Experimental setup for parallel measurements
The sample is 60 liters of tap water contaminated with sewage and measured over 14 days.
5 ColiMinder Devices have been measuring E. coli specific activity, in parallel IDEXX and Petrifilm has been used for culture-based evaluation (doublets of each).
The development of the enzymatic and the classical (IDEXX and Petri-film) results had been monitored.
The spikes that increasingly appear in the enzymatic results are caused because bacteria are agglomerating on particles – if such an agglomeration is brought into the measurement chamber the signal increases drastically.
By comparing different measurement approaches, it is important to understand the principle of indicator bacteria. E. coli, Enterococci and Total Activity (ALP) are used as indicators of fecal and microbiological contamination. The higher the contamination, the higher the risk of pathogens and all kinds of infectious germs that spread through the metabolic pathway. For this reason, water is generally tested for E. coli and others.
The question to be answered by testing for E.coli is:
How high is the level of fecal contamination of a sample? – because this indicates the level of risk
The classical method to evaluate the level of contamination is utilizing the growth of bacteria (E. coli, Enterococci or HPC) as a measurement instrument, and the number of colonies formed, to determine the level of contamination. The ColiMinder® is using the metabolic activity of target organisms (specific enzymatic activity) present in the sample, as a measure for how many living E. coli are present per volume of sample, to determine the level of contamination and risk.
Scientific Definition of enzymatic activity
One important fact may be mentioned here: enzymatic activity, for example the E. coli specific ß-Glucuronidase activity, is scientifically exactly defined.
An exact scientific definition of ß-Glucuronidase is given in Modified Fishman Units:
MFU (Modified Fishman Unit): One MFU will liberate 1.0 μg of phenolphthalein from phenolphthalein glucuronide per hour at pH 6.8 at 37°C.
As a direct consequence VWM´s ColiMinder® is calibrated and delivering a result on E. coli contamination in Modified Fishman Units per Volume, representing E. coli -specific enzymatic activity per volume of sample as a direct measure representing the number of living organisms per sample volume. As the measurement parameter of enzymatic activity is scientifically exactly defined it provides the base to adopt respective limits of tolerable contamination expressed in enzymatic activity, for all kinds of applications.
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