Colistin is the last resort for treatment of infections caused by multidrug-resistant Gram-negative bacilli. Objectives: This study aimed to evaluate colistin susceptibility among multidrug resistant E. coli and K. pneumoniae isolates, to determine the prevalence of mcr-1& mcr-2 genes carrying isolates and to evaluate the synergistic activity of colistin in combination with the sulfadiazine against colistin-resistant isolates. Methodology: 1632 E. coli and K. pneumoniae isolates were collected and identified. Antibiotic susceptibility& MIC for colistin were detected followed by investigating for mcr-1 and mcr-2 genes by PCR. The effect of colistin–sulfadiazine combination was tested by E-test/ agar diffusion method. Results: Among 1218 MDR E. coli and K. pneumoniae isolates, 34 (2.79 %) colistin resistant isolates were detected. The isolates were most frequently isolated from urine 10 (29.4%) and tracheal aspirate 5 (14.7%). Highest resistance rates were reported: ceftazidime (97.1%), cefepime (94.1%), and imipenem (82.3%). Least resistance rates were displayed for amikacin and sulfamethoxazole/trimethoprim (73.5%), and tobramycin (70.5%) respectively. The mcr-1 gene was detected in only one E. coli strain while the mcr-2 gene was not detected. By using colistin-sufadiazine combination, a decrease in colistin MIC ≥ 2 dilutions was observed for 26/34 isolates (76.5%) including the single mcr-1 positive isolate. Conclusion: The spread of colistin resistance threatens its use as the last resort for MDR E. coli and K. pneumoniae. The presence of mcr-1 alarms facing pan-drug resistance among Enterobacteriaceae. To its limit spread, a great concern should be paid to continuous surveillance, putting into practice an effective antibiotic stewardship program and enhanced infection control measures. The use of colistin/sulfadiazine combination represents an alternate and could be used for some cases of colistin-resistant Gram negative bacilli.