Combinations of three antimicrobial agents (III): In vitro combined effect of a β-lactam, aminoglycoside and tetracycline

We examined the in vitro combined effect of 19-lactams, aminoglycosides and tetracyclines against 118 strains of clinical isolates of six different species (S. aureus, E. coli, K. pneumoniae, E. cloacae, S. marcescens and P. aeruginosa). The in vitro combined effect was determined by a three-dimensional checkerboard dilution method, a modification of the method for two antimicrobial agents, using the Dynatech MIC-2000 system. The MIC and FIC₃ index of a 1:1:1 mixture of a two-fold serial dilution of the three antimicrobial agents against the above strains were determined. Also, the rate of change of the FIC₂ index was evaluated when minocycline (MINO) was added to the checkerboard of piperacillin (PIPC) and amikacin (AMK). The value of the mean FIC₃ index of the combination of PIPC+AMK+MINO and that of cefoperazone (CPZ)+AMK+MINO against all the strains were both 0.73, and that of ceftizoxime (CZX)+AMK+MINO 0.77. Though the FIC₂ indices of PIPC+ AMK against E. coil and K. pneumoniae gradually decreased when increasing concentrations of MINO were added to the checkerboard of PIPC+AMK, those against S. marcescens and P. aeruginosa remained almost the same. From the above results, we conclude that the combination of three antimicrobial agents exhibited a potent synergistic activity against clinical isolates, especially of E. coli and K. pneumoniae, without any antagonistic action. Our data thus suggest that the above combinations are indicated against severe infections.