Dilution Series Case Study

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2. You have been asked to set up a dilution series, and then use spread plates to determine the viable cell count. Why is it necessary to use a dilution series when isolating bacteria from a biological sample using spread plating? [5 MARKS]

It is vital to use a dilution series to reduce the concentration of the original biological sample so it is easier to count the number of isolated colonies which are present on the spread plate. The diluted samples will have a workable number of colonies on the spread plate which can then be used to calculate the number of bacteria which were present in the original undiluted sample.

3. You plate the dilutions onto nutrient agar and chromogenic agar plates. What are the differences between these types
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4. What temperature should the plates be incubated at and why? [2 MARKS]

Both the nutrient and chromogenic plates should be incubated at 37 degrees Celsius because this will mimic the temperature of the birthing pools where the sample is from as well as mimicking the temperature of the human body.
5. You use cUTI plates as the chromogenic media and find that some of the colonies have turned pink in colour. What information does this tell you about the bacteria in the sample, and why did the colony change colour? [2 marks]

This tells me that the bacteria in the sample contains coliforms such as Escherichia coli (E.coli). The colonies changed colour as the enzymes produced by the bacterial cells reacted with the red galactosidase in the agar medium, this reaction causes the colonies to turn pink making them easily

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