The purpose of this study is to assess the effect of bacterial lysates on respiratory tract microecology in patients with mechanical ventilation and the efficacy of prevention and treatment of ventilator associated pneumonia.

After being informed about the study and potential risks, all patients giving written informed consent will undergo a one-week screening period to determine eligibility for study entry. At week 0, patients who meet the eligibility requirements will be randomized in a double-blind manner in a 1:1 ratio to Bacterial Lysates( 14mg, once a day) or placebo(once a day).

Inclusion Criteria:

• Patients admitted to ICU for mechanical ventilation
• The expected mechanical ventilation time is more than 72h
• Sign the informed consent form when conscious and voluntary, and complete the questionnaire survey and follow-up as required; If it is unable to sign the informed consent form and complete the questionnaire survey and follow-up, the legal representative or guardian can act on behalf of it.

Exclusion Criteria:

• Ventilator associated pneumonia is known and confirmed
• Pregnant or lactating female
• Allergic to the active ingredients of bacterial lysates or any excipients listed in the ingredients
• Patients with autoimmune diseases
• Patients with acute intestinal infection
• Patients participating in other clinical studies at the same time
• Patients considered unsuitable by other researchers to participate in this study