Acute toxicity of (chloro-)catechols and (chloro-)catechol-copper combinations in Escherichia coli corresponds to their membrane toxicity in vitro

(Chloro-)catechols are toxic for bacteria and higher organisms, but the mod e of action is not yet clearly understood. We have compared the acute toxic ity of different chlorinated catechols to Escherichia coli with membrane to xic effects, namely narcosis and uncoupling that we have determined in an i n vitro assay. In vitro membrane toxicity was quantified by measuring the a ccelerated decay of the membrane potential of chromatophores isolated from Rhodabacter sphaeroides. Both acute and membrane toxicity increased with in creasing degree of chlorination. Analysis of dose-response curves, pH depen dence, and estimated membrane concentrations gave a consistent picture of t he mechanisms of membrane toxicity: At pH 7, the higher-chlorinated catecho ls acted as uncouplers of oxidative and photophosphorylation, and the lower -chlorinated catechols and catechol acted as narcotics. In the case of 3,5- dichlorocatechol and 4-monochlorocatechol at pH 8.8, both mechanisms appear ed to contribute to the overall toxicity. Copper exhibited a diverging effe ct on the toxicity of catechols and of (chloro-)catechols to E. coli. Where as the presence of copper increased the toxicity of catechol and 4-monochlo rocatechol, the toxicity of 3,5-dichlorocatechol, 3,4,5-trichlorocatechol, and tetrachlorocatechol decreased. Again, the results obtained with in vitr o assays agreed with the acute toxicity observed in E. coli: The presence o f copper accelerated decay of the membrane potential of catechol and 4-mono chlorocatechol; however, the effect was reversed by copper in experiments w ith 3,5-dichlorocatechol, 3,4,5-trichlorocatechol, and tetrachlorocatechol. We have proposed a mechanistic model to explain the diverging effects of c opper on the uncoupling activities of the different catechols.