mcr-positive Enterobacteriaceae* infection or colonisation

Clinical criteria
Not relevant for surveillance purposes.

Laboratory criteria
The identification of Enterobacteriaceae* carrying one or more of the mcr family of genes from any specimen, whether a diagnostic (invasive, non-invasive infection or colonisation, also known as carriage) or screening specimen (colonisation, also known as carriage).

mcr positive Enterobacteriaceae are bacteria of the family Enterobacteriaceae confirmed to have one of the family of mcr genes.

Confirmation of the presence of the mcr gene should be by a reliable method, such as a molecular method.

Epidemiological criteria
Not relevant for surveillance purposes.

Case classification
- Possible case: Not applicable
- Probable case: Not applicable
- Confirmed case: Any person meeting the laboratory criteria

An outbreak of mcr-positive Enterobacteriaceae is defined as two or more confirmed cases of mcr-positive Enterobacteriaceae (causing any of invasive or non-invasive infection or colonisation) that are linked epidemiologically in time and place.

Note: Given that chains of mcr-positive Enterobacteriaceae transmission are generally silent at the time of transmission and may involve multiple species of Enterobacteriaceae, assessment of linkage is challenging. If there is any concern, an outbreak control team should be convened to assess whether there is an outbreak and what action is required.

mcr gene: mcr is an acronym for mobilised colistin resistance. The mcr gene encodes for the production of an enzyme which alters the Gram-negative bacterial cell membrane, so that the antimicrobial colistin cannot damage it, rendering colistin inactive and enabling the bacteria to become resistant to colistin. The mcr gene is carried on a plasmid, which is a mobile genetic element readily transferrable between bacteria. 

Enterobacteriaceae: This is a family of bacteria. Due to recent technical changes in bacterial classification the strict meaning of the term Enterobacteriaceae has been narrowed. The term Enterobacterales now encompasses the group of bacterial genera formerly considered to be Enterobacteriaceae. In practical terms, at present, the terms are often used as interchangeable. This group of bacteria are found in the gut of humans and animals, where they make up a large part of the normal bacterial flora. They are shed in very large numbers in faeces and can contaminate the environment. Occasionally, they can cause infection or disease. Enterobacteriaceae include (but are not restricted to) the following genera: Escherichia, Klebsiella, Citrobacter, Enterobacter, Serratia, Proteus, Morganella, Salmonella, Shigella.

Colonisation or carriage = Detection of mcr-positive Enterobacteriaceae from a rectal swab or faeces or from a diagnostic specimen taken in the absence of evidence for active infection (e.g., midstream or catheter specimen of urine, swabs of wounds or chronic skin breaks, sputum etc.)

  • Non-invasive infection = Detection of mcr-positive Enterobacteriaceae from a diagnostic specimen taken in presence of evidence for active infection (e.g., urine, sputum, wound swabs etc.)
  • Invasive infection = Detection of mcr-positive Enterobacteriaceae from a diagnostic specimen taken from a normally sterile site (e.g., blood, cerebrospinal fluid (CSF), peritoneal fluid, pleural fluid etc.)

Note on colonisation/carriage: Because Enterobacteriaceae are normal residents of the human gut, once a person acquires mcr-positive Enterobacteriaceae they are considered to be colonised indefinitely. Some people may cease to be carriers in time, but criteria for definition of clearance have not yet been developed in Ireland and currently there is no clear international consensus on what would constitute clearance.

*Enterobacteriaceae are also known as Enterobacterales

Current as of: 5 February 2019