Symbolic representation of antibiotic resistance testing and the Colistin-MAC test.

Colistin Resistance: A Simple Test to Combat a Growing Threat

Theo Raines in Health & Wellness December 2025 • 4 min read.

"New phenotypic method offers rapid screening for MCR-1 mediated resistance, crucial for combating antibiotic resistance."

The rise of antibiotic-resistant bacteria poses a significant threat to global health. Polymyxins, including colistin, are often the last line of defense against multidrug-resistant Gram-negative pathogens. However, resistance to these antibiotics is increasing, driven in part by the spread of mobilized colistin resistance (mcr) genes.

A particularly concerning mechanism of resistance is mediated by the mcr-1 gene, which can transfer easily between bacteria, leading to rapid dissemination of colistin resistance. This highlights the urgent need for rapid and accessible screening methods to detect and control the spread of mcr-1-mediated colistin resistance.

Researchers have developed a novel phenotypic method called the Colistin-MAC test, designed for quick and easy screening of MCR-1-mediated colistin resistance. This test leverages the synergy between colistin and dipicolinic acid (DPA) to identify resistant strains, offering a promising tool for combating antibiotic resistance.

How Does the Colistin-MAC Test Work?

Symbolic representation of antibiotic resistance testing and the Colistin-MAC test.

The Colistin-MAC test is a broth microdilution method that assesses colistin MIC (minimum inhibitory concentration) in the absence and presence of dipicolinic acid (DPA). DPA is a chelator that can reduce colistin resistance in MCR-1 producing strains.

The test involves comparing the colistin MIC of a bacterial strain grown in a standard broth versus a broth supplemented with DPA. A significant reduction in the colistin MIC in the presence of DPA suggests the presence of MCR-1 mediated resistance. Specifically, a ≥8-fold reduction in colistin MIC with DPA indicates a positive result.

A ≥8-fold reduction of colistin MIC in the presence of DPA was observed with 59 mcr-1-positive strains.
Colistin MICs were unchanged, increased, or at most reduced by two-fold with the 13 mcr-negative colistin-resistant strains.

The research evaluated 74 colistin-resistant strains of Enterobacteriaceae, including Escherichia coli and other species. The test demonstrated high accuracy in identifying mcr-1-positive strains, offering a simple and presumptive method for detection, especially in resource-limited settings.

Why This Test Matters for Combating Antibiotic Resistance

The Colistin-MAC test offers a practical solution for identifying and controlling the spread of MCR-1 mediated colistin resistance. Its simplicity and low cost make it accessible to laboratories with limited resources, enabling widespread surveillance and prompt intervention.

By quickly identifying mcr-1-positive strains, healthcare professionals can implement targeted infection control measures and optimize antibiotic usage, slowing the spread of resistance. This is particularly crucial in low-income settings where advanced molecular diagnostics are not readily available.

While the Colistin-MAC test shows promise, further research is needed to evaluate its performance across a wider range of bacterial species and resistance mechanisms. Continuous refinement and validation will ensure its effectiveness in the ongoing fight against antibiotic resistance, ultimately safeguarding public health.

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Everything You Need To Know

What is the Colistin-MAC test and how does it identify colistin resistance?

The Colistin-MAC test is designed to quickly identify MCR-1-mediated colistin resistance in bacteria. It's a broth microdilution method that measures the minimum inhibitory concentration (MIC) of colistin both with and without dipicolinic acid (DPA). If the colistin MIC significantly decreases (≥8-fold reduction) in the presence of DPA, it suggests the bacteria has MCR-1 mediated resistance. The test's simplicity allows for easy screening, especially where resources are limited.

Why is the Colistin-MAC test important in combating antibiotic resistance?

The Colistin-MAC test is crucial because it offers a practical way to detect and control the spread of MCR-1 mediated colistin resistance. Identifying resistant strains promptly can inform antibiotic stewardship practices, helping to reduce the inappropriate use of colistin. This is important in slowing down the rise of antibiotic resistance and preserving colistin's effectiveness as a last-line treatment for multidrug-resistant Gram-negative pathogens.

What is MCR-1 and why is it a concern in the context of antibiotic resistance?

MCR-1 is a mobilized colistin resistance gene that enables bacteria to resist the effects of colistin, an antibiotic of last resort. The 'mobilized' part means this gene can transfer easily between different bacteria, leading to the rapid spread of colistin resistance. The concerning part is how quickly MCR-1 can disseminate, making infections harder to treat and posing a significant threat to public health, particularly for those with weakened immune systems or severe infections.

What role does dipicolinic acid (DPA) play in the Colistin-MAC test?

Dipicolinic acid (DPA) plays a crucial role in the Colistin-MAC test by acting as a chelator to reduce colistin resistance in MCR-1 producing strains. When DPA is added to the test broth, it helps to diminish the resistance caused by the MCR-1 gene. Consequently, a notable decrease in the colistin MIC in the presence of DPA indicates the presence of MCR-1-mediated resistance.

How accurate is the Colistin-MAC test based on the study of Enterobacteriaceae strains?

The study evaluated 74 colistin-resistant strains of Enterobacteriaceae. The test demonstrated high accuracy in identifying mcr-1-positive strains. This is significant because it validates the test's reliability in detecting MCR-1 mediated resistance, which is essential for effective antibiotic stewardship and infection control strategies.