Factors influencing capacitance-based monitoring of microbial growth

Microbiological impedance devices are routinely used by food and manufactur ing industries, and public health agencies to measure microbiological growt h. Factors contributing to increases and decreases in capacitance at the cu lture medium-electrode interface are poorly understood. To objectively eval uate the effects of temperature, cell density and medium conductivity on ca pacitance, admittance values from an impedance device were standardized; ca pacitance was converted to susceptance to allow unit comparisons with condu ctance. Although increases in temperature increased susceptance, a linear r elationship could not be established between the change of susceptance with temperature and conductance of the medium. Cell density by itself had no m easureable effect on susceptance or conductance, indicating that cells did not impede the movement of ions in the medium or around the electrode. In a low conductivity medium, increases in conductance by the addition of ions resulted in a concomitant increase of susceptance values. However, in a hig h conductivity medium, increases in conductance resulted in little or no in crease of susceptance values because ions saturated the electrode surface. Susceptance increased when Escherichia coli, Pseudomonas aeruginosa, Alcali genes faecalis and Staphylococcus aureus were grown in high conductivity me dia because protons produced by metabolically active bacteria balance more charge on the electrode than other ions. Increases in susceptance due to ba cterial growth and metabolism in low conductivity media were attributed to both increases in protons and ionic metabolites. These results indicate tha t capacitance may provide a better measure of microbial growth and metaboli sm than conductance. (C) 1999 Elsevier Science B.V. All rights reserved.