Colistin Resistance among Enterobacterales Isolates: Underlying Mechanisms and Alternative Treatment Options

Global dissemination of multidrug-resistant (MDR) and extensively drug-resistant (XDR) Gramnegative bacteria (GNB) such as carbapenemase-producing Enterobacterales has resulted in reviving colistin as a final therapeutic alternative. Colistin resistance foretold a catastrophe. We aimed to detect the rates of carbapenems and colistin resistance among hospital-acquired Enterobacterales species, verify the underlying mechanisms and provide antibiogram for colistin-resistant isolates. The collected Enterobacterales isolates were tested for their antimicrobial susceptibility by the disk diffusion method and agar dilution was utilized for both imipenem and colistin. The production of ES beta Ls and carbapenemases was phenotypically assessed by the combined disk (CDT) and modified carbapenem inactivation (mCIM) tests, respectively. Possible attributes for colistin resistance were explored by detection of both plasmid- and efflux pump-mediated mechanisms. By multiplex PCR assay, carbapenem resistance (blaNDM-1 & blaOXA-48) and mobilized colistin-resistant-1 (mcr-1) genes were identified. A total of 160 Enterobacterales isolates were obtained of which 68.8% were MDR, 25% were XDR and 6.3% were pandrug-resistant (PDR) isolates with no statistically significant difference among Enterobacterales species (P> 0.05). Carbapenems resistance was detected in 41.3% (66/160) while colistin resistance was detected in 22% (36/160) of isolates. Proteus mirabilis expressed the highest rate of colistin resistance (100%; 16/16), followed by Enterobacter aerogenes (23.1%; 6/26), E. coli (13%; 6/46) and K.pneumoniae (11.1%; 8/72). One hundred percent (36/36) of colistin-resistant isolates proved efflux pump activity for colistin. However; only 2% (2/100) of tested Enterobacterales carried mcr-1 gene through molecular analysis. Colistin-resistant isolates exhibited variable susceptibility to the tested antimicrobial agents of which fosfomycin was the highest (94.1%). Efflux pump activity played a major role for colistin resistance among Enterobacterales species and fosfomycin could be a promising therapeutic option.