Ventilator associated pneumonia (VAP) is one of the most commonly seen hospital-acquired infections that occurs in patients who receive mechanical ventilation. VAP usually occurs 48-72 hours after mechanical ventilation and is related increased incidence of multidrug-resistant infections, increased antibiotic use, extended mechanical ventilation time, increased patient stay in the intensive care unit (ICU), and increased patient stay in the hospital. One recent intervention is the probiotic therapy which is a nonantibiotic strategy for maintenance of the host's aerodigestive microbial balance and VAP prevention.Common causative pathogens of VAP include Gram negative bacteria such as Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter species, and Gram-positive bacteria such as Staphylococcus aureus. In this study, these Gram negative pathogens obtained from mechanical ventilated patients' tracheal aspiration culture and reference bacteria strains will be compared with Lactobacillus (LAB) members in the laboratory. Probiotics' antimicrobial properties and effects on these bacteria's (obtained from patients and reference strains) growth rate will be investigated.