The duration of MV and the prior exposure to antimicrobials significantly influence the distribution patterns of etiologic agents. In early onset VAP (<5 days), methicillinsensitive S. aureus, Streptococcus pneumoniae, and Haemophilus influenzae are the most common pathogens, whereas methicillin-resistant S. aureus (MRSA), P. aeruginosa, Acinetobacter baumannii and Stenotrophomonas maltophilia are more frequent in late onset VAP (≥ 5 days) [49, 50] . Also, MRSA, P. aeruginosa, A. baumannii and the other multi-resistant gram negative pathogens are the most common pathogens in the patients expose to prior antibiotic.