Using genotype to combat resistance: development of a potential treatment and simple phenotypic test during a ‘pan-resistant’ klebsiella pneumoniae mini-outbreak

Aims Aztreonam/clavulanic acid combination therapy is a potential treatment for multi-resistant Enterobacteriaceae containing MBLs and ESBLs. We describe an aztreonam/clavulanic acid based phenotypic detection and susceptibility test for MBL and ESBL-containing multi-resistant Enterobacteriaceae derived from initial genotypic characterisation. Methods Three phenotypically pan-resistant (by Etest and automated broth microdilution) K. pneumoniae isolates from a clinical colonisation mini-outbreak were shown by PCR to contain MBL and ESBL β-lactamases. These isolates and thirteen other Enterobacteriaceae containing MBLs and ESBLs (with three also containing AmpC β-lactamases) were assessed for susceptibility to aztreonam/clavulanic acid by either broth microdilution or Etest, and our phenotypic disc-diffusion assay. β-lactamases were confirmed by multiplex PCR/reverse line blot. Results Clavulanic acid restored susceptibility to aztreonam in all ‘pan-resistant’ outbreak isolates but not strains also carrying an AmpC β-lactamase. Our disc-diffusion assay predicted 16/16 MBLs, 15/16 ESBLs and 3/3 AmpC β-lactamases and showed susceptibility to aztreonam/clavulanic acid in all non-AmpC isolates. Discussion Pan-resistant Gram-negative isolates are likely to become increasingly common. We have described a potential treatment for an initially phenotypically pan-resistant organism that was predicted from a genotype and developed a simple combined phenotypic detection and susceptibility test. Both potential treatment and test could be employed in resource poor settings.