Inhibitory effects of combinations of oxytetracycline, dimethyl sulfoxide, and EDTA-tromethamine on Escherichia coli.

Antibacterial activity against Escherichia coli was obtained with subminimal inhibitory concentrations of oxytetracycline (OTC) and EDTA-tromethamine. Inhibitory effects were not observed using combinations of dimethyl sulfoxide and OTC or dimethyl sulfoxide and EDTA-tromethamine. Neither EDTA-tromethamine nor OTC used alone was capable of the same degree of inhibition. Using a 2-dimensional Microtiter checkerboard technique, the inhibitory activity of these combinations was studied and isobolograms were plotted. A synergistic effect was seen with combinations of OTC and EDTA-tromethamine. Kinetic studies of microbial death, using subminimal inhibitory concentrations of these agents, confirmed these findings.