Using absorbance readings and a standard curve relating absorbance to cell numbe
ID: 216422 • Letter: U
Question
Using absorbance readings and a standard curve relating absorbance to cell number, you suspect that a bacterial culture contains 7.4x 10° CFUs/ml. What dilution of this sample would give you a countable plate? . 6. You have observed the following data from an aerobic plate count. How many bacteria are in the undiluted sample? Dilution 10-3 Average CFUs/ml TNTC 334 10?? 10 27 10" 1.11 x 1ola Agar plates typically contain 1.5% agar by weight. To make 500 ml of TSA, how many grams of agar would you add to your flask before filling it with TS broth for a total volume of 500 ml? (Assume the density of TS broth is the same as H20). 7, You have a stock antibiotic solution with a concentration of 15 mg/ml. You need to make 100 ml of nutrient broth with an antibiotic concentration of 0.4 mg/ml. How much stock antibiotic solution would you add to how,much nutrient broth to 8. achieve this concentration?Explanation / Answer
5) The bacterial culture contains 7.4 X 109 CFUs/ml
If we dilute it 10-8 then the culture will contain 7.4 X 101 CFUs/ml, that means 74.
This is a countable number (plate).
6) If we consider, 111 CFUs/ml present in 10-8 dilution.
Then, in undiluted sample, the CFUs/ml will be 11.1 X 1011 = 1.11 X 1012.
7) 1000 ml of TSA contains 15 g of agar (agar is 1.5 %).
500 ml of TSA contains 7.5 g of agar.
So, we have to add 7.5 g of agar to the flask before filling it with TS broth.
8) Stock antibiotic solution contains concentration of 15 mg/ml.
We need to make it 0.4 mg/ml.
So, 15 / 0.4 = 37.5 (if we add 1 ml of antibiotic in 37.5 ml of nutrient broth then we will get the antibiotic concentration 0.4 mg/ml).
Total volume is 100 ml.
Therefore, 100/37.5 = 2.666 ml which is nearly 2.67
100 – 2.67 = 97.34 ml
We have to add 2.67 ml of antibiotic in 97.33 ml of nutrient broth to achieve this concentration.
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