Javier, the range of 15 to 150 CFU per plate. Many samples require a dilution step before plating.

Use a multiplication factor to convert the plate count to the number of colony forming units (CFU) of bacteria present per gram of sample. The multiplication factor is determined by dividing the dilution used by the volume plated.

http://multimedia.3m.com/mws/media/1035224O/petrifilm-evaluation-yeasts-molds-dairy-high-acid-food-article-reprint.pdf?fn=Petrifilm-Eval-Yeasts-Molds-Dair1035224http%3A%2F%2Fmultimedia.3m.com%2Fmws%2Fmedia%2F1035224O%2Fpetrifilm-evaluation-yeasts-molds-dairy-high-acid-food-article-reprint.pdf%3Ffn%3DPetrifilm-Eval-Yeasts-Molds-Dair

A notable challenge here (at least for the fecal indicators) is that they are typically in low numbers in drinking water, and the Petrifilm process analyzes small volumes (1mL versus the traditional 100 mL sample volume for drinking water analysis).  A non-detect on Petrifilm would correspond to count of < 1 per 1 mL, or

As noted by others, drinking water regulations typically specify limits based on a 100 mL sample volume. If you need to enumerate the bacteria, then membrane filtration is probably the best option. However, if you just want to measure compliance with guidelines you might consider a presence/absence test for E. coli using Colilert. Look to the WHO Guidelines for Drinking-water Quality, 4th Edition, for guidelines on microbial monitoring of drinking water.

I have done extensive field testing with Petrifilm (not yet published, unfortunately) and it does well in terms of accuracy compared with the Colilert Quantitray system, but I find there are significant usability issues. Petrifilm comes in packets of 25 units, and once opened, the plates absorb moisture. After a few days in a humid environment, the plates are no longer able to absorb the entire 1 mL sample, and it runs off the edges, past the foam dam. This makes it difficult to read results. As an alternative, you should look into the Compact Dry EC Plate from Hyserv (http://www.hyserve.com/produkt.php?lang=en&gr=1&pr=13). It costs about the same and works in a similar manner but is made of plastic and has a lid, so no leakage. It is used by the WHO and UNICEF in national water quality surveys.

 Gordon,

Thank you very much for your answer above. It is very helpful as I am also currently working on Petrifilm Coliform plates. But I am not aware of membrane filtration process inside petrifilm method. Please can you classify on this? I used membrane filtration as a process of its own without involving petrifilms.

Thank you.

Stephen.

1- None detected. (For drinking water in 1 ml sample, this is the only acceptable level.)

2- E. coli detected, but less than 2 colony forming units per Plate.

(Safe for contact recreation, Such as swimming.)

3- E. coli between 2 and 20 colony forming units per Plate.

(Not safe for contact recreation, but acceptable for non-contact Recreation, such as boating.)

4- E. coli greater than 20 colony forming units per plate.

(Not considered safe for non-contact recreation.)