Ventilator-associated pneumonia is the leading cause of nosocomial infection in the ICU. The pathogens responsible are multiple, but enterobacteria constitute a major source of pathogens involved. Within this family, Klebsiella spp. and Escherichia coli are the two most frequent genera, with Klebsiella spp. often present in severe forms. The factors associated with the occurrence of Ventilator-associated pneumonia and its adverse course depend on host defenses and the virulence of the pathogen. The virulence of Klebsiella spp. depends on several structures, notably the presence of a capsule, the particularities of its lipopolysaccharide, its adhesins (type 3 fimbriae), its capacity to capture iron (siderophores). The objective of this work is to evaluate the role of these different virulence mechanisms in the evolution of Ventilator-associated pneumonia and the hospital prognosis.