Molecular characteristics of emerging colistin resistant Gram-negative Bacilli in Lebanon

Abstract

Background: Gram-negative bacilli such as Acinetobacter baumannii and Escherichia coli are major causes of nosocomial infections worldwide. The misuse of broad-spectrum antibiotics contributed to the emergence of multidrug-resistant Gram-negative organisms (MDRGNs) that are of major threat to immunocompromised patients. The emergence of bacterial resistance against most classes of antibiotics and the shortage of antimicrobials against Gram-negative bacteria have led clinicians to reuse colistin, a polymyxin. Colistin targets the outer membrane of Gram-negative bacteria specifically anionic Lipopolysaccharide (LPS), a major component of their outer membrane. This leads to the disruption of the cell membrane and the displacement of magnesium (Mg+2) and calcium (Ca+2) ions leading to the leakage of the cell and consequently death. However, colistin-resistant isolates are increasingly reported with the usage of colistin, the last resort of antimicrobials. The recent emergence of colistin resistant isolates at the American University of Beirut Medical center led to the initiation of this study in order to assess the underlying mechanisms of resistance in colistin resistant Gram-negative bacilli. Methods: Thirty-three non-selected MDR Acinetobacter baumannii and 5 Carbapenem resistant Escherichia coli clinical isolates from the Department of Pathology and Laboratory Medicine, American University of Beirut Medical Center (AUBMC), that included 7 MDR A. baumannii from Makased Hospital, were collected and used in this study. In addition, one E.coli and one Enterobacter cloacae isolates obtained from poultry were used. The levels of resistance were measured using disc diffusion technique, broth microdilution assay, E-test and Vitek system. Capsule staining for A. baumanni was done. Differences in the three-dimensional topography for colistin sensitive and colistin-treated resistant A.baumannii and E.coli were visualized by Scanning Electron Microscope. The genomic relatedness of these bacteria was determined by Random A