phenol red

Yersinia yersinia at CYBERNEX.NET
Wed Jul 29 21:27:22 EST 1998

Chrystophylax writes,

<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?>

I'd be curious to know about your experiments and the media formulations 
you're using. I've spent the last year doing zillions of bacterial 
identifications, including many tests for nitrate reduction. To do these 
nitrate reduction tests, I inoculate a tube of nitrate broth, incubate it 
overnight and then add reagents (unless, of course I see bubbles in the 
inverted Durham tube, in which case I know that the organism reduced the 
nitrate to nitrogen) - sulfanilic acid and alpha naphthol. Well, when I 
saw your post here, I looked up the ingredients of nitrate broth, and it 
contains *no* phenol red - only beef extract, peptone and of course 
potassium nitrate.

Like I said, I don't know what experiments you're doing or what you're 
growing your bugs on, but if you want to know about nitrate reduction in 
general, nitrites (a product of nitrate reduction - it goes NO3  to NO2 
to N2) are colorless - but in an acid environment (such as my above 
mentioned nitrate reduction testing, when I add the sulfanilic acid and 
alpha naphthol, they will turn the broth red.

I'm familiar with phenol red as an additive to other media, usually to 
indicate whether or not an organism ferments a sugar to an alcohol to an 
acid, or an alcohol to an acid. Such characteristics are used to make 
identifications of microorganisms. As to why put phenol red in media 
rather than use a pH meter, I would think it's much easier and cleaner to 
look at the plate and see the yellow color than to contaminate a pH 
electrode measuring the pH of a medium containing bacteria just to see if 
the pH is acidic. Besides, I don't think they had pH meters at the time 
these media were originally being formulated.  ;-)

I hope I helped a little, anyway. Good luck with your experiments!


"If you're not part of the solution, you're part of the precipitate."

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