question archive A medical sample suspected of being infected is diluted via serial dilution to a total of three dilutions, and then streaked onto appropriate agar
A medical sample suspected of being infected is diluted via serial dilution to a total of three dilutions, and then streaked onto appropriate agar
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A medical sample suspected of being infected is diluted via serial dilution to a total of three dilutions, and then streaked onto appropriate agar. Given the description of the serial dilution method below and the streak results in the image below, calculate the CFU/mL of the original sample. The streak was performed with a 10uL calibrated loop. Serial Dilution method: 50uL of the original sample was added to 450uL of sterile media and mixed thoroughly 50uL of that was removed and added to 450uL of sterile media and mixed thoroughly 50uL of that was removed and added to 450uL of sterile media and mixed thoroughly Then, using each of these three dilutions, 10uL was plated via streak with a calibrated loop onto the appropriate agar and incubated appropriately
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30000 CFU/mL
Step-by-step explanation
To determine the Dilution factor, we use the formula;
Dilution factor (DF) = Initial volume ÷ Final volume, Total dilution factor (TDF)= DF1 x DF2 x DF3 x .....and so on
Therefore,
1. 50 microlitre was added to 450 microlitre, DF = 50 ÷ 500 = 0.1, TDF = 0.1
2. 50 microlitre was added to 450 microlitre, DF = 50 ÷ 500 = 0.1, TDF = 0.1 x 0.1= 0.01
3. 50 microlitre was added to 450 microlitre, DF = 50 x 500 = 0.1, TDF= 0.1 x 0.1 x 0.1= 0.001
CFU/mL in the original sample.
CFU /mL = (No. of colonies (counted from given diagram)/ amount plated) x Total dilution (just reverse the sign of dilution)
= (30 ÷ 10) x 10 = 30 CFU/ microlitre,
1 microlitre = 30 CFU
1000 microlitre (1 mL) = 30 x 1000 = 30000 CFU/mL
