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Using pH indicator in agar plates - (Feb/25/2012 )

Hi there, I'm about to conduct an experiment where I look at the effect of sugar on the shelf life of yoghurt. I plan to measure this by quantifying the number of lactic acid bacteria in each sample by using bromocresol purple pH indicator.

However, before I do the experiment, I was wondering what kinds of problems I can expect using bromocresol purple indicator in the MRS agar in terms of errors and practicality so I can minimise these problems.

I plan to use 0.025ml per litre.

Also, I've never used this indicator before so what can I expect in terms of the colour the agar will be when I add it to the plates? Will it be initially all purple and after incubation, there will be some yellow colonies signifying lactic acid bacteria?

Really really appreciate the help!

Many thanks!



Can you explain your protocol and criteria? You've apparently established a titer of "lactic acid bacteria" as indicative of shelf life - what is that level and why does it indicate shelf life?
MRS doesn;t typically include bromcresol purple ( ) though some have used it in this context and at 0.03% (as gram % s not ml's). It's used at 0.025% in other media and that should work in MRS as well. Supplemented medium should be purple and colonies of bacteria producing acid from glucose should produce yellow zones - but you really should validate qualitatively and quantitatively with representative isolates.

-Phil Geis-

Phenol red also will work.


Probably not phenol red - the MRS medium would be acid enough (pH 6.5) to have been yellow by phenol red as made. That's why bromcresol purple - it turns around .5

-Phil Geis-

Right. I use phenol red in mycoplasma medium, pH about 7.5.