Synergism between amikacin and cefazolin against Klebsiella: in vitro studies and effect on the bactericidal activity of serum.

Synergism between amikacin and cefazolin was studied in vitro in 20 strains of Klebsiella isolated from clinical material. Several techniques for the demonstration of synergism in vitro were employed. The checkerboard technique with use of bactericidal end points and the killing curves method showed synergism between amikacin and cefazolin for 13 and 15 strains, respectively. The two techniques were in accordance in 12 instances. The data obtained in vitro were correlated with the bactericidal activity in sera from 10 subjects who had received the usual doses of amikacin and cefazolin, alone or in combination. The sera of subjects who received amikacin plus cefazolin were bactericidal at significantly higher dilutions than sera of patients who received cefazolin alone. The increased serum bactericidal activity in subjects receiving amikacin plus cefazolin was observed only for strains against which these antibiotics were synergistic in vitro.