Hello everyone!
I am a student starting some experiments in the field of microbiology, and I can't figure out what is significant difference when it comes to number of cells (expressed in CFU/ml) and how to compare and interpet the results.
For example: If I have two samples with: 3*10^7 and 8*10^8 CFU/ml, I would say that both of the samples contain satisfactory number of bacteria. Yet, when I do simple calculations, it turns out that sample no 1 is only 3,75% of sample no 2. This especially bugs me when I do sporulation efficacy counts. Magnitudes of 10^7, or 10^8 are great number of spores achieved, but when I compare it to the number of viable cells (which is about 10^9) I get very low percentege for sporulation efficacy.
How do you deal with those huge numbers, how do you compare and interpret it when making a conclusion? Is there any way to simplify the data? What are your suggestions?
The answer doesn't need to be strictly in the field of microbiology, any solutions that work in any wide range (from 10^1 to 10^10) would be helpful.
I hope I explained my question properly. Thanks in advance.
Hi Elizabeth,
These are not easy questions. For starters it best if one does replicate dilutions and plates. If you have at least 3 replicates you can start to do a T test to determine if they are statistically different. In addition when counting plates you should target plates with the number of colonies in the right range. A good overview of how number of colonies relates to errors in plate counting can be found at
https://www.microbiol.org/wp-content/uploads/2010/07/Sutton.jvt_.2011.17_3.pdf
If you want to visualize data over large values it is often best to do a log transformation of the data before plotting.
I hope this is helpful.
Regards,
Frank
Thank you Mr Burns, that was helpful and I will definitely apply it in terms of getting the most accurate result.
But I still struggle when it comes to comparing the final number of cells. For me it is important to optimize the fermentation conditions in order to achieve the highest number of veg. cells and spores and I am not sure when can I say that experiment was successful?
Is 10^7 spores/ml bad result, only because another set of experiments achieved 10^8?
Or to speak metaphorically, if I have 1 million $ I am rich when compared to someone having 1000 $ and at the same time poor when compared to someone having 1 billion $ on the account. How can we say what is enough?
I apologize for writing so much, but all those big number are really confusing me.
Hi!,
I don't know if i really understand your question clearly, but I would try to suggest the following;
I don't think that you should worry about the high value of 107 because a result is a result and research is honesty and I don't even think that it is a bad result.
If i get you clearly, I think that your problem is simply a conversion (mathematical) problem. Then I suggest that you convert all the exponential values you have to the same level. For example, convert all your value to be at 10 7
That is the first value is 3 x 107; 8 x 108 can be simplified to be 80 x 107 and 109 can be simplify as 100 x 107, Then you can easily compare 3; 80 and 100 just as Dr Burns suggested you to transform all the data into Log.
That's how I understand your worry except otherwise.
Hope it will help.
Regards!
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