The goal of this clinical trial is to : 1. Test whether the combination of CAZ±AVI and FOS leads to superior clinical outcomes in patients with severe suspected Gram-negative bacterial infection, as demonstrated by a shorter time to normalization of quick Sequential Organ Failure Assessment (qSOFA) parameters and reduction in C-reactive protein (CRP) compared to standard of care (CAZ±AVI alone) 2. to further characterize the safety profile of this combination therapy In the presence of such an infection, standard treatment is to start intravenous antibiotic therapy. Ceftazidime (Fortam®), or ceftazidime-avibactam (Zavicefta®), is a standard antibiotic commonly used for this purpose (for very resistant bacteria, the compound avibactam is included to protect ceftazidime from being broken down by bacteria). Researchers will compare ceftazidime (or ceftazdime-avibactam) plus fosfomycin versus ceftazidime alone with placebo (i.e. a look-alike substance given with no drug) to examine whether the addition of another intravenous antibiotic, fosfomycin, would lead to faster recovery from severe infection and evaluate the tolerability of this combination. Participants will: * Be randomly assigned to either the intervention group or the control group. * The intervention group will receive ceftazidime (with avibactam if multidrug resistant bacteria are suspected, per routine practice) and intravenous fosfomycin, and the control group will receive ceftazidime (with avibactam if multidrug resistant bacteria are suspected) and placebo. * Either treatment will be given for 3 days, three times daily (or less frequently, depending on renal function). * Provide a few extra blood samples (for CRP measurement, a protein marker of inflammation) during the first few days of the study. * Be contacted in the hospital or by phone if discharged on day 28 to be asked a few questions.
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Time in hours from first study-medication administration to normalization of all qSOFA parameters and decrease of peak CRP by 50%.
Timeframe: From date of randomization until the date of first documented normalization of qSOFA and 50% decrease in CRP. We estimate median time to be of approximately 108h and 96h in the control and intervention groups, respectively (approx 4 to 5 days)