The study population is represented by adult hospitalised patients with severe nosocomial infections due to multi-drug resistant A. baumannii, susceptible to colistin. It will be performed in intensive or sub-intensive care units of 5 Italian clinical centres where MDR A. baumannii infection is endemic with epidemic phases. All adult subjects, irrespective of age, will be included in the study, thus also elderly subjects will be eligible. Large eligibility criteria are warranted by the pragmatic approach of the study, the severe prognosis of these patients and the lack of effective alternative treatments.