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Microbiology

Hello everyone,
I just wanted to know how to solve this question. A scientist wanted to estimate the number of bacteria in 1cm3 of fresh milk. A 1cm3 sample of the fresh milk was diluted by a factor of 100, by adding it to 99 cm3 of water. A sterile pipette was used to transfer 0.5cm3 of this dilution to a sterile petri dish containing nutrient agar. the same procedure was repeated with four other samples of the diluted milk. i) How many colonies was present in dishes 1,2 and 4. 1 is 24, 2 is 25 , 4 is 26 . Using the results from i) estimate the number of bacteria present in 1cm3 of fresh (undiluted) milk. The answer is 5000. If anyone could help with this question please help me.
Okay,
so when they diluted the sample that made no difference to the amount of bacteria, all it did was increase the total volume to 100cm^3 of solution
for each sample they took 0.5cm^3 which means you have 0.5/100 otherwise known as 1/200 of the original solution so 1/200 of the amount of bacteria,
next you need to look at the answers to part i and calculate a mean sample size, so 24+25+26=75 and 75/3=25
so for every 0.5cm^3 you have 25 bacterium
because 0.5cm^3 is 1/200 of the original amount we need to multiply our answer by 200
25*200=5000
hope this helps
Okay,
so when they diluted the sample that made no difference to the amount of bacteria, all it did was increase the total volume to 100cm^3 of solution
for each sample they took 0.5cm^3 which means you have 0.5/100 otherwise known as 1/200 of the original solution so 1/200 of the amount of bacteria,
next you need to look at the answers to part i and calculate a mean sample size, so 24+25+26=75 and 75/3=25
so for every 0.5cm^3 you have 25 bacterium
because 0.5cm^3 is 1/200 of the original amount we need to multiply our answer by 200
25*200=5000
hope this helps

Original post by saramahmoud1234
Hello everyone,
I just wanted to know how to solve this question. A scientist wanted to estimate the number of bacteria in 1cm3 of fresh milk. A 1cm3 sample of the fresh milk was diluted by a factor of 100, by adding it to 99 cm3 of water. A sterile pipette was used to transfer 0.5cm3 of this dilution to a sterile petri dish containing nutrient agar. the same procedure was repeated with four other samples of the diluted milk. i) How many colonies was present in dishes 1,2 and 4. 1 is 24, 2 is 25 , 4 is 26 . Using the results from i) estimate the number of bacteria present in 1cm3 of fresh (undiluted) milk. The answer is 5000. If anyone could help with this question please help me.
Original post by Gluconeogenesis
Okay,
so when they diluted the sample that made no difference to the amount of bacteria, all it did was increase the total volume to 100cm^3 of solution
for each sample they took 0.5cm^3 which means you have 0.5/100 otherwise known as 1/200 of the original solution so 1/200 of the amount of bacteria,
next you need to look at the answers to part i and calculate a mean sample size, so 24+25+26=75 and 75/3=25
so for every 0.5cm^3 you have 25 bacterium
because 0.5cm^3 is 1/200 of the original amount we need to multiply our answer by 200
25*200=5000
hope this helps

Thanks a lot, it really helped.

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