Background: Colistin resistance presents a challenge for microbiology laboratories, where broth microdilution (BMD) was recommended to be the standard method for testing the susceptibility. However, that phenotypic test is incapable of discriminate colistin resistance attributed to the gene of mcr-1 as opposed to chromosomal mechanisms of resistance. Adding a metallo-chelator agent like dipicolinic acid (DPA) to the BMD of colistin could potentially decrease the minimum inhibitory concentrations (MICs) of colistin for isolates that express the gene mcr-1.
Objective: The study aims to estimate the frequency of colistin resistant Enterobacteriaceae and to evaluate the impact of DPA on MIC of colistin as determined by BMD.
Methodology: This study was done on a total of 71 Enterobacteriaceae isolates. Confirmation of isolates and detection of colistin resistance was done then phenotypic detection of plasmid-mediated mcr-1 gene was done by measuring colistin MIC using DPA in BMD.
Results: Our study revealed that 74.6% of the isolates were colistin resistant, 7.4% of the isolates possessed mcr-1 gene. When DPA was added, all strains carrying the mcr-1 exhibited a decrease in the MIC of colistin by a minimum eightfold dilution (sensitivity 100%). However, 51% of the mcr-1 negative strains showed a similar reduction of MIC (specificity 48.98%). This test presented accuracy of 52.83%.
Conclusion: DPA-based microdilution test is not a sufficiently accurate test to rely on in the identification of the mcr-1 producers. However, it is a highly sensitive test, it could be used as an effective method to screen for mcr-1 presence.