Resistance to polymyxins: Mechanisms, frequency and treatment options.

Polymyxins act by binding to lipid A moiety of the bacterial lipopolysaccharide and subsequently disintegrating the bacterial membranes. The most important mechanism of resistance includes modifications of the bacterial outer membrane structure, including lipopolysaccharide. Lipopolysaccharide modification is mostly mediated by PmrA/PmrB and PhoP/PhoQ two-component regulatory systems. These mechanisms exist with some differences in many gram-negative bacterial species. Resistance to polymyxins is generally less than 10%. In specific regions, such as the Mediterranean basin, Korea and Singapore, they tend to be higher. Heteroresistance to polymyxins is associated with exposure to polymyxins and especially suboptimal therapeutic dosage. Polymyxin combination regimens, tigecycline and fosfomycin may be useful options for the treatment of polymyxin-resistant gram-negative infections.