Stimulating effects in bacterial toxicity tests—A source of error in evaluating the risk posed by chemicals

Abstract

In some cases stimulating effects—if not duly taken into consideration—lead to wrong toxicity assessments. In swarming inhibition assays, cells of Serratia marcescens showed a stimulation at low imugan concentrations. The fungicide cyclafuramid enhanced the swarming activity of Serratia marcescens Sm6 up to 10% at a concentration of about 0.05 mg/l and to 20% at 5 mg/l—thus exhibiting variations in swarming activity depending on the fungicide concentration. These phenomena were observed with different test substances and test organisms and seem to be caused by specific regulation processes of the respective microorganism. By using the herbicide aretit the toxic effect versus various physiological reactions of Serratia marcescens was comparatively studied. In prodigiosin synthesis and heat production stimulating effects could be found; swarming activity and ATP production showed a plateau in the activity-concentration graph, indicating stimulating effects as well. In contrast, growth inhibition of Serratia marcescens showed a normal dose-response relationship. Different reasons for the explanation of the stimulating phenomena are discussed.