We will conduct a Phase IIB, prospective, open-label, randomized-controlled trial in 4 major Singaporean hospitals, with balanced treatment assignments achieved by permuted block randomization, stratified by hospital. There will be 75 subjects per arm, with the subjects in the comparator arm receiving standard-dose polymyxin B while the intervention arm will receive a second antibiotic, doripenem, with polymyxin B against the bacterial isolate in question. Subjects with ventilator-associated pneumonia (VAP) will additionally receive nebulized colistin. The primary outcome is 30-day mortality while secondary outcomes include microbiological clearance, time to defervescence, and toxicity of therapy, presence of secondary infections due to new multi-drug resistant bacteria and length of ICU stay. Plasma drug levels will be measured by liquid chromatography-mass spectrometry.