Molecular characteristics of Colistin Resistance in Gram-negative Bacilli and assessment of Combination Therapy

Abstract

Background: Extensive and uncontrolled use of antibiotics, along with the natural evolution of bacteria, and the lack of discovery of new antimicrobial classes, has led to a rise in the number of multi-drug resistant (MDR) bacteria including Acinetobacter spp. and Escherichia coli, causing them to develop the ability to resist several classes of antimicrobials, including third generation cephalosporins and carbapenems. This rapidly growing phenomenon has led to increased reliance on the cyclic peptide antimicrobial agent colistin as a therapeutic option for MDR Gram-negative bacteria, making it the ‘last-line’ of defense against such infections. In recent years, the rise of colistin resistant strains has been reported worldwide at an alarming rate; and with that, so did the demand on the scientific community to provide alternative options for treatment of colistin resistant A. baumannii and E. coli. In a previous study, molecular characteristics of colistin resistant isolates from the American University of Beirut Medical Center (AUBMC) was determined. This study aims to assess the efficacy of different antimicrobials agents, in combination with colistin, against resistant A. baumannii and E. coli isolates, and to correlate synergistic effect in relation to encoding colistin resistant gene profiles. Methods: Broth microdilution in accordance with Clinical and Laboratory Standards Institute (CLSI) guidelines, was used to determine susceptibility to colistin on 33 A. baumannii and 6 E. coli MDR clinical isolates, obtained from the Clinical Microbiology laboratory at AUBMC, previously screened for colistin resistance by disc diffusion, E-test and Vitek®. Checkerboard and time-kill assays were performed on 5 resistant A. baumannii isolates to colistin, 2 CDC mcr-1 encoding E. coli strains, and 2 ATCC wild-type strains to investigate synergy between colistin and each of amikacin, rifampicin, and teicoplanin. Reverse transcription-quantitative PCR (RTqPCR) was performed on