A student has a pure culture of bacteria growing in a flask and wants to know how many living cells are in the culture at a given time. Which method should she employ to get an estimate?
Added by Robert P.
Step 1
This involves taking a small volume of the original culture and diluting it in a series of tubes or containers with sterile broth or saline solution to achieve different concentrations. Show more…
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The following set of questions goes through the steps you would follow to do a viable count. You would do serial dilutions of the culture and plate an aliquot of each dilution on nutrient agar. Some terminology: a 1:10 dilution = 1/10 dilution = 1x10^-1 dilution; a 1:100 dilution = 1/100 dilution = 1x10^-2 dilution, and so on. a) A bacterial culture contains 2 x 10^6 cells/ml. A 1:10 dilution of the culture is made by taking 0.1 ml out of the culture and adding it to a test tube that contains 0.9 ml of diluent. How many cells are present in the 0.1 ml sample taken from the culture? (units: # of cells) What is the new concentration of cells after you add the 0.1 ml sample to the tube with 0.9 ml of diluent? (units: cells/ml) You remove 0.1 ml from the 1:10 dilution and spread it on nutrient agar. How many colony forming units (CFU) do you expect to grow on the plate? b) A 1:100 dilution of the culture is made by taking 0.1 ml out of the 1:10 dilution made in (a) and adding it to a test tube that contains 0.9 ml of diluent. (1/10 * 1/10 = 1/100 or 1:100) How many cells are present in the 0.1 ml sample taken from the 1:10 dilution from (a)? What is the new concentration of cells after you add the 0.1 ml sample to the tube with 0.9 ml of diluent? (units: cells/ml) You remove 0.1 ml from the 1:100 dilution and spread it on nutrient agar. How many colony forming units (CFU) do you expect to grow on the plate? c) A 1:1000 dilution of the culture is made by taking 0.1 ml out of the 1:100 dilution made in (b) and adding it to a test tube that contains 0.9 ml of diluent. (1/10 * 1/10 * 1/10 = 1/1000 or 1:1000) How many cells are present in the 0.1 ml sample taken from the 1:100 dilution from (b)? What is the new concentration of cells after you add the 0.1 ml sample to the tube with 0.9 ml of diluent? (units: cells/ml) You remove 0.1 ml from the 1:1000 dilution and spread it on nutrient agar. How many colony forming units (CFU) do you expect to grow on the plate?
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