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The serum bactericidal activities of ceftriaxone, netilmicin, piperacillin and the combinations of ceftriaxone with each of the two other antibiotics were compared 1, 4 and 24 h after i.v. infusion in six volunteers. One hundred and one clinical isolates were used, including Staphylococcus aureus, Pseudomonas aeruginosa and various Enterobacteriaceae. The highest bactericidal titers were found against the Enterobacteriaceae, geometric means of 1:741 and 1:851 being obtained for ceftriaxone/netilmicin and ceftriaxone/piperacillin respectively. Against Staphylococcus aureus the geometric mean bactericidal titers of ceftriaxone/piperacillin (1:105) were markedly higher than ceftriaxone/netilmicin (1:35). Low bactericidal activity was exhibited by all drugs and combinations tested against Pseudomonas aeruginosa; a geometric mean bactericidal titer of 1:4.6 was achieved. The serum bactericidal activity of the double beta-lactam combinations was found to be at least equal to that of the combination containing a cephalosporin and an aminoglycoside.

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