In a critically ill patient, the gut integrity is rapidly compromised either by the treatments used (such as catecholamine or antibiotics) or by the disease itself. This gut alteration favours the adhesion and/or internalisation of bacteria by intestine cells which lead to the production of large amounts of cytokines that rapidly reach the blood compartment, inducing neutrophils activation and then organ damage. Moreover, the imbalance in the normal intestinal flora (demonstrated as early as 24 hours after ICU admission) is one of the mechanisms involved in the development of ventilator associated pneumonia (VAP). VAP is the leading cause of ICU-acquired infection and is responsible for prolonged ICU stay, increased mortality and costs. Probiotics, and especially Lactobacillus Rhamnosus GG ('LGG'), have been demonstrated to possess beneficial effects in terms of intestine flora imbalance and immune response.