Colistin Susceptibility Testing in Gram-Negative Bacteria

Colistin remains a critical agent against multidrug-resistant Gram-negative pathogens but poses unique challenges for susceptibility testing. Its cationic, amphipathic structure binds to plastic surfaces and divalent cations in media, leading to variable results across methods. The broth microdilution technique is the internationally recommended reference, yet reproducibility demands strict adherence to standards on inoculum density, medium composition and incubation conditions. Alternative phenotypic assays—such as rapid colourimetric tests, agar-based drop tests and commercial broth microdilution panels—have emerged to offer faster turnaround or ease of use, but often sacrifice essential agreement or struggle with low-level resistance. Molecular detection of mobile mcr genes provides genetic insight but does not substitute for phenotypic MIC determination. More recently, mass spectrometry-based lipid A profiling using matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) has enabled direct detection of colistin-induced lipid A modifications within minutes, heralding a new frontier in diagnostic speed and mechanistic clarity. Global implementation of any method must balance accuracy, cost and laboratory capacity, as colistin susceptibility testing underpins both clinical decision-making and surveillance of emerging resistance mechanisms.

Research from Nature Portfolio

Researchers have refined a MALDI-TOF-based lipidomic assay for Acinetobacter baumannii, detecting addition of phosphoethanolamine to lipid A—the hallmark of colistin resistance—in under 15 minutes. This workflow, applied directly to intact bacterial cells with minimal sample preparation, achieved accurate discrimination between susceptible and resistant isolates by profiling characteristic mass shifts, thereby bypassing the lengthy incubation of MIC assays. As a foundational exemplar of rapid, mechanism-based diagnostics, this work demonstrates how mass spectrometry can integrate into routine clinical microbiology to identify resistance phenotypes with unprecedented speed.

Colistin Susceptibility Testing in Gram-Negative Bacteria publication trend

The graph below shows the total number of articles in colistin susceptibility testing in gram-negative bacteria across all publications each year (not limited to Nature Index journals).

Technical terms

Broth microdilution: A reference method for determining the minimum inhibitory concentration of antibiotics by preparing serial dilutions in liquid medium.

Minimum inhibitory concentration (MIC): The lowest concentration of an antibiotic that prevents visible bacterial growth under defined conditions.

Lipid A: The membrane-anchored component of lipopolysaccharide; modification of lipid A often mediates colistin resistance.

MALDI-TOF MS: Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry, used for rapid identification and analysis of bacterial molecules.

MCR proteins: Plasmid-encoded phosphoethanolamine transferases that modify lipid A and confer transferable colistin resistance.

References

1. Low performance of Policimbac® broth microdilution in determining polymyxin B MIC for Klebsiella pneumoniae. Frontiers in Cellular and Infection Microbiology (2023).
2. Challenges in the Detection of Polymyxin Resistance: From Today to the Future. Microorganisms (2024).
3. Rapid detection of colistin resistance in Acinetobacter baumannii using MALDI-TOF-based lipidomics on intact bacteria. Scientific Reports (2018).
4. Effects of Microplate Type and Broth Additives on Microdilution MIC Susceptibility Assays. Antimicrobial Agents and Chemotherapy (2018).
5. Detection of Colistin Resistance in Escherichia coli by Use of the MALDI Biotyper Sirius Mass Spectrometry System. Journal of Clinical Microbiology (2019).