MICREDOX®, a research platform developed at Lincoln Technology, is a rapid, mediated assay for monitoring cellular respiration. The MICREDOX® assay accelerates the microbial oxidation of organic substrates by using a synthetic redox-active mediator, instead of oxygen, as the terminal electron acceptor in the cellular respiratory biochemical pathways. This substitution increases the rate of substrate degradation because the surplus of mediator added to the incubation allows the concentration of biocomponent to be boosted, which in turn increases the rate of substrate oxidation. An analytical signal that can be measured electrochemically is produced (Pasco et al., 2000). The method has been modified to provide a rapid direct toxicity assessment (DTA) assay (Pasco et al., 2001; Pasco et al., 2002; Webber et al., 2002) that can provide a toxicity assessment in less than one hour with early results indicating that effects are seen within 15 minutes.

Environmental monitoring is essential to safeguard communities from the accidental or deliberate release of contaminants into water. There is a growing demand for fast, portable and cost-effective monitoring methods that measure toxic biological effects. One of the new analytical tools with the potential to fulfil these analytical needs are DTA biosensors. DTA techniques seek to measure the impact, or effect, of toxic chemicals on biological materials resident in the environment, rather than provide an analysis of what toxic compounds are present.

When MICREDOX® is operating in toxicity mode, the signal produced by healthy cells is compared to that of cells subjected to a fixed level of toxin. Microbial respiration is compromised in the presence of toxin, thereby decreasing the analytical signal, and the ratio of electrochemical signal recorded in the presence, relative to the absence, of toxin provides an index of inhibition. Appropriate controls provide an upper and lower signal for the response of cells unchallenged by toxin.

The temporal variation in limiting currents recorded for the bacteria Bacillus subtilis spiked with various concentrations of 1-3 dichlorophenol (DCP) are shown below.

The toxic impact of the DCP is clearly well established by 15 minutes and suggests that a measure of toxicity could be made in an even shorter timeframe. This application of the MICREDOX® platform is being further evaluated to assess performance in measuring a range of common toxicants, relative to other methods.

The Lincoln Technology Biosensor Group is actively collaborating with Dr Richard John's research group at Griffith University, Gold Coast, Australia and Professor Ravi Gooneratne at Lincoln University on the toxicity application of MICREDOX®.