Six years ago the world woke up to the existence of an emerging antibiotic resistance gene dubbed New Delhi metallo-ß-lactamase-1, or NDM-1 (see NDM-1: A New Acronym To Memorize) which conveyed resistance to Carbapenums - drugs of last resort for treating difficult bacterial infections, including Escherichia coli (E. coli) and Klebsiella pneumoniae.
Of particular concern, this NDM enzyme was carried by a plasmid – a snippet of portable DNA - that can be easily transferred to other types of bacteria (see Study: Adaptation Of Plasmids To New Bacterial Species).
Since then, scattered variants of NDM-1 have begun to emerge around the globe, including NDM-2, NDM-4, NDM-5, NDM-7 and NDM-9.
Last November, in MCR-1: The Return Of The Plasmids,
we looked at the discovery of another resistance gene in China - dubbed mcr-1 - that conveys resistance to Colistin - essentially the last ditch drug available to treat many infections.
Since then we've followed that story closely, including:
Given the rapid genetic diversification of the NDM resistance gene, I suppose we shouldn't be too surprised by this, but today the ECDC Journal Eurosurveillance reports on the first detection of a variant MCR gene, which they have dubbed mcr-2.
Eurosurveillance, Volume 21, Issue 27, 07 July 2016
Identification of a novel plasmid-mediated colistin-resistance gene, mcr-2, in Escherichia coli, Belgium, June 2016
BB Xavier 1 2 3 , C Lammens 1 2 3 , R Ruhal 1 2 3 , S Kumar-Singh 1 3 4 , P Butaye 5 6 7 , H Goossens 1 2 3 , S Malhotra-Kumar 1 2 3
Correspondence: Surbhi Malhotra-Kumar (email@example.com)
Citation style for this article: Xavier BB, Lammens C, Ruhal R, Kumar-Singh S, Butaye P, Goossens H, Malhotra-Kumar S. Identification of a novel plasmid-mediated colistin-resistance gene, mcr-2, in Escherichia coli, Belgium, June 2016. Euro Surveill. 2016;21(27):pii=30280. DOI: http://dx.doi.org/10.2807/1560-7917.ES.2016.21.27.30280
Received:27 June 2016; Accepted:07 July 2016
We identified a novel plasmid-mediated colistin-resistance gene in porcine and bovine colistin-resistant Escherichia coli that did not contain mcr-1. The gene, termed mcr-2, a 1,617 bp phosphoethanolamine transferase harboured on an IncX4 plasmid, has 76.7% nucleotide identity to mcr-1. Prevalence of mcr-2 in porcine colistin-resistant E. coli (11/53) in Belgium was higher than that of mcr-1 (7/53). These data call for an immediate introduction of mcr-2 screening in ongoing molecular epidemiological surveillance of colistin-resistant Gram-negative pathogens.
(Continue . . . )
Bacteria that carry the mcr-1 or mcr-2 genes aren't necessarily untreatable, since they may still be susceptible to other antibiotics, including Carbapenums.
But last March The Lancet's Emergence of the mcr-1 colistin resistance gene in carbapenem-resistant Enterobacteriaceae, reported on two K pneumoniae isolates from China that carried both the mcr-1 gene and the gene for NDM-5 (Carbapenem-resistance), providing it near pan-drug resistance.
While still exceedingly rare, this is the kind of nightmare resistance combination that could someday propel us into a post-antibiotic era, one where even minor infections are no longer treatable.
Making the expansion of mcr-1 and NDM-1 very much much a public health concern.
For more on the implications of growing antibiotic resistance, you may wish to revisit: