Antimicrobial resistance (AMR) is a global health and development threat. World Health Organization (WHO) has declared that AMR is one of the top ten global public health threats facing humanity. Resistance to colistin is steadily rising in Gram-negative bacilli. Hence, there is a need to analyze their resistance profile, and mechanisms and look for alternate treatment options. The primary objectives are to detect the presence of the mcr-1 gene and its variants in Gram-Negative Bacilli which may lead to low/high-level colistin resistance and to evaluate various phenotypic tests for determining colistin resistance in comparison to Broth Microdilution. the secondary objectives are to determine susceptibility to currently available and newer treatment options as alternatives to colistin and to determine genes encoding for Carbapenemase and Extended-Spectrum Beta-Lactamase (ESBLs). The study will be carried out in the Department of Microbiology, Maulana Azad Medical College and associated Lok Nayak Hospital, New Delhi. Stocked Gram-negative bacterial isolates from all clinical specimens routinely submitted for bacterial culture and susceptibility testing will be included in the study. No patient data will be collected. No patient specimen will be collected for the study.