Scientists have for the first time isolated an extremely virulent strain of Klebsiella pneumoniae that is resistant to a class of highly effective antibiotic agents and presents a triple threat, from a patient in the US.

The previously reported hypervirulent forms were largely antibiotic susceptible. Carbapenem-resistant K. pneumoniae -- part of the carbapenem-resistant enterobacteriaceae (CRE) superbug family -- is considered an urgent (among top 3) threat by the US Centers for Disease Control and Prevention (CDC). "The problem of antibiotic resistance is becoming increasingly alarming. The combination of increased virulence and multidrug resistance makes the situation worse," said David Weiss, Director at the Emory University in Georgia, US.

The results, presented at the annual meeting of ASM Microbe 2018 in Georgia, revealed the K. pneumoniae isolate was heteroresistant to the last resort antibiotic colistin. This means that a small subpopulation of cells showed resistance. Heteroresistance is more difficult to detect with standard antibiotic susceptibility tests in clinical microbiology labs, and this isolate was classified as susceptible to colistin by standard methods.

This discrepancy is particularly important, as researchers have shown that such undetected colistin heteroresistance can cause antibiotic treatment failure in mice. The researchers are urging more monitoring for this form of bacteria, which have the potential for increased virulence and may be especially worrisome in healthcare settings.

For the study, the team examined 265 isolates of carbapenem-resistant K. pneumoniae using a simple "string test". "The string test is very low-tech. You take a loop, touch it to the bacterial colony, and pull back. The hypermucoviscous one looks like a string of cheese being pulled from a pizza," said Jessie Wozniak, graduate student at the varsity.

According to Wozniak, the isolate was approximately ten times more virulent in mice than other isolates of the same sequence type. Further, whole-genome sequencing discovered that the isolate carried several antibiotic resistance genes, along with a new arrangement of virulence genes, but not the same set seen in similar K. pneumoniae isolates from Asian countries, she said.