Phenol red, it is, as you said, a pH indicator. It is added to cultures of
nitrifying bacteria, at least ammonia oxidizers, to monitor the drop in pH
caused by oxidation of ammonia to nitrite. If the reaction is going OK, the
culture turns yellow. It is then normal to add a base (calcium carbonate) to
neutralise the medium so that growth is not inhibited by low pH.
Of course, you could use a pH meter, but that would be a lot more work than
just looking at the flask, and it would be pretty difficult not to
contaminate your cultures (which, with ammonia oxidizers, are difficult
enough to keep pure as it is!). Similarly, you could use pH indicator strips
or nitrite-indicator strips, but again, that is a lot of work. People have
been using phenol red in nitrifier cultures for a long time, so I don't think
it is doing them any harm, but maybe your friend would like to do a
comparison of methods. I know I wouldn't!
Best of luck to you both,
John Stephen
In article <6pnsc9$qi5 at enews4.newsguy.com>,
"Chrystophylax" <conner at wingate.edu*nospam*> wrote:
> An Internet friend (knowing that I am studying biology) asked the following
> question:
>> Why does media for nitrifying bacteria contain phenol red?
> I'm working with nitrifying bacteria. I've based my media for them on
> literature surveys; most media formulas contain phenol red. Why is this
> compound necessary when pH meters are available? How would phenol red
> affect bacteria?
>> I hate to tell him that I have no idea, can anyone offer any suggestion?
>>
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