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When to dilute the sample of water before filtration?

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Posted 29 March 2011 - 05:36 PM


I know this isn't food but it's somewhat related so hoping someone can help me out. I have recently seperated membrane filtration for faecal coliforms and enterococci and incubated each on MLSB and SBA respectively. In my lab manual I was instructed to dilute the sample of water before filtration for the coliform sample but not for the enterococci sample. Just wondering why this dilution is necessary in one but not the other?



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Posted 01 April 2011 - 06:26 PM

Hi Lou Lou.

Welcome to the site. :welcome:

Firstly, dilution of water samples are normally carried out if the samples are turbid (making filtration difficult) or may have a highed expected count.


The above link takes you to the Environment Agency Blue Book methods for coliforms/E. coli in waters.

For membrane filtration methods, the maximum recommended number of colonies is 100. After you regularly exceed this, then a dilution should be carried out. If you are diluting the samples, but never finding any growth, it may be an idea to question the need to dilute in the manual.

I have carried out water testing in the past and never diluted for either coliforms/E. coli or enterococci unless I had to for the stated reasons.

Hope this helps.


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