Toxicity testing of hydrophobic compounds with low aqueous solubility remains challenging. Dimethyl sulfoxide (DMSO) is widely used as a co-solvent for toxicity testing of hydrophobic chemicals, but it may modulate chemical toxicity patterns. In this study, we critically evaluated the suitability of DMSO as a co-solvent for toxicity testing of hydrophobic organic compounds in aqueous solutions. As the toxicity measure, we used growth inhibition of a natural bacterial community, and the test toxicants included phenol, BTEX (benzene, toluene, ethylbenzene and xylene) and transformation products of polycyclic aromatic hydrocarbons (PAHs). We found that dose-response curves for phenol were unaffected by DMSO concentrations up to 10% (v/v) and that DMSO (5% v/v) did not affect the degree of bacterial growth inhibition for any of the other test compounds in short-term experiments (3.5 h). By contrast, marked co-solvent effects of DMSO were observed in the long-term assay (25 and 27 h). We therefore conclude that DMSO has excellent co-solvent properties for short-term (≤3.5 h) toxicity testing of sparingly water-soluble compounds and its application provides a simple, inexpensive approach for screening of various environmentally relevant hydrophobic chemicals. Importantly, the use of DMSO allows for generation of full dose-responses that may otherwise not be attained.
