Liesegang-like rings of growth and inhibition of bacteria in agar caused by metal ions and chelating agents.

It was reported in previous studies that inhibitory concentrations of certain chelating or complexing agents did not inhibit several microorganisms when these agents were tested simultaneously (Feeney, 1951; Feeney and Nagy, 1952). Amounts of the metal-binding agents which were strongly inhibitory when tested alone were necessary in order to obtain these mutual preventions of inhibitory action. The mutual preventions did not occur when a metal ion specific for one of the complexing agents was also added. These effects were interpreted as possibly caused by the presence or absence of metal-ion imbalances induced by the complexing agents. In the present study we have attempted to delineate further the importance of relative concentrations of metal ions and chelating agents. This has been done by producing gradient zones of concentrations of ions and chelating agents by diffusion in agar media. Alternate zones or rings of growth and inhibition have been obtained by varying the concentrations of chelating agents or metal ions in agar plates and by placing filter paper discs containing different concentrations of chelating agents or metal ions on the agar and allowing diffusion to occur before incubation. These alternate zones appear to resemble the Liesegang rings obtained by allowing metal salts to diffuse through gelatin or agar (Gortner, 1944). In the case of the true Liesegang rings, however, a precipitation or chemical reaction is visible, whereas in the case of the observations reported herein, such rings remained invisible