A permitted facility I know recently had an odd experience when doing fecal coliform analysis. Using a membrane filler, they suddenly got TNTC (too-numerous-to count) results. Further examination found that the 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 side-by-side MF (membrane filter) and MPN (most probable number) tests, the MPN matched pretty well with the number represented by the major colonies alone.
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