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Sample Solutions: Serial Dilutions

Suppose your professor handed you a test tube with 4.o mL of an E.coli broth culture in it and told you to make a 10^-2 dilution of the entire culture. Explain how you would do this. Show your calculations.?

If you've taken 250 ul of plasma and added 750 ul of distilled water, you have achieved what dilution?

You begin serial dilution .  In tube 1,  you add 100 ul of plasma to 900 ul of distilled water, achieving what dilution? Taking 100 ul from tube 1's 1:10 dilution, and adding 900 ul of distilled water would achieve what dilution for tube 2?

You've diluted 100 ul of plasma with 300 ul of distilled water for a 1:4 dilution. The test still needs further dilution. You then take 100 ul of the 1:4 dilution and add 100 ul of distilled water. What is the resulting dilution factor?

If you add 43 g of food in 350 mL water and dilution of 1:6 is made, then from this dilution 1:90 is made, and from this 1mL is added to 60 mL of water, then 0.1 ml from this last dilution is plated and after incubation 143 colonies are counted/. What is the total count per g of original sample?

You were instructed to add 1.0 mL out of 5.0 mL of an undiluted sample to 99 mL of sterile diluent. Instead, you add all 5.0 mL to the 99 mL. What was the intended dilution and what was the actual dilution? Show your work.?

The plate with 45 colonies was inocculated with 100 µL Volume of a 10^-8 Dilution. Given the formula OCD = CFU/V x D, what as the Original Colony Density? 

The plate has 259 colonies, with a sample volume of 10^(-6) mL. What was the original concentration in the sample? How many colonies should be on the plate inoculated with a sample volume of 10^(-7) mL using the same sample?

A sample has a density of 7.9*10^9 CFU/ml. What sample volume should yield a countable plate? Which two dilution tubes could be used to produce this sample volume? How?

Solution Preview

1. Suppose your professor handed you a test tube with 4.o mL of an E.coli broth culture in it and told you to make a 10^-2 dilution of the entire culture. Explain how you would do this. Show your calculations.?

Ans: To make 10^-2 dilution, that is 1/100 dilution,
First take 4ml of culture and inoculate into 36 ml H20 => 4 /40 = 1/10 => 10^-1
Then take 4ml ( 10^-1) and inoculate into 36 ml H20 => 1/100 => 10^-2

2. If you've taken 250 ul of plasma and added 750 ul of distilled water, you have achieved what dilution?

Ans: You will have ¼ dilution because
250 ul + 750 ul = 1000 total
250 / 1000 = ¼

3. You begin serial dilution . In tube 1, you add 100 ul of plasma to 900 ul of ...

Solution Summary

Serial dilution is a series of dilution from the undiluted to minimize bacteria colonies for the ease of counting bacteria. It is a way to make bacteria easier to count because you minimize the number of cells in the original sample.

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