Evaluation of an SOS-Chromotest-based approach for the isolation and detection of sediment-associated genotoxins

A series of experiments was conducted to evaluate an approach advanced by the St. Lawrence Centre (SLC) of Environment Canada for assessing the genotoxic potential of sediments. The SLC method entails the extraction, isolation and solvent exchange of the organic constituents in sediment, and the testing of these solubilized extracts with the SOS Chromotest (Escherichia coli PQ37). A total of five sediments, three variously contaminated by organic compounds and two reference materials certified for persistent organic chemicals, were Soxhlet-extracted. Each of the five extracts was then split, with a portion remaining in crude form and another portion fractionated into two molecular-weight classes of organic contaminants, thus yielding a total of 15 extract samples. The ability of the SOS Chromotest to detect genotoxins in the various organic extracts was evaluated and compared with that of the Ames Fluctuation Assay (Salmonella typhimurium, strain TA100). The intra-laboratory variance associated with the SOS Chromotest was also assessed. Procedural details are presented and results are discussed. The SOS Chromotest results were in good agreement with those of the Ames Fluctuation Assay, especially after metabolic activation. However, the E. coli PQ37 system was slightly more sensitive than the Salmonella assay for detecting genotoxins in the sediment extracts. The SOS Chromotest was also the most discriminating of the two assays, generating SOS-induction factors that were consistent with the organic contamination gradient reported in the sediment samples. The removal of macromolecules from the dichloromethane extracts by size-exclusion chromatography prior to testing enhanced the sensitivity of both test systems. The intra-laboratory variance of the SOS Chromotest ranged from 0.24% to 23.82%, depending on the extract sample. As applied in this study, the SOS Chromotest can serve as a sensitive test for screening the genotoxic potential of uncharacterized sediment extracts. A more sensitive assay would be appropriate, however, as a confirmation for definitive investigations, especially for the detection of direct-acting genotoxins.