J. Schulenburg et T. Bergann, The use of the impedance measurement for total count determination problems and their reasons, FLEISCHWIRT, 80(4), 2000, pp. 146-150
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.