Toxicity of three phenolic compounds and their mixtures on the gram-positive bacteria Bacillus subtilis in the aquatic environment

Although phenolic compounds are intensively studied for their toxic effects on the environment, the toxicity of catechol, resorcinol and hydroquinone mixtures are still not well understood because most previous bioassays are conducted solely using single compound based on acute tests. In this work, the adverse effect of individual phenolic compounds (catechol, resorcinol and hydroquinone) and the interactive effect of the binary and tertiary mixtures on Bacillus subtilis (B. subtilis) using microcalorimetric method were examined. The toxicity of individual phenolic compounds follows the order catechol > resorcinol > hydroquinone with their respective half inhibitory concentration as 437, 728 and 934 µg mL−1. The power–time curve of B. subtilis growth obtained by microcalorimetry is in complete agreement with the change in turbidity of B. subtilis against time, demonstrating that microcalorimetric method agrees well with the routine microbiological method. The toxicity data obtained from phenolic compound mixtures show that catechol and hydroquinone mixture possess synergistic effect while the other mixtures display additive joint actions. Furthermore, the concentration addition (CA) and independent action (IA) models were employed to predict the toxicities of the phenolic compounds. The experimental results of microcalorimetry show no significant difference on the toxicity of the phenolic compound mixtures from that predicted by CA. However, IA prediction underestimated the mixture effects in all the experiments.