We are using a tool called QtNGS (quantitative targeted amplicon-based next-generation sequencing ) to measure the abundance of local pathogens in patients with ventilator-associated lower respiratory tract infections. We hypothesize that changes in pathogen abundance before and after treatment are related to patient outcomes. This study aims to evaluate the effectiveness of the tool by analyzing the changes in pathogen abundance and exploring the relationship between these changes and clinical outcomes.

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.

The study population description specifies individuals diagnosed with Va-LRTI (Ventilator-associated Lower Respiratory Tract Infection) who were enrolled based on culture-confirmed pathogens, including Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, and Staphylococcus aureus.