Applications of a new optical technique for measuring the dielectrophoretic behaviour of micro-organisms.

It is shown that the dielectrophoretic behaviour (motion in non-uniform a.c. electric fields) of micro-organisms can conveniently and reproducibly be measured by monitoring the decrease in optical absorbance of a cell suspension as the cells are collected at a micro-electrode array. The dielectrophoretic behaviour, as a function of the frequency of the applied electric field and conductivity of the supporting solution, can be determined more quantitatively and rapidly than by methods so far described in the literature. Results are presented for Micrococcus lysodeikticus, Bacillus subtilis and Escherichia coli for the frequency range 20 Hz to 4 MHz and theoretical considerations are presented for the effect of solution conductivity. A value of 0.2 S/m has been derived for the effective conductivity of M. lysodeikticus.