LETTER
Acquired 16S rRNA methyltransferase (16S-RMTase) has emerged as a mechanism of high-level aminoglycoside resistance among Gram-negative pathogens worldwide (1). RmtF is a 16S RMTase which was identified in a single Klebsiella pneumoniae isolate on La Réunion Island in 2011 (2). The isolate also produced NDM-1 and was resistant to all β-lactams and aminoglycosides tested. Subsequently, rmtF was identified in 34 of 140 aminoglycoside-resistant Enterobacteriaceae isolates collected in India (3). Twenty of these 34 isolates also carried blaNDM-1. Here, we report the identification of a K. pneumoniae strain coproducing NDM-7 and RmtF and an Escherichia coli strain producing NDM-7 from a patient who was admitted to a hospital in Minnesota.
In 2012, K. pneumoniae was isolated from the intra-abdominal abscess of a 69-year-old man who had been referred from India to a tertiary hospital in Minnesota. He had presented with abdominal pain and jaundice and had undergone endoscopic retrograde pancreatic cholangiography with biliary stenting twice while in India. In Minnesota, he underwent laparoscopic pancreaticoduodenectomy, and a diagnosis of adenocarcinoma of the pancreatic head was made. Six days postoperatively, he developed a complicated intra-abdominal infection for which he underwent percutaneous drainage of a fluid collection. The culture grew an E. coli isolate (EC1) that was resistant to ceftriaxone and susceptible to carbapenems. One week into treatment with piperacillin-tazobactam, another drain was put in place, and a repeat culture grew carbapenem-resistant K. pneumoniae (KP1). Six weeks later, the drains were adjusted, and repeat cultures grew carbapenem-resistant K. pneumoniae (KP2) and carbapenem-resistant E. coli (EC2). He initially received colistin, vancomycin, metronidazole, and caspofungin after KP1 was isolated, but treatment was limited by renal toxicity; therapy was completed with vancomycin, tigecycline, meropenem, and caspofungin. He ultimately made a complete recovery, and the abdominal drains were removed.
KP2 and EC2 were available for further analysis; they were resistant to all β-lactams tested, including carbapenems tested by broth microdilution (Sensititre GNX2F; Trek Diagnostics, Oakwood Village, OH) and interpreted by Clinical and Laboratory Standards Institute guidelines (Table 1) (4). KP2 was additionally resistant to all aminoglycosides tested. The sequence types (STs) of KP2 and EC2 were ST147 and ST617, respectively (5, 6). Both isolates were positive for blaNDM-7 and blaCTX-M-15 by PCR and sequencing of the entire genes (7). NDM-7 is a variant of NDM-1 with a two-amino-acid substitution that appears to have improved hydrolytic efficiency of carbapenems compared with NDM-1 (8). KP2 was also positive for rmtF by PCR and sequencing of an internal fragment of the gene. The plasmids of KP2 and EC2 were extracted by the standard alkaline lysis method and used to transform competent E. coli TOP10 cells to obtain transformants harboring plasmids with these genes using lysogenic agar plates containing ampicillin or gentamicin (9). As a result, KP2 yielded three different transformants demonstrating resistance to cephalosporins, carbapenems, and aminoglycosides, each with blaCTX-M-15, blaNDM-7, or rmtF, whereas EC2 yielded a transformant with blaNDM-7 (Table 1). The replicon could be determined only for the blaCTX-M-15-carrying plasmid of KP2, which was assigned to IncR (10). Nonetheless, the identification of blaNDM-7, a relatively infrequent blaNDM allele (11), in both KP2 and EC2 suggested that the blaNDM-7-carrying plasmids in these two strains may share an origin. The emergence of K. pneumoniae coproducing NDM metallo-β-lactamase and RmtF 16S-RMTase in the United States highlights the continuous threat of global dissemination of highly resistant enteric organisms by means of travel.
TABLE 1.
MICs of K. pneumoniae and E. coli isolates, their transformants, and control strains
| Drug | MIC (μg/ml) |
||||||
|---|---|---|---|---|---|---|---|
| K. pneumoniae KP2 |
E. coli TOP10 transformants for KP2 |
E. coli EC2 | E. coli TOP10 transformant for EC2 (pNDM-7-EC2) | E. coli TOP10 | |||
| pCTX-M-15 | pNDM-7-KP2 | pRmtF | |||||
| Imipenem | >8 | ≤1 | 2 | ≤1 | >8 | 2 | ≤1 |
| Ertapenem | >4 | ≤0.25 | 4 | ≤0.25 | 4 | 4 | ≤0.25 |
| Doripenem | >2 | ≤0.12 | >2 | ≤0.12 | >2 | >2 | ≤0.12 |
| Meropenem | >8 | ≤1 | 4 | ≤1 | 2 | 4 | ≤1 |
| Ceftazidime | >16 | >16 | >16 | ≤1 | >16 | >16 | ≤1 |
| Cefotaxime | >32 | >32 | >32 | ≤1 | >32 | >32 | ≤1 |
| Ticarcillin-clavulanate | >128/2 | 128/2 | >128/2 | ≤16/2 | >128/2 | >128/2 | ≤16/2 |
| Piperacillin-tazobactam | >64/4 | ≤8/4 | >64/4 | ≤8/4 | >64/4 | >64/4 | ≤8/4 |
| Cefepime | >16 | >16 | 16 | ≤2 | >16 | 16 | ≤2 |
| Aztreonam | >16 | >16 | ≤2 | ≤2 | 16 | ≤2 | ≤2 |
| Gentamicin | >8 | ≤1 | ≤1 | >8 | >8 | ≤1 | ≤1 |
| Tobramycin | >8 | ≤1 | ≤1 | >8 | 8 | ≤1 | ≤1 |
| Amikacin | >32 | 8 | ≤4 | >32 | ≤4 | ≤4 | ≤4 |
| Ciprofloxacin | >2 | ≤0.25 | ≤0.25 | ≤0.25 | >2 | ≤0.25 | ≤0.25 |
| Levofloxacin | 8 | ≤1 | ≤1 | ≤1 | >8 | ≤1 | ≤1 |
| Doxycycline | 8 | ≤2 | ≤2 | ≤2 | >8 | ≤2 | ≤2 |
| Minocycline | 16 | ≤2 | ≤2 | ≤2 | 8 | ≤2 | ≤2 |
| Tigecycline | 1 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 |
| Trimethoprim-sulfamethoxazole | >4/76 | >4/76 | ≤/0.5/9.5 | ≤0.5/9.5 | >4/76 | ≤0.5/9.5 | ≤0.5/9.5 |
| Colistin | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 |
| Polymyxin B | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 | ≤0.25 |
ACKNOWLEDGMENTS
The work of Y.D. was supported by research grants from the National Institutes of Health (R21AI107302 and R01AI104895).
Footnotes
Published ahead of print 20 August 2014
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