Background: The re-use of colistin (last-resort drug) to treat infections caused by resistant Gram-negative bacteria, has led to the emergence of a serious resistance against colistin. A new transferable plasmid-mediated colistin-resistance gene (mcr-1) has been described globally. Screening for such gene will provide an aiding step to explore the extent of colistin-resistance in Egypt. Objectives: To isolate the causative Gram-negative bacteria from different hospital-acquired (HA) and community-acquired (CA) infections, determine the antibiotic susceptibility pattern of the isolated bacteria, detect colistin-resistance and investigate the existence of mcr-1 gene in colistin-resistant isolates. Methodology: This study was carried out on 400 patients with HA- and CA-infections. Samples were taken from sputum, endotracheal aspirates, bronchoalveolar lavage, surgical and burn wounds, stool and blood. Bacterial isolation and identification were done by standard microbiological methods. Colistin-resistance was assessed by broth macrodilution method, then mcr-1 gene was detected in colistin-resistant isolates by conventional polymerase chain reaction (PCR). Results: Gram-negative organisms were the commonest isolates in both HA (67.6%) and CA (79.4%) infections. Colistin-resistance was detected in only 10 cases. mcr-1 gene was not detected in any of the tested colistin-resistant isolates. Conclusion: The prevalence of colistin-resistance in the study cases is still low and has not extended to the community yet. Colistin intake is not a prerequisite for the occurrence of resistance, but could be a supporting factor.