The effect of prior antibiotic therapy is still controversial. Sirvent et al. [40] reported that a short course of cephalosporin prophylaxis was associated with a lower rate of VAP in patients with structural coma. Also other investigators showed that antibiotics administered during the first days, reduced the risk of early-onset ventilator associated pneumonia [22, 37] . However, prior antibiotic exposure predisposes patients to subsequent colonization and infection with resistant pathogens [28, 41] .