 This is the MPN or most probable number method of determining the amount of contamination you'd have in a sample or so is to count the amount of microbes that are in an area. This is actually one of my favorite ones to do especially in environmental science because it's a great way to test water quality. So the MPN method is used when you have maybe really dilute samples or you have samples where you can't get the microbes to grow on solid media, but the most important time you would use this is if you're looking for coliforms, which E. coli is an example of a coliform in water quality samples. So it can be used in food testing as well, but so here you see a sample that has a pond water and we're looking for coliforms because that tells us there's E. coli there which tells us that there's fecal contamination in this water. All right so I'll we'll talk about the math part of the MPN or most probable number in just a moment, but first, why is this work so well with your coliform organisms? What has to do with these tubes? So the tubes that are used are actually called lactose broth tubes. So they have the sugar lactose in them and coliforms can ferment lactose. So if the coliforms are digesting the lactose in the sample, you're going to see a drop in pH. So these are basically like pH test strips. So you see on top you've got the 15 tubes that are reddish those will be your lactose broth tubes and then on the bottom after incubation you see that some of them are turning yellow. So that means that there are enough microbes there. There are microbes there that are digesting the lactose causing a pH change and then and that's why you see the color change. So here's how you do this. In order to do this experiment like I said before we use it and with water quality testing in our Biomedical Sciences classes, you start with 15 of these broth tubes and the principle behind this is the more microbes there are in a sample the more dilutions it would take to get rid of them. We're basically trying to see at what level can we eliminate or remove the coliform bacteria. So you start with your 15 lactose broth tubes. The first five you're adding 10 milliliters of your original sample. The second five you're adding 1 milliliter and the third you're adding .1 milliliters. So then you incubate them like it says here for 24 hours at 37 degrees Celsius which is a very important temperature in microbiology because that is 98.6 degrees Fahrenheit and that is the optimal growth temperature of your mesophilic organisms which are most of the pathogens we care about. Alright so then we come back in a day and we're looking for these color changes. So you'll see here with art with this original sample five of the five 10 milliliter tubes were positive turned yellow two of the 1 milliliter tubes were positive and turned yellow and zero of the .1 milliliter tubes were positive. So you take that information and I'm not asking to memorize this chart but using this table you'd be able to determine with about 95% confidence how many organisms were in a milliliter or 100 milliliters of this sample. So let's go ahead and find ours. So ours was I'll do the remembering for you. It was five two zero. So you come down here and you see of the 10 milliliter tubes five of them were positive 1 milliliter tubes two of them were positive .1 milliliter tubes zero of them were positive. So you find five two zero on the table and you'll see that with 95% confidence we can say that there are 49 organisms per 100 milliliters in this sample and you see the ranges and stuff like I said this is not this is far from perfect but it's a great way to make an educated guess about how much contamination you see. So obviously look at the top if it was zero zero zero and none of the tubes had lactose fermentation then you'd know that there were very few organisms. On the bottom you see five three three has a massive number I mean to get a sample that would actually be five five five it would just have to be so contaminated. I've seen it one time we got it from a a cow pond in in Denison Iowa and there was just obviously lots and lots of fecal contamination there. So okay so let me go back here so this is the MPN or most probable number a great way to assess water quality if you're looking for any chances of fecal contamination. All right have a wonderful day be blessed.