In vitro combined action of minocycline with dibekacin or with sulbenicillin against clinical isolates of gram-negative bacilli

In vitro combined action of Minocycline with Dibekacin or with Sulbenicillin against 86 clinical isolates of Gram-negative bacilli was examined by checkerboard technique and by analysis of the effects on the growth curves. The combinations of Minocycline with Dibekacin and Minocycline with Sulbenicillin, showed an apparent synergistic antimicrobial action against patient strains of Escherichia coli, Klebsiella pneumoniae and Enterobacter cloacae. Generally, an increase in antimicrobial activity of Sulbenicillin was observed when it was combined with Minocycline, although on rare occasion a slightly antagonistic effect was seen in small number of strains tested. The combination of Minocycline with Dibekacin was shown to be antagonistic against patient strains of Serratia species, while the combination of Minocycline with Sulbenicillin showed a marked synergistic activity against these strains. Anti-pseudomonas activity of Minocycline became intensified when it was combined with either of the two agents, Dibekacin or Sulbenicillin. The combination of Minocycline and Sulbenicillin revealed to be synergistic against patients strains of Acinetobacter calcoaceticus among glucose-nonfermentative gram-negative bacilli. As a rule, in the strains, against which the combination was shown to be synergistic by checkerboard technique, the synergistic effect was similarly observed on the growth in liquid medium. In contrast, in the strains in which an antagonistic action was demonstrated by the checkerboard technique, an antagonistic effect on the growth curves was rarely shown, and even a slightly synergistic action was observed in a portion of the strains.