Scientists from Emory Health Sciences have found "heteroresistance" to colistin, a last-resort antibiotic, in already highly resistant Klebsiella pneumoniae, a bacterium that is known to cause infections in the blood, soft tissue and the urinary tract. The work was published recently in mBio.
"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," says David Weiss, director of the Emory Antibiotic Resistance Center. "To our knowledge, this type of heteroresistant Klebsiella has not been observed in the United States before."
The team obtained bacterial isolates from two patients and found that the resistance was not detectable with current diagnostic tests. However, it was possible to see them after allowing the population to grow out for 24 more hours. Maintaining colistin resistance all the time is disadvantageous for bacteria, so using genetic techniques, the group was able to see the genes turn off and on.
In a peritonitis mouse model, an infection with heteroresistant isolates was lethal and untreatable by colistin.
"Clinical laboratories should consider testing for heteroresistance to colistin if this last-line antibiotic is required for treatment," the authors say. "However, the extra time required is a downside. Novel diagnostics that rapidly and accurately detect colistin heteroresistance are needed."