The use of the impedance measurement for total count determination problems and their reasons

The impedance measurement is a rapid method applicable for both quantitativ e and qualitative examinations. Provided that sample material consists of h eterogeneous bacterial Flora, this method is prone to yield invalid results in total count determinations. Hitherto, the reasons for the afore were ne ver clearly elucidated. In model trials we used the detection rime as a mar ker for the impedance effectiveness. Hereby all investigated strains were i ncubated at equal initial concentrations. Thus, strains of Enterobacteriace ae, Aeromonas, Enterococcus and Bacillus cereus proved to be highly impedan ce effectiv, whereas some strains (e.g. Micrococcus, Acinetobacter and Broc hothrix) did not produce any change in the media impedance. This heterogene ity in the impedance effectiveness of (food relevant) strains can result in either false-negative (i.e. underestimations) or false-positive (i.e. over estimation) counts. Reasons for this were Found in differences concerning t he ability of ionic production, generation time as well as different lag ph ase durations of the microbes. In conclusion, we recommend the impedance me asurement for the total enumeration of aerobic bacteria only for mass inves tigations of uniform sample material.