Polymyxin heteroresistance in Klebsiella oxytoca

This study highlights that heteroresistance—where a small subpopulation of bacteria is much more resistant than the majority—occurs in Klebsiella oxytoca, a pathogen linked to hospital infections. All tested isolates showed significant resistance differences (8–16-fold) between the main bacterial population and resistant subgroups, which were stable and unaffected by growth conditions or human serum. The resistance was linked to specific changes in the bacterial outer membrane (lipid A modification). These findings suggest that such hidden resistance can be missed by standard diagnostics and may contribute to treatment failure, complicating the effective use of last-resort antibiotics like polymyxins.