Microbial diffraction gratings as optical detectors for heavy metal pollutants

Abstract

As a significant industrial pollutant, cadmium is implicated as the cause of itai-itai disease. For biological detection of cadmium toxicity, an assay device has been developed using the motile response of the protozoa species, Tetrahymena pyriformis. This mobile protozoa measures 50 μm in diameter, swims at 10 body lengths per second, and aggregates into macroscopically visible patterns at high organism concentrations. The assay demonstrates a Cd+2 sensitivity better than 1 μM and a toxicity threshold to 5 μM, thus encouraging the study of these microbial cultures as viable pollution detectors. Using two-dimensional diffraction patterns within a Tetrahymena culture, the scattered light intensity varies with different organism densities (population counts). The resulting density profile correlates strongly with the toxic effects at very low dosages for cadmium (<5 ppm) and then for poison protection directly (with nickel and copper antagonists competing with cadmium absorption). In particular, copper dosages as low as 0.1–0.5 mM Cu have shown protective antagonism against cadmium, have enhanced density variability for cultures containing 1 mM Cd+2, and therefore have demonstrated the sensitivity of the optical detection system. In this way, such microbial diffraction patterns give a responsive optical measure of biological culture changes and toxicity determination in aqueous samples of heavy metals and industrial pollutants.

Analysis of Freely Swimming C. elegans Using Laser Diffraction

“…Many modern imaging methods such as photothermal microscopy [12,13] and 4Pi-STED Microscopy [14,15] require species to be constrained to two-dimensional focal planes with a typical depth of field, DoF, on the order of micrometers [16] since the DoF is inversely related to the magnification m. In addition, the f-number N has a reciprocal relationship with the circle of confusion (c) so that a short focal length is associated with a large c [17,18]:…”