Besides the IMPs (colistin and meropenem) all patients will receive levofloxacin is considered non IMPs so that it is provided by the study pharmacy according the available product approved by its own providers (dose of 500 mg every 12 hours). According to investigator criteria, vancomycin or linezolid are even permitted as empiric treatment if methicillin-resistant Staphylococcus aureus is suspected. Any antibiotic prescribed in the 14 days previous to randomization, and any other antibiotic received as concomitant medication (described inList of concomitant medication) once the laboratory results are obtained, will be gathered in the eCRF.