The causative organisms vary according to the patients' demographics in the ICU, methods of diagnosis, the durations of hospital and ICU stays, and the antibiotic policy. Gram negative bacteria are the most common pathogens cause VAP in several studies [7, 17, 44] . In NNIS data, although the most frequent reported isolate was Staphylococcus aureus (17%), 59% of all reported isolates were gram-negative. The most common gram-negative species were Pseudomonas aeruginosa (15.6%), Enterobacter species (10.9%), and Klebsiella pneumoniae (7.0%) (24) . In recent years, gram-positive bacteria have become more common in ICU and in the EPIC study, S. aureus accounted for 31% of the 836 cases with identified microorganisms (46) . NNIS data from medical ICU, also, reported high percentage (20%) of S. aureus [25] . Polymicrobial infection rate is usually high in VAP [12, 17, 47, 48] .