Prabhav AGGARWAL, Sonal SAXENA, Anugula AMRITHA, Manisha KAIM
Infectious Diseases and Clinical Microbiology - 2026;8(2):139-143
Objective: Accurate colistin susceptibility testing is critical but remains challenging, especially in resource-limited settings with a high workload. Broth microdilution (BMD) is the reference standard; however, it is time- and resource-intensive. The colistin agar test (CAT) offers a viable alternative; however, the additional step of preparing a 1:10 dilution can limit routine workflow. This study aimed to evaluate a simple modification of the CAT for Enterobacterales. Materials and Methods: Enterobacterales isolates were tested using BMD, the standard CAT, and modified CAT (mCAT). The mCAT uses an undiluted 0.5 McFarland inoculum with a reduced inoculum volume (1 µL), instead of the Clinical and Laboratory Standards Institute (CLSI)-recommended 10 µL of a one-tenth-diluted suspension, simplifying the protocol while maintaining an equivalent bacterial inoculum load. Results: A total of 306 Enterobacterales isolates (102 colistin-resistant and 204 intermediate) were evaluated. The mCAT method showed 100% categorical agreement with CAT and 99.3% concordance with BMD (304/306 isolates). Among all isolates, two very major errors were observed only in Enterobacter cloacae, with none detected in Escherichia coli or Klebsiella pneumoniae. The mCAT reduced procedural steps and resource use without compromising test performance, saving up to 30 minutes daily spent preparing additional saline tubes and 1:10 dilutions in a high-throughput laboratory. Conclusion: The mCAT method offers a reliable, simplified, and cost-effective alternative to standard colistin susceptibility testing for Enterobacterales. It holds particular promise for microbiology laboratories in low- and middle-income countries, where routine BMD testing is limited, and sample volumes are high.
