Does this serial dilution question make sense?

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Discussion Overview

The discussion revolves around a serial dilution question involving the viability of bacteria in a sample. Participants are examining the implications of the given concentration and the resulting colony count after plating a diluted sample on agar. The scope includes conceptual understanding of bacterial viability and statistical considerations in microbiological experiments.

Discussion Character

  • Debate/contested, Conceptual clarification, Exploratory

Main Points Raised

  • One participant questions whether the term "total number of bacteria" can be applied when it includes both viable and non-viable cells, suggesting a need for clarity in definitions.
  • Another participant expresses confusion about the statistical validity of plating 0.1 of a single bacterium, indicating that the sample size may be too small for reliable conclusions.
  • Several participants speculate that the question might contain a typo or be a trick question, with suggestions that the observed colonies could be due to contamination or poor mixing of the dilution.
  • Concerns are raised about the apparent contradiction of obtaining 122 colonies from a dilution that theoretically contains an average of 0.1 bacterium.

Areas of Agreement / Disagreement

Participants generally agree that there are issues with the numbers presented in the question, but there is no consensus on the exact nature of the problem, whether it be a typo, contamination, or misinterpretation of the dilution process.

Contextual Notes

Participants highlight potential limitations in the question's formulation, including assumptions about bacterial viability, the statistical implications of small sample sizes, and the clarity of terminology used in microbiological contexts.

Kalibasa
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"You have a sample with a concentration of 1x106 bacteria/ml and you wish to know how many of these bacteria are viable. You dilute the sample down to a single bacterium/ml and plate 100 microliters of this diluted sample onto an agar dish. After incubating the dish for two weeks you count 122 colonies. What percentage of the bacteria in the initial sample were viable?"

The number at the beginning is supposed to be the total number of bacteria in the sample, including dead and damaged cells (i.e. the number of bacteria you would get from physically counting the sample). But can you talk about a "total" number of bacteria in a sample like this, or does the number of bacteria we assign to samples always refer automatically to the number of viable bacteria?

I'm just trying to figure out the standards on these kind of questions...

Thanks!
 
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Maybe I am misreading the question, but it appears very odd. The way I figure, you are plating (on average) 0.1 of a single bacterium, so your sample size is too small to make any statistical generalization.
 
Must be one of those trick questions where the answer is contamination. :-p

...probably a typo?
 
BoomBoom said:
...probably a typo?

I would assume so also. Otherwise I'd agree that you're measuring contamination, not actual bacteria in your sample. That, or it was a really poorly mixed dilution. :rolleyes:
 
How can you get 122 colonies out of 0.1 bacterium? As mentioned above, there is something wrong with the numbers in the initial question.
 

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