Microbiology at NDSU: Dilutions

Microbiology at NDSU: Dilutions


Hello and welcome to our lab today where we’re going to be performing a series of dilutions. Now when it comes to dilution, or a series of dilutions, many different sciences use solutions. Whether it be to dilute a chemical in chemistry or in a micro lab to dilute cells. Now if you kind of take a look at this dilution, or this stock vial, it’s very difficult to see or count how many bacteria would be present in this stock solution. So in order to count how much would be in here, or if you’re curious about how much bacteria is in milk or on hamburger we would need to set up a series of dilutions and we’d have our stock bottle. So on our stock bottle we’d label that as 10 to the 0, because anything to the 0 power is 1. So this is our original one stock solution. Our protocol today requires that we draw one mL from our stock bottle and put it into our first dilution. Making sure that our bottle is evenly distributed, we’re going to shake it a little bit. Make sure you shake away from your face, and then we will aseptically take off that lid as you learned at the beginning of the semester. Very carefully, and once again we’re taking 1 mL up, so we’re going to draw up to the zero line. Beautiful. We’re going to cap our stock bottle up, so that nothing falls into there causing any contamination or some false counts. We’ll then open up our test tube that’s labeled 10 to the negative 1 and dispensing by either rolling down or pressing the plunger. As you can see, we’re pressing down into the liquid, we’ll cap are liquid up very carefully. We’ll set that into our test tube – rack never on the counter, and disposed are pipette into our biohazard bin. Now this is a one to 10 dilution because we’re taking one mL from our stock bottle and placing it into nine mLs of solution. So in this bottle, or in this vial, we have a total of ten mLs, we have one mL of our stock in a total of ten mLs, so one out of 10, a one to ten dilution, or for those who prefer to work in exponents 10 to the negative 1. So we’ve shaken this up very well, we’re going to repeat what we just did. We’re going to add one mL from our 10 to the negative one tube however and put it in our 10 to the negative 2. Give that 10 to the negative 2 a few shakes and if we compare from our stock bottle to our 10 to the negative 1 bottle, or test tube, to our 10 to the negative 2 test tube, notice how that as we go down in dilutions our solution gets less and less colorful, means that we are diluting out that color. For bacteria, we’re diluting the bacteria and it’s becoming less and less turbid. So if you’ve got a dark solution, or a very turbid solution, going to a less turbid solution it means that you more than likely completed your dilution series correctly.

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