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[Microbiology] Re: phenol red fermentation tests

myrehfuss from ucdavis.edu via microbio%40net.bio.net (by myrehfuss from ucdavis.edu)
Wed Feb 21 13:56:11 EST 2007

On Feb 20, 7:09 am, "Lisa Moore" <lmo... from usm.maine.edu> wrote:
> I teach a basic microbiology lab class and have included the phenol red
> fermentation test in the lab after the students do the
> oxidation-fermentation glucose test.  I am trained as a phytoplankton
> ecophysiologist, so I am not familiar with this test outside of teaching
> this lab.  From the various lab books and websites that I have looked
> at, the test is done in test tubes under aerobic conditions.  My
> question is whether a non-fermenter can oxidize the sugars (such as
> lactose and sucrose) and turn the medium yellow (indicative of acid
> production), thus giving a false positive?  It seems to me that this is
> possible, making the test rather limited.  Of course, I could be missing
> something in understanding the protocol or chemistry behind the test.  
> If someone has a quick answer or a good reference, I would appreciate
> the help.
> Thank you,
> Lisa
> Lisa R. Moore
> Assistant Professor
> Department of Biological Sciences
> University of Southern Maine
> 96 Falmouth Street
> Portland, ME  04103
> email:  lmo... from usm.maine.edu
> phone:  (207) 780-4261
> fax:  (207) 228-8116

With prolonged incubations, bacteria that do not oxidize the
particular carbohydrate you have added to your phenol red broth can
begin growing oxidatively on the peptone.  In this case, the indicator
is neutralized and the broth will appear reddish.  As far as false
fermentation positives, this shouldn't occur.  Do you mean complete
oxidation of carbohydrates (glycolysis -> TCA -> e- transport chain)?
If this occurred there wouldn't be a buildup of organic acids anyway,
since glycolysis isn't the end point.  In any case, if a bacterium
cannot ferment a particular sugar, it will not cause acid buildup and
yellowing of the medium, at least not significantly.

Also, you are going to put in Durham gas tubes, correct?  A true
positive is a yellow solution and trapped CO2 in the inverted tube.


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