The primary objective of the study is to demonstrate that colistin is not inferior to meropenem in the empirical treatment of VAP regarding the primary endpoint (mortality at 28 days of follow-up). As secondary endpoints,clinical healing for intention to treat in clinically evaluable patients and comparison of microbiological efficacy are considered for both treatments arms. The safety of treatment with colistin compared to meropenem in VAP will be followed for all patients.