Part a. Two plates received 100 µL from the same dilution tube. The first plate
ID: 11691 • Letter: P
Question
Part a. Two plates received 100 µL from the same dilution tube. The first plate had 293 colonies, whereas the second had 158 colonies. Suggest reasonable sources of error.Part b. Two parallel dilution series were made form the same original sample. The plates with sample volumes of 10^-5 mL from each dilution series yielded 144 and 93 colonies. Suggest reasonable sources of error.
Part c. What are the only circumstances that would correctly produce countable plates from two different dilutions?
Explanation / Answer
a) since the question tells us that two plates were made from the same tube, the only source of error could be during transfer or plating of the solution containing bacteria. So the answer could be: contamination during streaking, incorrect streaking technique.
b) here, they are expanding the possibilities of error being introduced because they tell you to trace the process all the way from the beginning of dilution series. So the answer could include answers from part a) + incorrect dilution technique or contamination during dilutions.
c) the only circumstances would be when the concentration of the bacteria in the original solution are just so right as to produce the one before last diluted tubes with around from 100 to 300 colonties (whatever is high enough to be reasonably countable). So your 10-x dilution would count, say 200 colonies, and your last 10-(x+1) dilution would count around 20 colonies, give or take, consequently. So the only limitation is the countability of the one before last dilution tube colonies - if it's too high, your last dilution would contain too many colonies that need to be diluted once more, and if it's too low, your last dilution will have too few colonies and thus be statistically unreliable in count.
Hope this helps!