Our study aims to evaluate the relationship between the heterogeneity of pulmonary microbiota and clinical and outcome variables among critically ill patients admitted to the intensive care unit (ICU). In patients undergoing invasive mechanical ventilation, an aliquot of bronchoalveolar lavage (BAL) fluid will be used in the microbiology laboratory for the analysis of respiratory microbiota through next-generation sequencing technologies and validate computational techniques.

Traditionally, microbiological investigations and clinical trials have contributed to the definition of lower airways as a physiologically sterile district, whose microbiological balance is altered when a respiratory infectious process occur. Actually, the introduction of molecular study methods aiming at the identification of pathogens through genomic sequencing questioned the pardigm of "one bug-one disease", according to which we usually tend to consider a bronchial or pulmonary infectious event as due to the pathogenic role of a single exogenous microorganism. In such a contest, there are truly few data dealing with the characterization of respiratory microbiota in human BAL as well as with the major determinants of this phenomenon and the possible impact on clinical and microbiological outcomes. Our study, although it's a pilot one, aims to evaluate these aspects in a larger cohort of critically ill patients, observing the relationship between the heterogeneity of pulmonary microbiota and clinical and outcome variables. In patients undergoing invasive mechanical ventilation, an aliquot of BAL fluid will be used in the microbiology laboratory for the analysis of respiratory microbiota through next-generation sequencing technologies and validate computational techniques. For each enrolled patient, we will register demographic, clinical and laboratory variables. The benefits deriving from this study lay in the possibility of improving the understanding of characteristics of critical patient's pulmonary microbioma and its clinical impact. Such an information meets the increasingly topical need to customize medical interventions, especially in the context of critically ill patients.

Inclusion Criteria:

• execution of bronchoalveolar lavage sampling, either for clinical indications or in the context of surveillance programs
• acquisition of an informed consent

Exclusion Criteria:

• presence of significant coagulation abnormalities and/or severe respiratory failure
• clearly bloody BAL sample
• small quantity of BAL sample (<5 ml)

All patients admitted to the ICUs and undergoing invasive mechanical ventilation (through oro-tracheal tube or tracheotomy)