I run a lot of Enterolert assays each year and don't believe that I seeing too many false positives (based on comparisons with membrane filtration methods). In fact the Enterolert results tend to be a bit lower. Are there specific conditions under which you think you are getting false positives?

Hi Elizabeth: Thank you for answering. The water samples we run give different intensities post the 24 hours of incubation. For example, in one tray, wells can have a strong fluorencense and others a medium flouorencense while others have none. The others are negatives, however the medium ones are always questionable.

I tend to have a limited number of sites that have what I record as "faint fluorescence" either the entire tray or individual cells. The samples with overall faint fluorescence I usually run again at a lower sample (usually 10 ml) which usually seems to eliminate the faint response. I've discovered that very urbanized sites with higher salt content (not marine sites for which you can only use 10 ml but fresh water impacted by road runoff), sites with cyanobacteria blooms or which have been recently treated with herbicides or other chemicals often have overall  faint fluorescence. Thus for those sites I run smaller sample volumes (25 - 10 ml) and that seems to reduce the problem. For individual faint cells, I often do an initial count after 24 hours, marking and counting strongly fluorescing cells. Then I allow the tray to incubate for a few more hours (usually ~4 hrs) to see if those cells brighten. If they do I count them - if not (and they often don't) I do not count them. Based on my past experience with membrane filtration, I know that bacteria grow at different rates so this process allows me to ensure that enterococci that got a later start are still accounted for.  Hope this helps...

thanks!