Neutral red based colorimetric microorganism bioassay for direct toxicity assessment of toxic chemicals in water

In this work, we develop a reconfigured colorimetric bioassay for direct toxicity assessment (DTA) of toxic chemicals in water based on neutral red (NR) as a colorimetric indicator. Confocal laser scanning microscopy and UV-vis absorption spectroscopy are employed to prove the feasibility of the proposed assays using P. fluorescens as the model microorganism. The presence of toxicants can damage cells, and this results in an enhanced adsorption of NR by the cells. This fact provides a new strategy for DTA of toxic chemicals by comparing the absorption peak intensities of residual NR after incubation in the presence of toxicants. Four important toxicants such as 3,5-dichlorophenol (DCP), As3+, Hg2+ and Cr6+ are tested, and the toxicity order is DCP > Hg2+ > Cr6+ > As3+, which is identical with that obtained by the ferricyanide-mediated toxicity alerter (FM-tox) assay. The RC50 value of DCP is estimated to be 12.5 mg L−1, which is comparable with that of other DTA assays. The responses of E. coli and BOD seed (multi-species cultures) to DCP are further examined successfully, suggesting this NR based colorimetric assay may be a general one to measure the response of microorganisms to toxic chemicals. The low cost of NR, easy and simple operation procedure and reliable results make this assay a promising one in DTA.