Which of the following is the most common pathogens responsible for nosocomial pneumonias in the ICU?

Most common organisms causing nosocomial pneumonia are gram negative bacteria in 50-70% cases, which reach the lower airways by aspiration of gastric contents or by microaspiration of upper airway secretions. Most common organism is pseudomonas aeruginosa followed by enterobacteriacea. Staphylococci aureus is second to gram negative bacteria and accounts for 15 -30%. Anaerobic bacteria is found in 10-30% cases. Ref: Infectious Diseases By Sherwood L. Gorbach, 3rd Edition, Page 477