The continued evolution of highly resistant Gram-negative bacteria is an ongoing public health threat worldwide.  The combination of increasing resistance to such antimicrobial classes as the carbapenems, with the limited development of new antimicrobials, has resulted in the reintroduction of antimicrobials previously considered last resort. Although historically the polymyxins have had limited use in human infections primarily due to reports of nephrotoxicity, the increasing prevalence of otherwise pan-resistant Gram-negative organisms has led healthcare providers to revisit use of these antimicrobials to treat human infections–in particular, infections due to carbapenemase-producing Gram-negative bacilli.

The polymyxins are polypeptide antibiotics; the class contains five compounds, polymyxin A-E, two of which have been used clinically (polymyxin B and E). Colistin (polymyxin E) disrupts the outer membrane of bacteria by displacing magnesium and calcium, thus resulting in cell death. Acquired resistance to the polymyxins through chromosomal mutation has long been reported in organisms such as Pseudomonas aeruginosa and Acinetobacter baumannii. In November of 2015 Liu et al. published the first case of plasmid-mediated colistin resistance, conferred by the mcr-1 gene; since the initial report, mcr-1 has been reported on all continents from human, animal, and environmental sources (1).  The mcr-1 gene has been found primarily in E. coli but it has also been reported in other members of the Enterobacteriaceae including Klebsiella pneumoniae. Recently, bacteria harboring a second plasmid-mediated colistin resistance mechanism–the mcr-2 gene–have been reported from surveillance studies of E. coli isolates obtained from stool of livestock with diarrhea (2).

Since the initial discovery of mcr-1, four reports of mcr-1-positive isolates from U.S. patients have been published. The first case, described in May 2016 from an E. coli urinary tract isolate from a patient in Pennsylvania, demonstrated a colistin minimum inhibitory concentration (MIC) of 4 µg/ml as measured by broth microdilution (3). The isolate had an extended spectrum beta-lactamase (ESBL) phenotype but tested susceptible to carbapenems, amikacin, nitrofurantoin, and piperacillin-tazobactam. The second isolate was discovered in May of 2015 as part of a retrospective analysis of E. coli and K. pneumoniae isolates with elevated (MICs) to colistin (≥4 µg/ml) through the SENTRY Antimicrobial Surveillance program (4). The 2015 isolate was susceptible to amikacin, ertapenem, imipenem, meropenem, nitrofurantoin, piperacillin-tazobactam, cefepime, gentamicin, tobramycin, fosfomycin, and tigecycline. Additionally, the 2015 isolate was tested for ceftolozane-tazobactam and ceftazidime-avibactam and was susceptible to both. The colistin MIC of this U.S. isolate was 8 µg/mL.  The third case was identified from a urine sample of a U.S. patient whose E. coli isolate harbored both mcr-1 and blaNDM-5  (5). This isolate tested resistant to the carbapenems and fluoroquinolones and tested susceptible to amikacin, aztreonam, gentamicin, nitrofurantoin, tigecycline, and trimethoprim-sulfamethoxazole. The fourth case was a pediatric patient with diarrhea; the mcr-1 gene was identified from non-Shiga toxin–producing E. coli O157 isolated from stool.  This last case appears to be travel associated; there was no identified transmission and the patient was transiently colonized (6). Importantly, none of the cases reported thus far from the US have been pan-resistant.

The SENTRY project showed that of the 21,006 E. coli and K. pneumoniae isolates collected in 2014-2015, only nineteen were positive for mcr-1 a total prevalence of only 0.1%. These data represent a large global collection of clinical isolates and demonstrate that the overall prevalence of mcr-1 is very low.

