The present invention provides for methods of treating a Gram negative bacterial infection comprising a co-administration regimen of an effective amount of fosfomycin together with at least one antimicrobial agent selected from the group consisting of piperacillin-tazobactam, ceftazidime and meropenem to an infected subject. The present invention further provides for a method of treating a subject with a bacterial infection that includes infection with a "resistant" mutant subpopulation selected from the group consisting of Staphylococcus aureus, Enterococcus faecalis, Pseudomonas aeruginosa, Acinetobacter baumannii and E. coli, the method comprising (a) obtaining a sample from a subject suffering from a bacterial infection； (b) identifying the presence of the "resistant" mutant subpopulation in said sample； and (c) co-administering fosfomycin and at least one antimicrobial agent to the subject, wherein after the co-administration, the bacterial density is effectively reduced and the "resistant" mutant subpopulation is inhibited.
