Background Nosocomial infection Nosocomial infection, especially those caused by bacteria resistant to antibiotics, is a major threat to critical care medicine. A systemic review revealed that 10-20% of patients receiving mechanical ventilation (MV) more than 48 hours will develop ventilator associated pneumonia (VAP), patients who develop VAP are twice as likely to die compared with the similar patients without VAP, and patients who develop VAP have in additional hospital costs more than $ 10,019 [1].