CFU Calculations Worksheet for BIOL1007 2020 PDF

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Summary

Guided worksheet for practicing CFU calculations, including answers. Relevant to Tutorial and Lecture Work from Week 8, microbiology module....

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CFU calculations

The following exercise is to give you an opportunity to practice CFU calculations You will do the following calculation with images of agar plates inoculated with diluted soil samples in Practical 10. This exercise is being provided to you in Practical 9 so you can practice using the calculation which will form part of the proficiency assessment. Using the resources on canvas, Investigation 9.4, to complete the following calculation.

SOURCE OF SOIL: SOIL SAMPLE WEIGHT = VOLUME OF BUFFER SUSPENTION = 1. Select a suitable dilution and count the total number of colonies on the plate. Colonies counted =

2.

cfu per plate

The number counted in the previous step is the cfu in 100 µl of that particular dilution. How do we convert this back to the number in our original sample? Follow these steps:

(a) Calculate the cfu in 1 ml of the diluted sample. To do this, multiply the raw cfu count by 10. (remember, we only plated 100 µl of the dilution, which is 0.1 ml)

Cell concentration in diluted sample =

cfu / mL

(b) Calculate the cfu concentration in the original soil suspension. To do this, multiply the number from (a) above by the reciprocal of the dilution factor, i.e. if you used the 10 -4 sample for counting, then multiply by 104

Practical 9 and Practical 10

(10,000).

Cell concentration in original soil suspension = mL

cfu /

(c) Calculate the total cfu in your soil suspension by multiplying the number from (b) above by the suspension volume you used. This converts the cfu from a concentration to an absolute number.

Cell number in original soil suspension =

cfu

(d) Calculate the number of cells per gram of soil by dividing the number from (c) above by the weight of soil (g) used in the experiment.

Cell density in soil =

cfu / gram

Remember to present your final calculation in scientific notation i.e. 3.7 x108 cfu/g (not 370000000 cfu/g)

If for example the method initially suspended 2 grams of soil in 5 ml of buffer, using that same 10-4 agar plate, the calculations would be as follows: a) Calculate the cfu in 1 ml of the diluted sample - Multiply the raw cfu count by 10 b) Calculate the cfu concentration in the original soil suspension - Multiply the number from (a) above by 104 (10,000) c) Calculate the total cfu in your soil suspension - Multiply the number from (b) above by the suspension volume = 5 ml

Practical 9 and Practical 10

d) Calculate the number of cfus per gram of soil - Divide the number from (c) above by the weight of soil (g) used in the experiment = 2 g

Practical 9 and Practical 10...