Laboratory Testing

Challenges in laboratory testing and reporting of colistin are related to specific physiochemical properties of this antimicrobial agent and the relative lack of clinical data to correlate with isolate MICs. The large size and amphipathic (hydrophobic and hydrophilic) nature of the polymyxins hampers performance of disk diffusion and agar gradient diffusion. The polymyxins are also cationic, which increases adsorption of these agents to plastic surfaces. The addition of Polysorbate-80 (P-80) to broth microdilution panels decreases adsorption and leads to more accurate MICs, but it is unclear whether P-80 itself interferes with the assay. Investigations of the most accurate method for testing of the polymyxins are ongoing, but there are currently no commercially available U.S. FDA-cleared test methods for polymyxin B or colistin. However, if laboratories receive requests for colistin testing, we recommend using a validated broth microdilution method and reporting the MIC value with no interpretation or sending to a reference laboratory.   

Due to technical challenges associated with susceptibility testing and the variability of colistin in vivo concentrations in patients with altered renal function, breakpoint-setting organizations are currently reassessing interpretive criteria for the polymyxins. In a recent alert to healthcare facilities concerning mcr-1, the Centers for Disease Control and Prevention (CDC) advises that laboratories which test Enterobacteriaceae for colistin resistance should confirm the presence of the mcr-1 gene in isolates with an MIC of 4 µg/ml or greater. If needed, laboratories may send the isolate to the CDC for testing.  It is not necessary to test isolates that are intrinsically resistant to colistin (e.g. ProteusProvidenciaMorganella, and Serratia). Additionally, the presence of mcr-1 should be confirmed for Enterobacter spp. with elevated colistin MICs of 2 µg/ml or greater. The Clinical and Laboratory Standards Institute (CLSI) has reevaluated colistin interpretive criteria, and upcoming changes will be outlined in the M100 Performance Standards for Antimicrobial Susceptibility Testing to be published in 2017.

Epidemiology and Infection Control

mcr-1 has demonstrated natural interspecies spread within the Enterobacteriaceae.  In addition, in vitro transmission to P. aeruginosa has been shown through experimental models (7)Given the potential consequences of widespread colistin-resistance, it is imperative that every effort be made to control the spread of this resistance mechanism. As such, in their alert to healthcare facilities, the CDC offered several infection control recommendationsIn this document they state that healthcare providers should follow standard and contact precautions for any patient colonized or infected with strains found to harbor mcr-1. If Enterobacteriaceae with mcr-1 are identified from patients, local and state public health authorities should be notified immediately. The CDC also states that all mcr-1-harboring isolates can safely be handled in a biosafety level-2 (BSL-2) laboratory.

At this time, very little is known about the risk factors for infection or colonization with mcr-1-positive isolates. However, point-prevalence studies have identified isolates with mcr-1 in cases of true infection as well as asymptomatic colonization. Of note, a Hong Kong-based point-prevalence study identified mcr-1 positive isolates from the stool of two asymptomatically colonized individuals (8).  Other studies have also demonstrated human fecal carriage of mcr-1 (9, 10). Colonization with mcr-1-positive organisms is not surprising, but it is none-the-less concerning because it suggests that mcr-1 may follow the same rapid pattern of spread that has already been observed for other plasmid-borne mechanisms of resistance, such as the Klebsiella pneumoniae Carbapenemase (KPC) and the New Delhi Metallo Beta-Lactamase (NDM).


The polymyxin antibiotic class serves an important role in treating infections caused by multi-drug resistant Gram-negative organisms. The emergence and spread of plasmid-borne colistin resistance threatens to eliminate this treatment option, and all efforts should be made to control mcr-1 dissemination through detection and proper infection control precautions for patients harboring mcr-1-positive bacteria. Unfortunately, laboratory methods for susceptibility testing of colistin are limited.  Laboratories that choose to test for colistin susceptibility should validate a broth microdilution method and report MIC values only. Laboratories should look to the upcoming CLSI M100 standards for additional guidance on colistin susceptibility testingIn the event that laboratories detect isolates with elevated colistin MICs (4 µg/ml or greater) in the Enterobacteriaceae it is strongly recommended that those strains be forwarded to public health laboratories for testing for the mcr-1 gene.


