56. Ms. Marle g ane was given a 10.0ml suspension of bacteria culture and was as

Question

56. Ms. Marle g ane was given a 10.0ml suspension of bacteria culture and was asked to determine the titer (CFU/m), She perfomed the spread plate method and was able to read only the dilutions listed below after she plated 300 ul of each dilution in quadruplicates. 10-9 0,3,2,1 Use the data above to answer the following questions. Show all calculations. a. What is the titer of the original stock bacterial suspension when 300 ul was plated in 10-8 20, 19, 22,34 10-7 207, 193, 220, 180 quadruplicates? (5 points) b. What is the total number of bacterial in the original sample given to the student? (5 points) c. Calculate the concentration of bacteria in cfulm in 10 -s dilution.(5 points) d. What dilution would give you 25000 colonies/0.25ml? (5 points) fssstiol medium is commonly used in testing the presence of coliform bacteria in water?

Explanation / Answer

At 10-7 dilution Marle gets her countable colonies in all 4 plates. So we must take a mean of these no. And the mean is (207+193+220+180) /4 =200. So no . Of bacteria present in 300 ul is 200×107.

a.So bacteria present in stock at (200 x 107 x 1000)/300 =6.667x109 cfu/ml .

b. So total 10ml suspension given to student. So total no of bacteria is 6.667x1010.

C. So bacteria present in 105.9 dilution will be. (6.67x10-5.9)/10-9 =5.298×103

D. At 104.2   dilution would give 25000 colonies in 0.25 ml

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