Dielectrophoretic analysis of microbes in water

Traditional microbiological methods are still used extensively for analysis of micro-organisms in water. However, they are inefficient due to a high l abour input requirement, a low sample capacity, and often a long time lag b efore results are available. Analytical stages involving incubation and gro wth (enrichments and colony isolation) contribute the greatest delay in rep orting, although subsequent identification can also be protracted. The use of electrometric growth analysers (measuring impedance, conductance or capacitance changes) is now more common in water microbiology. Although these instruments can provide more rapid results and provide increased han dling capacity, the bacterial generation times required to provide detectab le changes cause delays and suitable selective media are not fully develope d for all microbes of interest. Most other recent methods have equally disa ppointing drawbacks and thus extensive research continues in order to reali se the ambition of 'real-time' analytical microbiology. Several research groups have demonstrated the potential of dielectrophoresi s in providing microbial concentration, separation and identification syste ms which are not limited by bacterial growth and are therefore extremely; r apid. Dielectrophoresis occurs when cells are placed in nonuniform electric field s. The cells move towards the electrodes (regardless of the direction of th e applied field) as determined by their dielectric properties (conductivity and permittivity) rather than by their charge as occurs in electrophoresis . Also, the polarisability of the cells, and therefore the polarity and mag nitude of the dielectrophoretic force, varies as a function of the electric field frequency. Because the dielectric properties of a particular cell ty pe have characteristic frequency-dependent components, if cell collection a t electrodes is observed across a frequency range, the collection spectrum produced is distinctive for the cell type under investigation. This can be exploited for analytical and separation applications in microbiology. This paper will describe rapid analytical techniques based on electrokineti c phenomena under research and development at York. These include dielectro phoretic enrichment, concentration and characterisation systems for the ana lysis of water bacteria and protozoa.