A modified SOS-Chromotest procedure to test for genotoxicity and cytotoxicity in sediments directly without extraction

A modified SOS-Chromotest bioassay using a chromogenic pad (pad procedure) was developed to test for genotoxicity in sediments directly without extraction. This test is based on the synthesis of β-galactosidase enzyme by a genetically-engineered E. coli strain PQ37. In the bioassay, an exponential growth phase antibiotic-containing culture of the test bacterium is introduced into a series of tubes with the first tube containing 0.1 gram of sediment. Serial dilutions are then made and the tubes of sediment plus bacterial culture are incubated at 37°C for four hours, followed by placing a drop of each mixture on a chromogenic pad and additional incubation for 20 hours at 37°C. The solid particulates are then washed off with tap water and positive (genotoxic) activity is noted by the presence of a distinctive blue colour on the pad. The SOS-Chromotest pad procedure may be best used as a relative measure of genotoxicity by comparing results to a reference sample. In addition it can also determine sediment cytotoxicity by comparing samples spiked with a genotoxic standard (i.e., 4-nitroquinoline-N-oxide). Preliminary results suggest that this new bioassay is highly sensitive, consistent and discriminating.