Bactericidal effect of combinations of nalidixic acid and various antibiotics on Enterobacteriaceae

The activity of nalidixic acid combined with each of 10 other antimicrobial agents on 95 strains of Enterobacteriaceae was studied. Synergism was found less often than antagonism, and the commonest outcome was indifference. Combinations of nalidixic acid with kanamycin, gentamicin, or colistin were more often synergistic than antagonistic, whereas with the other antibiotics the combination was more often antagonistic than synergistic. Synergistic effects were more common with Shigella than with other genera. In vitro examination for synergism or antagonism appears to be advisable before nalidixic acid is used therapeutically in combination with other antimicrobial agents.