# Delineating genetic determinants of polymyxin resistance in Serratia marcescens

> **NIH NIH R21** · COLUMBIA UNIVERSITY HEALTH SCIENCES · 2021 · $255,393

## Abstract

This proposal aims to define the molecular mechanisms of resistance to polymyxins in Serratia marcescens.
The emergence of carbapenem resistance in this nosocomial pathogen poses a treatment dilemma as few
treatment options exist. S. marcescens has long been considered intrinsically resistant to polymxyins, a
mainstay for treatment of carbapenem-resistant Enterobacteriales. However, we unexpectedly noted that a
substantial proportion of carbapenem-resistant S. marcescens (CR-SM) are fully susceptible to polymyxin. In
addition, resistant isolates displayed two distinct phenotypes (low-level (R1) and high-level resistance (R2).
Polymyxins are cationic peptides targeting negatively charged components of the bacterial outer membrane,
notably Lipid A. Chemical modification of Lipid A through enzymatic transfer of L-Ara4N to Lipid A is the major
contributor to polymyxin resistance and is catalyzed by the aminoarabinose transferase ArnT. Lipid A analysis
of isolates representing the 3 phenotypic groups demonstrated a complete lack of L-Ara4N modification in
susceptible isolates and distinct lipid A profiles between R1 and R2 phenotypes. Through whole genome
sequencing we have begun to establish putative genetic determinants of polymyxin susceptibility. This includes
a unique arnC-like gene only present in susceptible isolates and a flippase related to arnE only encoded in
highly-resistant R2 isolates. In addition, R1 and R2 isolates harbored unique arnD and eptA alleles compared
to susceptible isolates. Here, we aim to define the mechanisms underlying polymyxin susceptibility in detail to
provide critical information on the potential suitability of polymyxins as a treatment option for CR-SM infections.
We have assembled a highly skilled team with complementary expertise in microbiology, bacterial genomics
and membrane protein biochemistry. In Aim 1, we will determine the prevalence of polymyxin susceptibility in a
comprehensive collection of clinical multi-drug resistant (MDR) and non-MDR S. marcescens isolates. Through
whole genome sequencing we will refine putative genetic markers of PR phenotypes and through phylogenetic
reconstruction determine if the loss or PR occurred repeatedly or represents a stable sublineage. We will then
validate candidate genetic markers on PR through a combination of complementation, gene editing and
deletion experiments. In Aim 2, we will assess LPS structure and lipid A modifications of isogenic mutants
generated in Aim 1 using radiolabeling techniques and mass spectrometry. We will evaluate gene expression
of lipid A modifying enzymes and overall differences in transcriptional profiles across phenotypes. Lastly, we
will examine the impact of PR mutants on bacterial fitness and virulence. Findings from our study will fill
significant gaps in knowledge of the genomic and molecular determinants of intrinsic PR, provide information
on the mechanisms of disruption of lipid A modifications, deliver molecular markers for rapid...

## Key facts

- **NIH application ID:** 10317863
- **Project number:** 1R21AI156727-01A1
- **Recipient organization:** COLUMBIA UNIVERSITY HEALTH SCIENCES
- **Principal Investigator:** Anne-Catrin Uhlemann
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $255,393
- **Award type:** 1
- **Project period:** 2021-08-05 → 2023-07-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10317863

## Citation

> US National Institutes of Health, RePORTER application 10317863, Delineating genetic determinants of polymyxin resistance in Serratia marcescens (1R21AI156727-01A1). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10317863. Licensed CC0.

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