Thursday November 19 2015
The resistant gene may have originated in pigs
"The last line of antibiotic defence against some serious infections is under threat," The Guardian reports, after researchers found that E.coli bacteria from food products in China has developed resistance to colistin – a polymixin antibiotic.
This antibiotic is, in a sense, a weapon of last resort in the antibiotics armoury, and is sometimes used to treat serious infections that have become resistant to other strong antibiotics.
The researchers found that colistin resistance was caused by a gene called MCR-1. This gene was found on a piece of bacterial DNA that can be transferred between bacteria.
They took a number of samples from animals in abattoirs, and raw meat from open markets and supermarkets in China to identify how frequently the MCR-1 gene is found in bacteria.
The study found the MCR-1 gene in E. coli collected from 15% of raw meat samples and 21% of animals tested from 2011-14. The gene was also found in E. coli from 1% of hospital inpatients in China.
As this study was conducted in China, we do not know whether the situation is the same in the UK. However, antibiotic resistance is a global concern that could potentially advance more quickly than new antibiotics can be developed.
An editorial accompanying the study recommends that the use of polymixin should be restricted in agriculture, as we could end up with a situation where doctors are forced to say, "Sorry, there is nothing I can do to cure your infection".
Where did the story come from?
The study was carried out by researchers from a number of institutions, including the South China Agricultural University and the China Agricultural University.
It was funded by the Chinese Ministry of Science and Technology, and the Chinese National Natural Science Foundation.
The study was published in the peer-reviewed medical journal, The Lancet Infectious Diseases.
This research has been reported on widely and accurately by the UK media, but we do not know if the findings and level of risk apply to the UK population.
What kind of research was this?
This laboratory study aimed to investigate the cause of resistance to one of the strongest "last resort" group of antibiotics.
During routine surveillance of E.coli bacteria isolated from livestock in China, researchers observed an increase in resistance to the antibiotic colistin.
Colistin is a very strong polymixin antibiotic. It is given directly into the vein (intravenously) to treat serious infections – such as lung or urinary tract infections – where other strong injected antibiotics are not effective, mostly because bacteria have developed resistance to them.
The finding that bacteria seem to be developing resistance to colistin is therefore a major concern. The researchers wanted to find out how the bacteria developed this resistance.
This type of study is useful to investigate how the antibiotic resistance developed and how it can be transferred between bacteria cells. It also gives some indication of how common these resistant bacteria are in China. Whether there has been a wider spread of the resistance needs to be investigated further.
What did the research involve?
This study aimed to investigate the cause of a major increase in E. coli resistance to the class of antibiotics known as polymyxins seen in livestock in China.
The researchers selected one E. coli strain (SHP45) for investigation, as this strain had shown colistin/polymixin resistance. The researchers identified that the cause of the resistance seemed to be a gene called MCR-1, which was found on a piece of DNA called a plasmid.
Bacteria are capable of transferring plasmids to other bacteria, which could help antibiotic resistance spread. The researchers therefore investigated the possibility that these bacteria could transfer plasmid-mediated colistin resistance. Pig strains of colistin-resistant E. coli and another type of bacteria called K. pneumoniae were chosen for this investigation.
To test how widespread this resistant gene was, samples of bacteria called clinical isolates were collected from inpatients at two hospitals in China and screened for the presence of the MCR-1 gene.
Further samples were collected from pig abattoirs and raw meat from 30 open markets and 27 supermarkets located in seven regions of Guangzhou from 2011-14. One isolate was collected from each animal and retail meat sample, and was then screened to look at the spread of MCR-1 in animals and food.
Mice were used to investigate whether the colistin-resistant E. coli collected from an inpatient would be able to resist the antibiotic in injected mice if they were given the equivalent of human colistin dosing.
What were the basic results?
The researchers found that the cause of colistin resistance was the MCR-1 gene. The resistance gene was found to be transferred between bacteria cells through a process called conjugation, where the plasmid are passed from one bacterium to another. Researchers found that this transfer was able to take place across bacterial species, from E. coli to K. pneumoniae.
Between 2011 and 2014, the MCR-1 gene was found in E. coli isolates collected from 78 (15%) of 523 samples of raw meat, 166 (21%) of 804 animals, and 16 (1%) of 1,322 samples from hospital inpatients with infection.
How did the researchers interpret the results?
The researchers concluded that the MCR-1 gene is able to cause resistance to colistin and is transferred between bacterial cells by the process of conjugation.
Although currently confined to China, MCR-1 is likely to spread further. More surveillance and molecular epidemiological studies on the spread of this gene are urgently required.
This Chinese study followed on from previous routine surveillance, which found that some livestock were carrying E. coli bacteria resistant to one of the "last resort" groups of antibiotics used in humans.
Here, the researchers investigated how this resistance developed and how it can be transferred between bacterial cells. They found it is caused by the MCR-1 gene, which is found on a piece of DNA that can be transferred between bacteria. This gene was found in E. coli isolated from a number of raw meat and animal samples taken by the research team.
The prevalence of MCR-1 found in E. coli cells was found to be quite high, which is of some concern and suggests it may already be widespread among livestock in China. However, as the researchers acknowledge, they took a relatively small number of samples, and caution against the results being extrapolated too far.
As China is the world's largest producer of poultry and pig products, this is of great concern to their population and economy. The researchers suggest a possible reason for this antibiotic resistance is the use of colistin in animal feed in China.
It is unclear whether the situation may be similar in other countries. Antibiotic resistance is a global concern that could potentially advance more quickly than new, stronger antibiotics can be developed.
Without effective antibiotics, infections we regard as non-serious and routine operations could carry a much higher risk of serious complications. Further study to investigate the ways bacteria develop resistance and how we can tackle this problem are needed.
There are several things you can do to help prevent the development of antibiotic or other antimicrobial resistance. These include recognising that many common respiratory and gastrointestinal infections are viral and do not need – and will not respond to – antibiotics.
If you are given a course of antibiotics for any condition, it is very important to take the full course as prescribed, even if you begin to feel better. Doing this prevents bacteria being exposed to a dose of antibiotics that is too small to eradicate them, but gives them a taste of the antibiotic and allows them to develop resistance.