Two novel rapid colorimetric methods for determination of colistin susceptibility in Enterobacterales isolates.

PURPOSE

Colistin is an antimicrobial used as a last resort in the treatment of resistant gram-negative infections associated with high morbidity and mortality. Therefore, the rapid and accurate detection of colistin susceptibility is of vital importance. However, the current methods used to determine colistin susceptibility face various challenges. The purpose of this study was to evaluate two novel rapid colistin susceptibility methods, namely resazurin-based rapid modified colistin disk elution (RrmCBDE) and nitrate reductase-based rapid modified colistin disk elution (NRrmCBDE), alongside the established modified colistin broth disk elution (mCBDE), against the reference broth microdilution (BMD) method for determining colistin susceptibility in Enterobacterales isolates.

METHODS

The colistin susceptibility of 215 multidrug-resistant or extensively drug-resistant Enterobacterales isolates was determined using RrmCBDE and NRrmCBDE with 0.5 and 3 McFarland variations, and mCBDE test. The RrmCBDE and NRrmCBDE tests yielded results in approximately 6 h, whereas the mCBDE test required 16-20 h. The findings were then compared with the reference BMD method.

RESULTS

According to BMD results, the mCBDE method showed 98.6% categorical agreement (CA) with 1.39% very major errors (VME). It demonstrated 97.86% sensitivity and 100% specificity. For RrmCBDE, the 0.5 McFarland variation had 91.2% CA, 8.8% VME, 86.4% sensitivity, and 100% specificity, while the 3 McFarland variation achieved 97.2% CA, 2.79% VME, 95.7% sensitivity, and 100% specificity. Similarly, NRrmCBDE, the 0.5 McFarland variation exhibited 88.37% CA and 11.6% VME, 82.1% sensitivity, and 100% specificity, while the 3 McFarland variation demonstrated 98.6% CA, 1.39% VME, 97.86% sensitivity, and 100% specificity.

CONCLUSION

Based on these results, it was concluded that the novel RrmCBDE and NRrmCBDE tests, particularly with the 3 McFarland variation, provided results within approximately 6 h, making them a rapid alternative to conventional methods.