Acinetobacter Infections/microbiology; Acinetobacter baumannii/drug effects; Bacteremia/microbiology; Carbapenems/pharmacology; Cross Infection/microbiology; Acinetobacter Infections/drug therapy; Acinetobacter Infections/epidemiology; Acinetobacter baumannii/isolation & purification; Adult; Aged; Anti-Bacterial Agents/pharmacology; Anti-Bacterial Agents/therapeutic use; Bacteremia/drug therapy; Bacteremia/epidemiology; Carbapenems/therapeutic use; Catheter-Related Infections/drug therapy; Catheter-Related Infections/epidemiology; Catheter-Related Infections/microbiology; Cross Infection/drug therapy; Cross Infection/epidemiology; Drug Resistance, Bacterial; Female; Greece/epidemiology; Humans; Intensive Care Units; Male; Microbial Sensitivity Tests; Middle Aged; Pneumonia, Ventilator-Associated/drug therapy; Pneumonia, Ventilator-Associated/epidemiology; Pneumonia, Ventilator-Associated/microbiology; Prospective Studies; Regression Analysis; Risk Factors