Elution methods to evaluate colistin susceptibility of Gram-negative rods.

Recently it was developed the Colistin Broth Disk Elution test which uses colistin disks as a source of these antibiotics. The aim of this study was to evaluate the performance of protocols that used diminished volumes of the reagents: the Colistin Broth Microelution (CBM) (1 mL) and the Microelution-Plates Test (MPT) (200 µL), as well as the Colistin Susceptibility Test Tube (CSTT), which uses only one colistin disk added to a tube containing broth. The tests were performed with 85 Gram-negative isolates collected from surveillance studies. The CBM, MPT, and CSTT tests presented a good Categorical Agreement (CA), Essential Agreement (EA), sensitivity and specificity to Enterobacterales isolates, however the ME and VME were less satisfactory. The results for non-fermentative isolates were not satisfactory. In conclusion, the proposed methods, mainly the CSTT, can be used as screening tests to detect colistin resistant among Enterobacterales, as they are an easy and inexpensive option to the reference method.