Colistin-Resistant (Plasmid-Mediated) Gram Negative Bacteria
Reportable by Laboratories and ProvidersAbout this Reportable Disease
Infectious agent: The mcr-1, mcr-2, mcr-3 and mcr-4 genes cause resistance to colistin, a last-resort antibiotic used to treat resistant infections.
Description of illness: The mcr gene is particularly worrisome because it is found on plasmids, small pieces of mobile DNA that carry genetic instructions from one bacterium to another. This means that plasmids carrying resistant mcr genes can make other bacteria become resistant to colistin, including carbapenem-resistant Enterobacteriaceae (CRE). The term mcr stands for “plasmid-mediated colistin resistance,” describing the gene’s ability to avoid the effects of colistin and share this ability with other bacteria. The descriptions -1, -2, -3 and -4 indicate different DNA sequences.
Laboratory Reporting
Reporting requirements apply to all laboratories located within Tennessee, as well as laboratories outside of Tennessee that test residents of Tennessee, including laboratories located within healthcare facilities. Healthcare providers and laboratories in the same healthcare facility both have a duty to report. The type of organisms and analytes laboratories must report to TDH for 2026 are indicated, and there are several ways laboratories can report results to TDH.
- Manually report results to TDH by faxing or mailing a completed PH-1600 form to your local health department, or faxing to the state health office at (615) 741-3857
- Automatically submit results to TDH via electronic laboratory reporting (ELR), which automates the process of sharing data with TDH using interoperability standards.
- See the ELR Onboarding Handbook for details on the onboarding process, checklist, frequently asked questions, business rules, message format, and vocabulary.
- To initiate the ELR onboarding process with TDH, register in the Trading Partner Registration (TPR) system TPR provides documentation for Promoting Interoperability (PI) attestation and milestone letters to document onboarding progress. Contact MU.Health@tn.gov for assistance.
- Submit online via NBS. NBS is TDH's reportable disease system. To request an NBS account for reporting Complete this user survey to request an NBS account for reporting.
- Blood lead levels can be sent via fax ( (615) 741-3857), entered online, or reported using the instructions at this link.
Information about this Reportable Disease for Healthcare Providers
Clinical Summary
The mcr‑1 gene is a plasmid‑mediated resistance factor found in Gram‑negative bacteria (especially Enterobacterales) that confers resistance to Colistin, a “last‑resort” antibiotic for multidrug‑resistant infections.
Because it’s carried on plasmids, the gene can move horizontally between different bacterial strains and species, increasing the potential for broad dissemination of colistin resistance.
The emergence of mcr‑1 raises serious clinical concern because it limits treatment options in already resistant infections and could lead to pan‑resistance when combined with other resistance genes.
Initial U.S. detection occurred in 2016 in an E. coli urinary isolate with no recent travel, marking domestic presence of the gene in clinical settings.
Current response hinges on strong antimicrobial stewardship, infection‑control practices, and enhanced laboratory surveillance to detect mcr‑1 in isolates with elevated colistin minimum inhibitory concentrations, and immediate public‑health notification when found.
Healthcare Provider Reporting
Healthcare reporting requirements apply to all providers located within Tennessee, as well as providers whose patients reside in Tennessee.
Providers must report cases of all diseases and conditions listed through one of these methods:
• Mail or fax a completed PH-1600 form to your local health department or fax to the state health office at (615) 741-3857
• Send automatically via electronic case reporting (eCR). See this TDH webpage for more information on eCR, register at the Trader Partner Registration website, or contact MU.Health@tn.gov for assistance.
• Submit online via NBS. NBS is TDH's reportable disease system. To request an NBS account for reporting Complete this user survey to request an NBS account for reporting.
• Blood lead levels can be sent via fax ( (615) 741-3857), entered online, or reported using the instructions at this link.
Information about this Reportable Disease for the Public
What it is
The mcr‑1 gene is a plasmid-mediated antibiotic resistance gene found in Gram-negative bacteria, especially Escherichia coli and Klebsiella species. It confers resistance to colistin, a last-resort antibiotic used for multidrug-resistant infections. The gene can spread between bacteria through horizontal gene transfer due to its plasmid location. Its presence increases the risk of infections that are very difficult to treat with existing antibiotics. mcr‑1 has been detected globally, including in the U.S., in both human and animal bacterial isolates.
Types
There are no “phases” like in viral infections, but mcr‑1 may be present in different bacterial strains and species.
It can appear in Enterobacterales from humans, livestock, food, and the environment.
Bacteria carrying mcr‑1 may be multidrug-resistant, compounding the challenge of treatment.
Plasmid-mediated resistance allows rapid spread within bacterial populations.
Surveillance focuses on detecting presence in clinical isolates, animals, and food sources.
Signs and Symptoms
mcr‑1 itself is a gene and does not cause symptoms directly; it is part of the bacterial pathogen.
Clinical manifestations depend on the bacterial infection in which it is present (e.g., urinary tract infection, bloodstream infection).
Patients may experience fever, pain, dysuria, sepsis, or other signs typical of Gram-negative bacterial infections.
Severe infections may progress rapidly if bacteria are resistant to multiple antibiotics, including colistin.
In healthcare settings, colonization may be asymptomatic but still facilitates spread to vulnerable patients.
Treatment
Standard colistin therapy may fail in infections caused by mcr‑1–positive bacteria.
Treatment requires antibiotics to which the bacteria remain susceptible, guided by susceptibility testing.
Combination therapy may be considered for multidrug-resistant infections.
Supportive care is critical for severe infections such as sepsis.
Infection control, antibiotic stewardship, and early detection are essential to prevent spread.
This Page Last Updated: March 25, 2026 at 8:53 PM