hey everybody doctor oh here this is the M PN or most probable number method of determining the amount of contamination you'd have in a sample or so as to count the amount of microbes are in an area is actually one of my favorite ones to do especially in environmental science because it's a great way to test them 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 as if you're looking for coliform x' which eco lies 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 coliform x' because that tells us there's an e coli there which tells us that there is fecal contamination in this water alright so I will 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 well has to do with these tubes so the tubes that are used are actually called lactose broth tubes so they have that they have the the sugar lactose in them and cola forms can ferment lactose so if the if the Cola forms 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 that are reddish those will be your lactose broth tubes and then on the bottom after incubation you see 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 I 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 bioscience 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 delusions 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 5 you're adding 10 milliliters of of your original sample the second 5 you're adding one milliliter and the third you're adding point 1 milliliters then you incubate them like it says here for 24 hours at 37 degrees Celsius which is a 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 the pathogens we care about all right 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 ten milliliter tubes we're positive turned yellow two of the one milliliter tubes were positive and turned yellow and zero of the point one milliliter tubes were positive so you take that information and I'm not asking you 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 our so ours was I'll do the remembering for you it was five to zero so you come down here and you see of the ten milliliter tubes five of them were positive one milliliter tubes two of them are positive 0.1 milli tubes zero of them are positive so you'll find five to zero on the table and you'll see that with 95% confidence we can say that there are 49 organisms per hundred milliliters in this sample and you see the ranges and stuff like that this is not this is far from perfect but it's a great guesstimate way to make an educated guess about how much contamination you see so obviously look at the top if if it was zero zero zero and none of the two 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 his so contaminated I've seen it one time we got it from 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 alright have a wonderful day be blessed