Abstract: OBJECTIVE To explore the correlation between bacteria in ventilator circuits and time of lower respiratory tract infections so as to provide guidance for prevention of ventilator-associated pneumonia (VAP).METHODS A total of 122 patients with type Ⅱ respiratory failure who were hospitalized for mechanical ventilation in the intensive care unit (ICU) from Jun 2015 to Jun 2016 were enrolled in the study.The specimens were collected from lower respiratory tract secretions, humidifier, condensation water collectors of air inlet and outlet, and interior walls of outlet tube for bacterial culture after the ventilators were used for different time periods.The growth of bacteria in different sites was observed; mechanical ventilation duration, length of ICU stay, and incidence of VAP were recorded.RESULTS Totally 1 898 specimens were collected from the 122 mechanical ventilation patients.The rates of bacterial growth in humidifier, condensation water collectors of air inlet and outlet, and interior walls of outlet tube were positively correlated with the mechanical ventilation duration (r=0.8545,0.9636,0.9636,0.9394; P＜0.05).The bacteria emerged earliest in condensation water collectors of air outlet, latest in humidifier.Gram-negative bacteria were dominant among the bacteria that were cultured from the various sites and lower respiratory tract secretions.Of all the patients, 19 had VAP, with the incidence of VAP 15.57%; the incidence of VAP increased with the mechanical ventilation duration, and there was significant difference in the incidence of VAP between the patients with mechanical ventilation duration less than 5 days and the patients with mechanical ventilation duration more than 7 days (P＜0.05).Among the 19 patients with VAP, The consistencies of condensation water collectors of air outlet, interior walls of outlet tube, condensation water collectors of air inlet, and humidifiers with bacterial culture of lower respiratory tract secretions were 66.48%, 63.45%, 40.26%, and 54.32%,respectively.The time of occurrence of bacteria in air inlet of condensation water collectors and humidifier was basically same with the time of occurrence of pathogens in lower respiratory tract.CONCLUSIONThe bacterial contamination of ventilator circuits and incidence of VAP are associated with the mechanical ventilation duration, the bacterial contamination of ventilator circuits may be one of the causes of VAP, and regular replacement of ventilator circuits is recommended.