Predicting Ventilator-associated Lower Respiratory Tract Infection Outcomes Using Sequenced-based Early Microbiological Response

Inclusion Criteria: * Aged 18 years and above. * Previously relied on mechanical ventilation (endotracheal intubation or tracheotomy) for breathing assistance, and the duration of mechanical ventilation was more than 48h. * Lower respiratory tract infection based on at least two of the followings: abnormal temperature (body temperature greater than 38.5°C or less than 36.5°C), leucocyte count abnormality (leucocyte count greater than 12\*10\^9/L or less than 4\*10\^9/L), and the presence of purulent tracheal secretions. Exclusion Criteria: * Bronchoscopy and respiratory specimen collection were not performed at screening (Day 1) and after 3 days of treatment (Day 4). * Refusal of patients or families to participate in the study * After initial screening, bronchoscopy was performed to obtain BALF for bacterial culture. The results of the culture showed no evidence of infection by study-associated lower respiratory pathogens. Note: The evidence of infection was defined as a single positive bacterial culture (pathogen quantification ≥10\^4 cfu/ml or "++" and more) on Day1. And the study-associated causative pathogens are Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, and Staphylococcus aureus. Additionally, the included patient must have single infection with one of these pathogens.