2189. Colistin Heteroresistance in Enterobacter due to Base Heterozygosity at Certain phoPQ Locations

BACKGROUND: Enterobacter species are major nosocomial pathogens, and the clinical implications and mechanisms of colistin heteroresistance (CHR) in Enterobacter remain unclear. METHODS: We used the population analysis profile (PAP) assay to determine the presence of CHR in Enterobacter. To examine whether CHR leads to treatment failure, we conducted in vitro time-killing assays and in vivo assays using murine intra-abdominal infection models. To determine the genetic mechanism for the CHR phenotype, we conducted whole-genome sequencing and ultra-deep second-generation sequencing. A schematic outline of methods and main results [Figure: see text] RESULTS: In this study, we found that 30% of Enterobacter clinical strains causing bloodstream infections at West China Hospital exhibited CHR, which was associated with treatment failure and fatal infections, as demonstrated by in vitro time-kill tests and in vivo mouse infection modeling. We identified base alterations in the phoP-phoQ gene as the main resistance mechanism in Enterobacter CHR and that this heterogeneity originated from colistin selection and base heterozygosity, as detected by ultra-deep second-generation sequencing. We also found that several different resistance subpopulations existed simultaneously in the same strain with different resistance mechanisms. Time-kill experiments in Enterobacter isolates with CHR or susceptible to colistin [Figure: see text] CHR strains lead to in vivo colistin treatment failure [Figure: see text] Phenotypic and genomic study of a CHR strain [Figure: see text] CONCLUSION: We presented evidence for "genetic heterogeneity" in Enterobacter CHR strains, which is consistent with "phenotypic heterogeneity" and genetically explains the unstable and transient HR. This provides important insights and a fresh perspective into the mechanisms and clinical implications of Enterobacter CHR and underscores the importance of deep sequencing, prompting the research of high-throughput microbial single-cell sequencing as a method for detecting "genetic heterogeneity." Frequency of bases that mediate colistin resistance at the same locus of phoP-Q in the parental and resistant strains [Figure: see text] DISCLOSURES: All Authors: No reported disclosures