ust tte presence of bacena ete, 56. Ms. Marle was given a 10.0ml suspension of b
ID: 218056 • Letter: U
Question
ust tte presence of bacena ete, 56. Ms. Marle was given a 10.0ml suspension of bactería culture and was asked to determine the titer (CFU/m). She performed 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 quadruplicates? (5 points) 10-8 20, 19, 22, 34 10-7 207, 193, 220, 180 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 cfu/ml in a 10-59 dilution. (5 points) d. What dilution would give you 25000 colonies/0.25ml? (5 points) 57 Which differential medium is commonly used in testing the presence of coliform bacteria in water?Explanation / Answer
A-
10-9
Average of all colonies = 0+3+2+1/4 = 1.5 colonies per plate.
CFU/ml = Number of colonies * 1 ml/ total volume plated * 1/Dilution factor.
= 1.5 *1000/300 *1/10-9
= 1.5*3.3*109
= 5 *109 CFU/ml
For 10-8
Average of all colonies = 20+19+22+34/4 = 95/4 = 24 colonies per plate.
CFU/ml = Number of colonies * 1 ml/ total volume plated * 1/Dilution factor.
= 24 *1000/300 *1/10-8
= 24*3.3*108
= 79 *108 CFU/ml
or 7.9 *109 CFU/ml
For 10-7
Average of all colonies = 207+193+220+180/4 = 800/4 = 200 colonies per plate.
CFU/ml = Number of colonies * 1 ml/ total volume plated * 1/Dilution factor.
= 200 *1000/300 *1/10-7
= 200*3.3*107
= 666 *107CFU/ml
or 6.66 *109 CFU/ml
B- Total volume is 10 ml.
It is best to measure the CFU/ml from the plate which has colonies ranging from 30-300 colonies per plate
So the total number of bacteria present in the total stock solution is 10*6.66 *109
or 6.66 *1010 bacteria.
C- 105.9 = 794328.
We have CFU/ml = 6.6*109 and we need to dilute it 794328 times.
So the total CFU/ml after dilution will be 6.6*109 /794328 = 8384 CFU/ml
D- CFU/ml = Number of colonies * 1 ml/ total volume plated * 1/Dilution factor.
25000/0.25 ml = 100000 coloines per/ml
6.6*109 = 25000*1000/250 *1/ Dilution factor
Dilution factor = 25000*4/6.6*109
= 1.5*10-5
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