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RE: f. coliform analysis

John Sansalone — Mon, 28 Apr 2008 16:07:53 GMT

Thanks all.

I'm not the lab guy ( I'm a regulator !), but when this was reported to me, I thought it unusual enough to warrant a posting. I'll pass this along to the involved party.

RE: f. coliform analysis

Sabir — Sun, 27 Apr 2008 00:18:31 GMT

There could be a number of explanations for your observation but if it is one time observation; the problem can be linked either to filter or agar media. Filter might be damaged during filtration (high pressure or dry sucking) or during transfer step or some thing wrong with the lot (less likely). In case of any doubt about filters lot, representative samples from the batch can be assessed for its ability to retain the organism (usually Serratia marcescens is recommended for 0.45micron pore size filters) from a 100-ml suspension containing about 1X10³ cells. There should not be detectable bacteria downstream in the filtrate.

The second reason could be abused media, either by overheating during sterilization or re-melting this may disrupt the gelling properties. Other reason could be the change in pH of the agar media (may be) due to lower pH value of the filtered sample has weaken the gelling structure at the point of junction between agar and filter that cause oozing out of its watery content on the filter leading to ‘satellite colonies’

RE: f. coliform analysis

Old lag — Fri, 25 Apr 2008 14:01:32 GMT

SM9222D gives you the option to use agar M-FC.

RE: f. coliform analysis

dsmith — Fri, 25 Apr 2008 11:44:35 GMT

I am not familiar with any membrane filtration method for fecal coliform that uses any agar. The only one I have heard of uses the M-FC broth which is purple and produces blue fecal colonies. If there is some sort of gel forming that sounds like something being introduced by the sample. Was the same sort of gel layer in the blank and did it have any colonies in it?

RE: f. coliform analysis

MoscowLabAnalyst — Fri, 25 Apr 2008 11:00:08 GMT

dhmh brings up an excellent point.

What were the results on the blanks?

MLA

RE: f. coliform analysis

dhmh — Fri, 25 Apr 2008 08:19:03 GMT

They should perform a student's t test (use test) on their membrane filters, media, and dilution (or rinse) water immediately to prove or disprove any possible contamination as required by Standard Methods. I never used the agar for my MF method, I used M-FC broth (SM 9222D) for f. coli and M-Endo for T Coli. I have not come accross the problem you described.

f. coliform analysis

John Sansalone — Thu, 24 Apr 2008 18:14:42 GMT

A permitted facility I know of, recently had an odd experience when doing f. coli. analysis. Using membrane filter, they suddenly got TNTC results. Further examination found that agar medium was wicking up through the filter, forming a gel type layer on top (where it is not supposed to be), and allowing "satellite" colonies to form. It was these extra colonies that resulted in high counts. When they resampled, and did a side-by-side MF and MPN tests, the MPN matched pretty well with the number represented by the major colonies alone.

Wondering if this represents a bad batch of filters, or media, or something else, and wondering if anyone else has run into something similar. If so, was it determined what was the cause ? Just curious as to experience of others.