Bacteria resistant to last-resort antibiotic found

Scientists have detected "heteroresistance" to colistin, a last-resort antibiotic, in Klebsiella pneumoniae, a bacterium that causes blood, soft tissue and urinary tract infections.
Researchers from Emory University School of Medicine in the US had observed heteroresistance to colistin in other bacteria, called Enterobacter, previously.
Carbapenem-resistant Enterobacteriaceae (CRE), which include Klebsiella, were listed as one of the top three urgent antibiotic resistant threats in 2013 by the US Centers for Disease Control and Prevention.
Various types of Klebsiella are estimated to be responsible for 10 per cent of infections acquired in health care facilities.
"This is concerning because Klebsiella is a more common cause of infection than Enterobacter, and these isolates were carbapenem-resistant, which means that they might actually be treated with colistin," said David Weiss, professor at Emory University School of Medicine.

"To our knowledge, this type of heteroresistant Klebsiella has not been observed in the US before," said Weiss.

The bacterial isolates came from urine samples from two patients in Atlanta-area hospitals, researchers said.
Heteroresistance means that bacterial resistance to particular antibiotics is harder to monitor, they said.
Heteroresistance is caused by a minor subpopulation of resistant bacteria which are genetically identical to the rest of the susceptible bacteria.
The bacterial isolates described in the journal mBio were not detectable with current diagnostic tests, although it was possible to see them by waiting an extra 24 hours for the resistant population to grow out, researchers said.

It appears that maintaining colistin resistance all the time is disadvantageous for bacteria, they said.
Probing the mechanism of heteroresistance, Weiss and his colleagues were able to see a signature of colistin resistance, in terms of genes turned on and off.
In a mouse model of peritonitis (body cavity infection), infection with the heteroresistant isolates was lethal and untreatable by colistin.
Colistin is viewed as a last resort measure for bacterial infections that are resistant to other drugs, partly because it is poisonous to the kidneys.
"Clinical laboratories should consider testing for heteroresistance to colistin if this last-line antibiotic is required for treatment," researchers said.

"However, the extra time required is a downside. Novel diagnostics that rapidly and accurately detect colistin heteroresistance are needed," they said.