Synergism between amikacin and cefazolin against Staphylococcus aureus: a comparative study of oxacillin-sensitive and oxacillin-resistant strains

Synergism between aniikacin and cefazolin was studied in vitro on 9 oxacillinresistant and 10 oxacillin-sensitive isolates of Staphylococcus aureus. Minimal bactericidal concentrations of amikacin and cefazolin alone or combined in a fixed proportion were determined. The results were compared with those obtained from the study of bacterial killing curves using the same drugs alone or in combination and in concentrations similar to those achieved in sera of patients treated with usual doses of these antibiotics. These two techniques demonstrated a synergistic activity of amikacin and cefazolin against most oxacillin-resistant strains. As far as oxacillin-sensitive strains are concerned, a synergistic effect was seen only during the early part of the bactericidal process. The bactericidal activity of the sera of 5 patients who were given usual doses of amikacin and cefazolin alone or in combination was also measured against the 19 strains. For the oxacillin-resistant strains, the bactericidal activity of sera containing both drugs was significantly higher than that of sera containing cefazolin alone. In contrast, the oxacillin-sensitive strains were equally susceptible to sera containing cefazolin alone or in combination with amikacin. These in vitro data suggest that a combination of a cephalosporin with an aminogiycoside might be useful for the treatment of infections caused by oxacillin-resistant staphylococci.