Colistin is the last-resort antibiotic used to treat highly drug-resistant bacterial infections

A small study involving 65 stool samples taken from patients from a single hospital in Chennai found 51% of them harbour colistin-resistant bacteria. This reflects the presence of such bacteria in the gut as stool samples represent gut colonisation. This is the first study from India which has found indirect evidence of colistin-resistant bacteria in the gut and was published in the journal Diagnostic Microbiology & Infectious Disease.

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Colistin is the last-resort antibiotic used to treat highly drug-resistant bacterial infections.

Colistin-resistant bacteria can be of hospital origin or food origin. Colistin-resistant bacteria of hospital origin do not respond to any of the antibiotics, including carbapenem while colistin-resistant bacteria of food origin will respond to carbapenem.

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Stool samples

Of the 65 stool samples studied, 33 samples were found to have colistin-resistant bacteria, the team led by Dr. Abdul Ghafur, consultant in infectious diseases at Chennai’s Apollo Cancer Institute found. And of the 33 samples that had bacteria resistant to colistin, 77% were found to be food-acquired and the remaining 23% had an antibiotics resistance pattern suggestive of hospital origin.

In a paper published in the Journal of Global Antimicrobial Resistance in 2018, Dr. Ghafur and his team found that colistin-resistant Klebsiella bacteria were widely present in poultry and raw vegetables. The main cause of colistin resistance in food is due to the rampant use of colistin in poultry. Since poultry litter is used as manure to grow vegetables, colistin-resistant bacteria are found in vegetables as well.

Usage in poultry

“Most of the colistin-resistant Klebsiella bacteria in the gut is from food and not hospital-acquired,” says Dr. Ghafur. “Colistin usage in poultry plays a bigger role than its usage in hospitals for the bacteria to develop widespread colistin resistance.”

Dr. Ghafur adds: “So even if hospital usage of colistin is rationale and scientific that will hardly make any difference in the generation of colistin resistance. The only way out is to control the use of colistin in poultry.”

How resistance is conferred

In clinical practice, it is the mutation in the mgrB gene or other chromosomal genes that confers colistin resistance to Klebsiella bacteria. In their 2018 study, the authors found mgrB gene mutation in food Klebsiella bacteria.

Till date, there is no evidence to suggest that the mgrB gene mutation spreads from food to human Klebsiella bacteria. The only colistin resistance mechanism that is known to spread from food to human Klebsiella bacteria is through mcr gene transfer.

However, only a minority (less than 3%) of colistin-resistant Klebsiella bacterial infections in humans is contributed by the mcr gene. “Majority of infections by colistin-resistant Klebsiella in humans are due to mutations in the mgrB gene or other chromosomal genes,” Dr. Ghafur says.

The gene and its mutation

The mcr gene is located in the plasmid and so can spread quickly to other bacteria. The mgrB gene mutation too can spread quickly but through insertion sequences.

The cause of colistin resistance in majority of bacteria of food origin in human gut is due to mgrB gene mutation.

“Our study provides additional evidence to the hypothesis that colistin resistance due to mgrB mutation in food Klebsiella has the potential to spread to human Klebsiella in the gut and hence may contribute colistin-resistant infections in humans,” he says.

In light of that, the finding that a large number of individuals carry colistin-resistant bacteria of food origin in the gut is therefore worrying.