Characterization of an agar component which influences antibiotic activity

Bacto agar reversed the effect of basic antibiotics while acidic and amphoteric antibiotic were not markedly affected. The reversal of activity was shown to be due to the presence of a diffusable, non-dialyzable sulfated polyanionic carbohydrate. This substance was shown to bind cationic dyes and form insoluble complexes with some basic antibiotics. Antibiotic activity was not markedly affected by the pore size of the agar or by the presence of diffusable, physiologically active substances in the agar. Such of the antagonism of Bacto agar was removed by precipitation of the agar with ethanol or by extraction of polymerized agar with ammonium hydroxide. It is possible that the ammonium hydroxide extraction may be commercially feasible and will yield an agar product suitable for the evaluation of antibiotic activity.