Primary objective of this study is to measure the change of concentration of faecal bifidobacteria between critically ill patients who receive enteral formula with and without prebiotics during enteral nutrition.Our null hypothesis is that there is no difference in the concentration of faecal bifidobacteria between critically ill patients who receive enteral formula with and without prebiotics during enteral nutrition (EN). Three faecal samples will be taken from the patient. First faecal sample is the first stool after initiation of EN and second sample is taken seven days after the initial sample and the third faecal sample is taken at day 14 after initial sample. Patient will be randomized to receive either of described formula after baseline (first) stool sample is obtained. Patient will be monitored up to 14 days after the initial stool sample is obtained.