Dear Colleagues,
I counted some bacteria colonies on EMB and Maconkey Agar after 7-fold dilutions due to numerous count across several samples. I noticed that a particular sample was uncountable(zero count). Please how could I represent it in log form. If I am to use their assay minimum limits to represent zero, what are their limits?
Thank you.
A negative result should be recorded as 0 CFU/ml. In this case, logarithm is not used, because there is no need.
Dear Tenebe,
While counting colonies, if the number of colonies is above 300 then we call it as Too Numerous to Count (TNC) if it is below 30 we call it as Too Few to Count (TFC). As far i know we cant represent zero count in log form since log value of zero is infinity.
For bacterial dilution upto 10^4 to 10^6 only preferred. Since you have used EMB media which is a selective media for coliforms you might have missed some bacterial population. If you are looking for Gram negative especially the coliforms then only you can use EMB and MacConkey. If yes try lower dilutions. If not use basal media like Nutrient Agar.
Hope this would be useful.
Regards
Sam Ebenezer
Dear Sam,
Thank you for your useful comments. But I also want to inform you that those samples in question initially show no trace of gas when multiple fermentation test was carried out. Bearing this in mind, is it not still possible to have zero count?
Dear Pawel,
Thank you for your answer as well. In the case of your response, his do I report such information if I had to report the range of my count values.
Thank you.
Dear Dr.Tenebe,
It may be possible, but at this stage we cannot conclusively mention it as zero count since you have used EMB and MacConkey only. Try some lower dilutions. Even you can try undiluted sample directly onto Basal media like Nutrient Agar.
Regarding multiple fermentation test , absence of gas only cannot infer anything. Have you not observed tubidity? Have you observed change in the colour of media to yellow color (indication of acid production)?
if no turbidity and acid production and gas is observed then we can consider it as absence of microbes. Still for the conclusive result we should do spread plating onto basal media.
Another hint, try anaerobic culture methods. There are chances for this also. (anyhow i am not sure whether this will suite your objective)
Thanks
Thank you Dr. Sam,
I have observed both gas production and color change. They were absent for those particular samples. Thank you for your general comments. I will do exactly as you have suggested. I also want to mention that there were no growth on Nutrient Agar as well . Thank you
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