The primary objective of this study is to demonstrate a low rate of emergence of antibiotic resistance in P. aeruginosa and Acinetobacter spp during the treatment of hospitalized patients with pneumonia requiring mechanical ventilation treated with PD optimized meropenem administered as a prolonged infusion in combination with a parenteral aminoglycoside plus tobramycin by inhalation (Group 1) compared to therapy with meropenem alone (Group 2 - control arm).