• Characterisation of the combined effects of physicochemical parameters and toxicants on microbial cells

      Bhatia, Radhika (University of BedfordshireUniversity of Bedfordshire, 2005-02)
      This thesis reports on the combined effects of toxicants and physicochemical factors on micro-organisms. The main objective of the project was to use multi-sensing systems such as mediated and non-mediated sensor systems, growth tests and physicochemical sensors to investigate novel stressor-toxicant-assay combinations. Screen-printed, disposable, developmental-phase, physicochemical sensor constructs (conductivity and dissolved oxygen) were validated under conditions compatible with microbial bioassays, to ascertain their potential role in toxicity testing. The conductivity sensor construct could be used to indirectly inform on the osmolality of the test samples, but the dissolved oxygen sensor construct was not found to give reproducible results. The results were thought to be compromised by in-house screen-printing using a complex carbon ink formulation for the working electrode. Escherichia coli and a consortium with ammonia oxidation capacity (CAOC) were used as the test species for the bioassays. The combined effects of four inorganic salts (NaCl, NaN03, KCl and KN03) and two toxicants (3,5-DCP and HgCh) on E. coli were investigated using the CellSense™ biosensor system, Clark oxygen electrode and microtitre plate growth assays. A variety of trends were observed with each salt-toxicantbioassay combination, emphasising a need for better understanding of the assay media and factors such as bioavailability, to interpret the toxicity data. The results also suggested the importance of using multiple bioassays with varied end points, for toxicity testing. The CAOC, which was isolated from the activated sludge, was tested for physicochemical stressor and toxicant effects using the mediated biosensors. The results were very different from those obtained with E. coli, indicating that each species reacts to toxicants and changes in physicochemical factors differently. Although the full potential of disposable, physicochemical sensors, at the point of toxicity testing was not achieved, the study did investigate previously uncharacterised, combined effects of salts and toxicants on microbial cells. It highlighted the need for development of hybrid systems and also offered a route towards integration of physicochemical and biological sensing systems for simultaneous monitoring of both environmental and biological elements.