This is known as the "gastropulmonary hypothesis" and it postulates the following sequence. First, the stomach is colonized by potentially pathogenic microorganisms, either from an exogenous source (contaminated liquid injected into a nasogastric tube), or from an endogenous source (Detection of gastrointestinal complications in mechanically ventilated critically ill patients and its relation to oropharyngeal and respiratory infections in relation to oropharyngeal and gastric PH duodenogastric reflux). This is followed by retrograde colonization of the oropharynx . Finally, the lower respiratory tract is colonized from sustained microaspiration of contaminated oropharyngeal (or gastric) secretions around the endotracheal tube cuff.