While checking acid tolerance of bacteria, the growth of the culture in acid media is monitored by plating the culture on agar plates and then counting the CFUs. Why isn't it done spectrophotometrically taking OD at 600 nm, as in case of bile tolerance test?
Dear Hossain,
There is no reason why acid tolerance could not be investigated using optical density measurements, and for many reasons this approach is easier than determining viable plate counts. However, optical density measurements will not distinguish between dead but intact cells and live cells. You could determine whether this is a problem by sampling test cultures at different acid concentrations (or pH) and dotting or diluting and spreading on to agar plates and determining whether there is a correlation between optical density measurements and viable cell counts.
Regards, Andrew
Dear Hossain,
There is no reason why acid tolerance could not be investigated using optical density measurements, and for many reasons this approach is easier than determining viable plate counts. However, optical density measurements will not distinguish between dead but intact cells and live cells. You could determine whether this is a problem by sampling test cultures at different acid concentrations (or pH) and dotting or diluting and spreading on to agar plates and determining whether there is a correlation between optical density measurements and viable cell counts.
Regards, Andrew
Thanks Dr. Spiers.
However, in all the articles (around 20) I read, the acid tolerance was analyzed by counting CFUs; after several washing of the cells and then growing in acidic media and plating. Whereas bile tolerance was examined by measuring OD directly!
Dear Hossain
I think, acid (acidic pH) can kill some microbial types, but bile only inhibits microbial growth. OD do not distinguish between dead and viable microbial cells, but plate count(CFU) determine only viable microbial cells.
If you can afford the time I would also still recommand plate counting to get the viable cells. You could combine this with OD top get a ratio between plate conting vital cells and total OD cell number. Alternative, cell counts on a filter via microscopy, eventually combined with LifeDead or CTC staining could give aditional information about vital and dead cells.
Thanks Dr. Nasr and Dr. Böllmann. Yes it is possible that the acidic media might kill some cells.
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