Determining Cell Number Flashcards
If have an undiluted sample of 2563 cfu ml-1, what would the 1/10 dilution be
256 cfu ml-1
Is the sample volume important when preparing a dilution
Any sample volume can be used to prepare a dilution.
It had the same effect as long as ratios stay the same
when do errors in pipetting become significant
errors when pipetting become greater with smaller volumes
what are the two things we need to do when sampling on dilutions ?
we need to do replicable samples and multiple dilutions
what is meant by accuracy ?
when all of the same things cluster together
what is meant by reproducibility ?
when we do it 2 or more times to see how close our answers are
what is the sample volume used in the spread plate method ?
0.1ml or 0.2ml
what is the sample volume used in the pour plate method?
1ml or more
what is the sample volume used in the Miles and Misra method ?
30 ul or less
why is standard error and standard deviation normally given on a graph ?
its normally given to show robustness of data / shows us the variation
why don’t we consider colonies that total to less than 30 ?
small numbers relative to large sample volume introduces greater variation
= subject to variability
why don’t we consider >300 colonies ?
this is because crowding on plate affects colony development and obscures counting
If a culture has 300,000 cfuml-1 and you need to dilute it to 30 cfuml-1. How many 10-fold dilutions need to be performed ?
10^4
30/300,000 = 1/10^4 = dilution factor of 10^-4
what is the formula to work out the no. of cells in the original sample ?
no. in the sample (or plate) x 1/dilution factor
what is the formula used to work out No. cfuml^-1 ?
No. (in countable range) x 1/dilution factor x 1/sample volume
