New test could rapidly identify colistin-resistant bacteria

Researchers have found a method to identify bacteria showing one of the most dangerous forms of antibiotic resistance, in a test that’s both quick and cheap to run.

Dr Laurent Dortet, an Associate Professor at South Paris University and his colleagues at Imperial College London, Dr Gerald Larrouy-Maumus and Professor Alain Filloux, presented their work at the 27th European Congress of Clinical Microbiology and Infectious Diseases (ECCMID) in Vienna, Austria, held from April 22nd-25th.

Antibiotic resistance has become an increasingly prevalent threat, with many bacteria gaining ‘immunity’ from the drugs we’d normally use to treat them. It’s reached the stage where some bacterial strains have become multi-drug resistant, and to have a hope of dealing with these infections we need to rely on ‘last resort’ antibiotics.

Colistin is an example of one of these last resorts. Used to treat gram-negative bacteria, such as Pseudomonas aeruginosa and Klebsiella pneumoniae, colistin works by binding to important proteins in the bacterial outer membrane, causing the leakage of bacterial cell contents and eventually leading to cell death.

Unfortunately, we’ve started to see bacteria also show a resistance to colistin. Since this drug can sometimes be the only remaining treatment option, if it no longer works then dealing with the bacteria becomes extremely difficult – making an already dangerous infection even more threatening.

Colistin is a last resort drug which is the only option left to treat some bacteria… but the number of colistin-resistant strains is on the rise. Image credit:

Dr Dortet and his team tested 134 colonies of K. pneumoniae and Escherichia coli, bacteria which can cause urinary tract infections, pneumonia and sepsis. When they passed the organisms through a machine called a mass spectrometer, a machine commonly found in hospitals which can be used to separate and identify molecules in a sample, they found that the mass spectrometer could show not only which bacteria were resistant to colisitin and which weren’t, but also identified which bacteria carried ‘plasmid-encoded resistance’.

There are two main ways that bacteria can get a hold of resistance to a drug. One way, ‘chromosome-encoded resistance’, happens when a bacterial cell already has a way to resist an antibiotic, and passes this trait down to its daughter cells when it grows and divides. This can be problematic but since the resistance trait sticks to the same bacteria, it’s still somewhat controllable.

The more dangerous method that bacteria can gain resistance is by picking up little circles of DNA called plasmids. Crucially, plasmids can be shared between totally unrelated types of bacteria, making this form of resistance extremely unpredictable.

“This plasmid-encoded resistance is particularly worrying because it has the potential to spread quickly and easily and, if that happens, last resort drugs like colistin could also become obsolete,” Dr Dortet said. “If, on the other hand, we are able to rapidly identify bacteria that have this type of resistance, we can take measures to stop its spread. This might include isolating the patient in a separate room where they are treated by dedicated medical staff.”

It’s hoped that by being able to identify which patients are infected by bacteria with plasmid-encoded colistin resistance, hospitals will be able to treat them more effectively and have better control on the transmittance of these bugs.

The test can be carried out in around 15 minutes, would cost less than $1 per sample to run and, since the machines needed are already in place in most hospitals, putting the test into practise should be fairly straightforward.

“The exciting thing about this technique is that it relies on technology that is already available in most hospitals,” Dr Larrouy-Maumus commented. This means that it could rolled out quickly and cheaply, and potentially have a rapid impact on tackling drug resistance.”

The test could also potentially be used to test whether new drugs restore bacterial sensitivity to colistin, and would also be useful in screening veterinary samples, where colistin resistant bacteria are known to be high.

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