Polymyxin heteroresistance in Klebsiella oxytoca

<b>Introduction.</b> Antibiotic heteroresistance presents a growing public health concern, as the phenotype is associated with treatment failure and is hard to detect using conventional diagnostic testing.<b>Gap Statement.</b> Heteroresistance in <i>Klebsiella oxytoca</i>, an opportunistic pathogen associated with hospital-acquired infections, has not been characterized.<b>Aim.</b> In this study, we characterized polymyxin B heteroresistance in a collection of six clinical and environmental isolates of <i>K. oxytoca</i>.<b>Methodology.</b> We assessed heteroresistance using population analysis profile assays and LPS modifications using matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF).<b>Results.</b> All six isolates tested exhibited heteroresistance, indicated by an 8-16-fold difference between the MIC of the bulk population and the MIC of the resistant sub-population, as determined using population analysis profiling. Heteroresistance was found to be due to the presence of a stable sub-population of resistant bacteria, the size of which was unaffected by growth phase or the presence of host antimicrobial factors present in human serum. MALDI-TOF analysis revealed 4-amino-l-arabinose modifications of the lipid A of resistant sub-populations.<b>Conclusion.</b> This pilot study identifies that polymyxin heteroresistance in <i>K. oxytoca</i> may complicate the treatment of infections caused by this organism.