  1. Skov RL, Monnet DL. 2016. Plasmid-mediated colistin resistance (mcr-1 gene): three months later, the story unfolds. Euro Surveill 21.
  2. Xavier BB, Lammens C, Ruhal R, Kumar-Singh S, Butaye P, Goossens H, Malhotra-Kumar S. 2016. Identification of a novel plasmid-mediated colistin-resistance gene, mcr-2, in Escherichia coli, Belgium, June 2016. Euro Surveill 21.
  3. McGann P, Snesrud E, Maybank R, Corey B, Ong AC, Clifford R, Hinkle M, Whitman T, Lesho E, Schaecher KE. 2016. Erratum for McGann et al., Escherichia coli Harboring mcr-1 and blaCTX-M on a Novel IncF Plasmid: First Report of mcr-1 in the United States. Antimicrob Agents Chemother 60:5107.
  4. Castanheira M, Griffin MA, Deshpande LM, Mendes RE, Jones RN, Flamm RK. 2016. Detection of mcr-1 among Escherichia coli Clinical Isolates Collected Worldwide as Part of the SENTRY Antimicrobial Surveillance Program in 2014 and 2015. Antimicrob Agents Chemother 60:5623-5624.
  5. Mediavilla JR, Patrawalla A, Chen L, Chavda KD, Mathema B, Vinnard C, Dever LL, Kreiswirth BN. 2016. Colistin- and Carbapenem-Resistant Escherichia coli Harboring mcr-1 and blaNDM-5, Causing a Complicated Urinary Tract Infection in a Patient from the United States. MBio 7.
  6. Vasquez AM, Montero N, Laughlin M, Dancy E, Melmed R, Sosa L, Watkins LF, Folster JP, Strockbine N, Moulton-Meissner H, Ansari U, Cartter ML, Walters MS. 2016. Investigation of Escherichia coli Harboring the mcr-1 Resistance Gene - Connecticut, 2016. MMWR Morb Mortal Wkly Rep 65:979-980.
  7. Liu YY, Wang Y, Walsh TR, Yi LX, Zhang R, Spencer J, Doi Y, Tian G, Dong B, Huang X, Yu LF, Gu D, Ren H, Chen X, Lv L, He D, Zhou H, Liang Z, Liu JH, Shen J. 2016. Emergence of plasmid-mediated colistin resistance mechanism MCR-1 in animals and human beings in China: a microbiological and molecular biological study. Lancet Infect Dis 16:161-168.
  8. Wong SC, Tse H, Chen JH, Cheng VC, Ho PL, Yuen KY. 2016. Colistin-Resistant Enterobacteriaceae Carrying the mcr-1 Gene among Patients in Hong Kong. Emerg Infect Dis 22:1667-1669.
  9. Stoesser N, Mathers AJ, Moore CE, Day NP, Crook DW. 2016. Colistin resistance gene mcr-1 and pHNSHP45 plasmid in human isolates of Escherichia coli and Klebsiella pneumoniae. Lancet Infect Dis 16:285-286.
  10. Olaitan AO, Chabou S, Okdah L, Morand S, Rolain JM. 2016. Dissemination of the mcr-1 colistin resistance gene. Lancet Infect Dis 16:147.

Acknowledgements: Brought to you by Paula Revell, Ph.D., D(ABMM), Clinical Consultant, Microbiology, Virology, Molecular Diagnostic Laboratories, Texas Children’s Hospital, Houston, TX; Christopher Doern, Ph.D., D(ABMM), Associate Director of Clinical Microbiology, Virginia Commonwealth University Medical Center, Richmond, VA; Audrey Schuetz, MD, MPH, D(ABMM),  Senior Associate Consultant , Division of Clinical Microbiology, Mayo Clinic, Rochester, MN ; and ASM’s Public and Scientific Affairs Board Committee on Laboratory Practices in Microbiology. October, 2016