Polymyxin Combinations Combat
Escherichia coli
Harboring
mcr-1
and
bla
NDM-5
: Preparation for a Postantibiotic Era
Authors: Bulman Zackery P. , Chen Liang , Walsh Thomas J. , Satlin Michael J. , Qian Yuli , Bulitta Jürgen B. , Peloquin Charles A. , Holden Patricia N. , Nation Roger L. , Li Jian , Kreiswirth Barry N. , Tsuji Brian T. 
Published: July 2017

The rapid increase of carbapenem resistance in Gram-negative bacteria has resurrected the importance of the polymyxin antibiotics. The recent discovery of plasmid-mediated polymyxin resistance (
mcr
-
1
) in carbapenem-resistant
Enterobacteriaceae
serves as an important indicator that the golden era of antibiotics is under serious threat. We assessed the bacterial killing of 15 different FDA-approved antibiotics alone and in combination with polymyxin B in time-killing experiments against
Escherichia coli
MCR1_NJ, the first reported isolate in the United States to coharbor
mcr-1
and a New Delhi metallo-β-lactamase gene (
bla
NDM-5
). The most promising regimens were advanced to the hollow-fiber infection model (HFIM), where human pharmacokinetics for polymyxin B, aztreonam, and amikacin were simulated over 240 h. Exposure to polymyxin B monotherapy was accompanied by MCR1_NJ regrowth but not resistance amplification (polymyxin B MIC from 0 to 240 h [MIC
0h
to MIC
240h
] of 4 mg/liter), whereas amikacin monotherapy caused regrowth and simultaneous resistance amplification (amikacin MIC
0h
of 4 mg/liter versus MIC
240h
of >64 mg/liter). No MCR1_NJ colonies were observed for any of the aztreonam-containing regimens after 72 h. However, HFIM cartridges for both aztreonam monotherapy and the polymyxin B-plus-aztreonam regimen were remarkably turbid, and the presence of long, filamentous MCR1_NJ cells was evident in scanning electron microscopy, suggestive of a nonreplicating persister (NRP) phenotype. In contrast, the 3-drug combination of polymyxin B, aztreonam, and amikacin provided complete eradication (>8-log
10
CFU/ml reduction) with suppression of resistance and prevention of NRP formation. This is the first comprehensive pharmacokinetic/pharmacodynamic study to evaluate triple-drug combinations for polymyxin- and carbapenem-resistant
E. coli
coproducing MCR-1 and NDM-5 and will aid in the preparation for a so-called “postantibiotic” era.

IMPORTANCE
A global health crisis may be on the horizon, as the golden era of antibiotics is under serious threat. We recently reported the first case in the United States of a highly resistant,
Escherichia coli
so-called “superbug” (MCR1_NJ), coharboring two of the most worrying antibiotic resistance genes, encoding mobile colistin resistance (
mcr
-
1
) and a New Delhi metallo-β-lactamase (
bla
NDM-5
). Worryingly, the medical community is vulnerable to this emerging bacterial threat because optimal treatment strategies are undefined. Here, we report the activity of an optimized combination using simulated human doses of commercially available antibiotics against MCR1_NJ. A unique triple combination involving a cocktail of polymyxin B, aztreonam, and amikacin eradicated the MCR-1- and NDM-5-producing
E. coli
. Each antimicrobial agent administered as monotherapy or in double combinations failed to eradicate MCR1_NJ at a high inoculum. To our knowledge, this is the first study to propose 3-drug therapeutic solutions against superbugs coharboring
mcr-1
and
bla
NDM
, seeking to prepare clinicians for future occurrences of these pathogens.