Methods Of Measuring Growth Flashcards
Why is estimating the growth of population of bacteria important?
Environmental health officers inspect food premises, water authorities check water supplies and food manufactures check that food is safe to eat
What are the direct ways the size of microorganism population in liquids can be measured?
Viable counts count living cells, total counts count living and dead cells.
What are the indirect ways the size of microorganism population in liquids can be measured?
Measuring turbidity (cloudiness) of the culture.
What does plating and counting rely on?
It isn’t possible to count whole populations so dilutions have to be made, which relies on each live cell forming a colony so it provides a viable count.
How are cultures diluted?
when 1cm3 of suspension is added to 9cm3 of medium a 10-1 dilution has been made. If this process is repeated 10-2,10-3,10-4 etc can be made. If 0.1cm3 is added to 9.9cm3, the first dilution is 10-2, then 10-4, then 10-6 etc.
How are cultures plated and counted after dilution?
1cm3 of dilutes sample is spread over an agar plate and incubated at 25 degrees for 2 days. A dish containing 20-100 colonies is chosen and the colonies are counted. To count viable colonies, the number of colonies is multiples by the appropriate dilution factor.
Give an example of how a viable count is done.
Colonies/amount added to plate x dilution = total x dilution = viable count
If 0.5cm3 of a 10-7 dilution produces 129 colonies
129 (colonies) / 0.5 (amount added to plate) x 10^7 (dilution) = 258(total) x 10^7(dilution) = 2.6x10^9
What are the issues with the culture dilution being too great or weak?
If it’s too great, there will be too few plate colonies
If it’s too inefficient, colonies merge, referred to as ‘clumpingM and counting may be inaccurate.
What is a haemocytometer?
A specialised microscope slide, making it possible to distinguish between living and dead cells.
How is turbidity measured?
A colorimeter measures turbidity as cell numbers increase. The absorbance of the suspension is found, then a reading is taken from a standard graph of light absorbance plotted against the number of bacterial cells. The result is a total cell count.
