After incubation at 37oC in a 5% CO2 incubator for 24 hours, a colony count was done and expressed as number of colony forming units per ml (CFU/ml). The microorganisms isolated at a concentration of more than 105 CFU/ ml were considered as VAP pathogens and were identified based on standard bacteriological procedures including Gram’s stain, colony morphology on blood agar and Mac Conkey agar, and biochemical reactions .