Patients who mechanically ventilated at least 48 hours will be included. Patients' tracheal aspirate cultures will be used for study. Pseudomonas aeruginosa, Acinetobacter baumanii, Staphylococcus aureus and Klebsiella pneumoniae will be isolated from tracheal aspiration cultures.Antimicrobial effects of Lactobacillus members will be investigated on the bacteria's growth rate in the laboratory.