceftriaxone | >128 | 1 | >128 | >128 | >128 | >128 | 0.064 |
| Cefepime | >128 | 1 | 32 | >128 | >128 | 32 | 0.064 |
| Aztreonam | 64 | 1 | 16 | >128 | 16 | 16 | 0.125 |
| Amikacin | 64 | 1 | 64 | 64 | 2 | 2 | 0.25 |
| Gentamicin | 16 | 1 | 16 | 16 | 16 | 1 | 0.5 |
| Tobramycin | 16 | 1 | 16 | 16 | 16 | 1 | 0.5 |
| Ciprofloxacin | 0.5 | 0.25 | 0.25 | 8 | 0.25 | 0.25 | 0.032 |
| Levofloxacin | 1 | 0.25 | 0.25 | 8 | 1 | 0.25 | 0.032 |
| Ertapenem | 16 | 0.5 | 8 | 16 | 4 | 8 | 0.064 |
| Imipenem | 16 | 1 | 16 | 16 | 4 | 16 | 0.125 |
| Meropenem | 16 | 0.5 | 16 | 8 | 4 | 8 | 0.064 |
| Tigecycline | 1 | 0.5 | 0.5 | 1 | 0.25 | 0.25 | 0.125 |
| Colistin | 4 | 4 | 0.5 | 4 | 4 | 0.5 | 0.5 |
| Polymyxin B | 4 | 2 | 0.5 | 4 | 4 | 0.5 | 0.25 |
| Species . | EC1002 . | Transconjugants . | EC2474 . | Transconjugants . | J53 . | ||
|---|---|---|---|---|---|---|---|
| MCR-1 . | NDM-1 . | MCR-1 . | NDM-1 . | ||||
| Resistance genes | MCR-1 blaNDM-1 blaCTX-M-14 aac(6′)-Ib blaTEM-1 | MCR-1 | blaNDM-1 aac(6′)-Ib blaCTX-M-15 blaTEM-1 | MCR-1 blaNDM-1 blaCTX-M-15 blaCTX-M-55 rmtB | MCR-1 blaCTX-M-15 | blaNDM-1 blaCTX-M-55 | NA |
| Sequence type | ST2373 | NA | NA | ST131 | NA | NA | NA |
| Plasmid(s) size | ∼80 kb ∼105 kb ∼125 kb ∼210 kb | ∼80 kb | ∼125 kb | ∼110 kb ∼130 kb ∼250 kb | ∼250 kb | ∼110 kb | NA |
| MIC of antimicrobial drug, μg/mLa | |||||||
| Ampicillin | >128 | 8 | >128 | >128 | >128 | >128 | 2 |
| Piperacillin-tazobactam | 64 | 4 | 64 | >128 | 64 | 64 | 1 |
| Cefazolin | >128 | 2 | >128 | >128 | >128 | >128 | 1 |
| Cefoxitin | >128 | 2 | >128 | >128 | >128 | >128 | 1 |
| Ceftriaxone | >128 | 1 | >128 | >128 | >128 | >128 | 0.064 |
| Cefepime | >128 | 1 | 32 | >128 | >128 | 32 | 0.064 |
| Aztreonam | 64 | 1 | 16 | >128 | 16 | 16 | 0.125 |
| Amikacin | 64 | 1 | 64 | 64 | 2 | 2 | 0.25 |
| Gentamicin | 16 | 1 | 16 | 16 | 16 | 1 | 0.5 |
| Tobramycin | 16 | 1 | 16 | 16 | 16 | 1 | 0.5 |
| Ciprofloxacin | 0.5 | 0.25 | 0.25 | 8 | 0.25 | 0.25 | 0.032 |
| Levofloxacin | 1 | 0.25 | 0.25 | 8 | 1 | 0.25 | 0.032 |
| Ertapenem | 16 | 0.5 | 8 | 16 | 4 | 8 | 0.064 |
| Imipenem | 16 | 1 | 16 | 16 | 4 | 16 | 0.125 |
| Meropenem | 16 | 0.5 | 16 | 8 | 4 | 8 | 0.064 |
| Tigecycline | 1 | 0.5 | 0.5 | 1 | 0.25 | 0.25 | 0.125 |
| Colistin | 4 | 4 | 0.5 | 4 | 4 | 0.5 | 0.5 |
| Polymyxin B | 4 | 2 | 0.5 | 4 | 4 | 0.5 | 0.25 |
Abbreviations: MCR-1, plasmid-mediated colistin resistance; MIC, minimum inhibitory concentration; NA, not applicable; NDM-1, New Delhi metallo-β lactamase 1.
a In vitro antimicrobial susceptibility was performed by broth microdilution method, and the MICs were interpreted according to Clinical and Laboratory Standards Institute criteria, except for tigecycline, colistin, and polymyxin B, for which interpretations were performed according to the European Committee on Antimicrobial Susceptibility Testing guidelines.
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