Frequently asked questions about colistin resistance

What does resistance to colistin antibiotic mean?

Colistin resistance means that bacteria are resistant to colistin antibiotic. Colistin resistance is a result of a genetic change in either the chromosome of the bacteria, that is the more permanent part of its genome, or in the plasmids, which are an additional DNA fragment within the bacteria cell.

What is plasmid-mediated, transferable colistin resistance, and why is it considered more significant than the resistance genes in the bacterial chromosome?

Transferable colistin resistance refers to the colistin resistance genes found in the plasmids of bacteria. Plasmids can be transferred between different bacteria, which means that the resistance gene may also be transferred from one bacterial species to another. This makes it possible for the resistance determinants to be transferred from a bacterial species that is harmless to humans into a pathogenic one. In contrast, the genes in the chromosome are only inherited by the daughter cells when bacteria multiply.

When was transferable colistin resistance discovered for the first time?

In November 2015, the respected medical journal Lancet published an article reporting the first discovery of the plasmid-mediated colistin resistance gene mcr-1 in China (source: Liu et al., 2015). The gene in question was found in E. coli bacteria isolated from both humans and animals. This was a new, significant discovery of a mechanism that allows colistin resistance to be transferred from one bacterium to another.

Where else has transferable colistin resistance been found?

After the Chinese article, the resistance gene mcr-1 has been found in animals and food around the world (source: Shen et al., 2016). In Europe for example, the mcr-1-gene has been found in E. coli from livestock in the Netherlands, Belgium, France, Germany, and Italy. In Finland the gene has be found in an ESBL carrying E. coli bacterium in 2017 from a dog that was brought from abroad.

Are colistin-resistant bacteria also resistant to other antibiotics?

Colistin-resistant bacteria may also be resistant to other antibiotics. Several publications have reported finding ESBL and mcr-genes among others in the same strains (source: Skov & Monnet, 2016). However, colistin resistance in bacteria can also occur without resistance to other antibiotics.

How does an animal get bacteria with colistin resistance?

The resistance may come from the medication of the animal, the food they eat, or other animals.

Are the resistant bacteria in animals a threat to human health?

Transferable colistin resistance becoming increasingly common is considered to be a very great concern globally, because colistin is a so-called reserve antibiotic, meaning that it is used to treat human infections when other antibiotics are no longer effective. The most serious consequence of colistin resistance discovered in animals is the resistance being transferred from animals to humans. For example, the high incidence of ESBL in rescue dogs imported from abroad, combined with the discovery of colistin resistance, poses a clear threat to the health of the humans and animals that come into contact with these animals.

How is colistin resistance studied in Finland?

In Finland, the antimicrobial susceptibility of bacteria isolated from food-producing animals and food is monitored in the FINRES-Vet programme. Resistance to colistin in indicator E. coli from food-producing animals has been studied since 2010 and no resistance to colistin has been found. Colistin resistance has also been studied in salmonella since early 2014. Some salmonella serotypes, such as S. Enteritidis, are naturally slightly less susceptible to colistin, but the exact mechanism is unknown. Such strains of salmonella have also been isolated in Finland, but the mcr-1 gene has not been detected in them.

How is colistin used?

Colistin is a critically important antibiotic in human medicine, and it is often the last option for treating severe infections caused by multidrug-resistant, gram-negative bacteria.

Is colistin used in Finland to treat animals?

Colistin is not used for animals in Finland, but it is used in veterinary medicine in many other countries.

How long does colistin-resistant bacteria remain in the intestines of dogs?

There is no existing research on the subject at the moment.

Sources:

Liu, Y.Y., Wang, Y., Walsh, T.R. et al. : 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. 2015. 

Shen, Z., Wang, Y., Shen, Y., Shen, J. & Wu, C. (2016): Early emergence of mcr-1 in Escherichia coli from food-producing animals. Lancet Infect Dis, 16:161 - 168.

Skov, R.L. & Monnet D.L. (2016): Plasmid-mediated colistin resistance (mcr-1 gene): three months later, the story unfolds. Euro surveillance 2016. 

Wang, R., van Dorp, L., Shaw, LP., Bradley, P., Wang, Q., Wang, X., Jin, L., Zhang, Q., Liu, Y., Rieux, A., Dorai-Schneiders, T., Weinert, LA., Iqbal, Z., Didelot, X., Wang, H. & Balloux, F. (2018), The global distribution and spread of the mobilized colistin resistance gene mcr-1. Nat Commun. 9(1): 1179.