Treatment of infections caused by XDR-GNB presents a considerable challenge for clinicians. Monotherapy polymyxins - commercially available as polymyxin B or polymyxin E (colistin) - are currently the gold standard of treatment for severe XDR-GNB infections. However, they are associated with significantly more adverse effects and may be less effective clinically compared to other antibiotics, such as the beta-lactams. In one tertiary center in Korea, retrospective analysis suggested that mortality of XDR-A. baumannii bacteremia was not reduced when colistin was used for treatment compared to other antibiotics (to which the organisms were resistant). Individual treatment failures with polymyxins have been reported, either due to the development of resistance in vivo or inherent heterogeneous polymyxin resistance - a phenomenon where many isolates that appear susceptible to the drug may actually harbor polymyxin-resistant subpopulations. Tigecycline susceptibility in XDR-GNB is variable and clinical failures have also been reported, particularly for bloodstream infections due to the low achievable concentrations of the drug in serum as well as the potential for development of resistance during treatment. This has led to some experts advocating combination antibiotic therapy as an alternative.