Cross-over cluster randomization : an individual randomization is almost impossible to achieve, because of the emergency setting of any decision of tracheal intubation in the ICU. Therefore we chose to ascribe random periods during which every tracheal tubes of the ICU will be those of the study protocol for the given period. The random plan will be different in each participating ICU. Each ICU will have to perform periods of 10 patients. Each ICU intend to recruit 10 patients in periods of around 2 months. The physicians in charge of the patients will be kept blind of the bacteriological samples results. This will prevent them to modify their clinical practices according to these results and particularly to influence their clinical suspicion of VAP and/or stridor.