Summary: A recently discovered antibiotic (CDA; calcium-dependent antibiotic) of A3(2) was found to be effective against a wide range of Gram-positive bacteria only in the presence of calcium ions. Producer and non-producer strains were identified and several media tested for their ability to support antibiotic production. The action of calcium was not simulated by any of the other cations tested. The antibiotic was found to induce discrete conductance fluctuations in planar lipid bilayer consistent with a channel-forming action. The electrical potential difference caused by a concentration difference of various salts across the CDA-containing bilayer, showed the channel to be cation-selective but of a size that discriminated against tetramethyl ammonium and choline ions. The data indicate that the antibiotic activity of CDA is due to its action as a calcium-dependent ionophore.


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