Inclusion Criteria:  age ≥ 18 years; Pneumonia with undetermined pathogen and lower respiratory tract specimens can be obtained ; signed informed consent; the expected length of staying in ICU is more than 3 days  Exclusion Criteria:  pregnant women lactating women Those who specimens of lower respiratory tract cannot be obtained; Those who have submitted for other microbiological examination within 72 hours before enrollment; The main responsibility of infection was not in the lung, but outside the lung; Clinical diagnosis of non-bacterial pneumonia, such as Pneumocystis carinii pneumonia, viral pneumonia and fungal pneumonia; Those who are estimated to die or give up treatment within 72 hours; patients have participated in other clinical studies.