Need help on MR-VP test

Tue Mar 7 15:56:41 EST 1995

Pong Nim writes:

>I am having problem obtaining a positive test for Methy Red (MR)
>and Voges-Proskauer (VP) reaction.  
>I have grew Enterobacter aerogenes & Serratia marcescens in
>MR-VP broth at 35 C for 48 hrs.  Five drops of MR were added.
>on one set of MR-VP broth.  Twelve drops of VP reagent I and
>three drops of VP reagent II were added to the second set of
>MR-VP broth.  All reagents were made fresh on the day of the test.
>Still I can't seem to get any positive reaction at all.  What
>am I doing wrong.  Your helpful advice would greatly be appreciate.

In reply:

The Enterobacter, of course, should be negative for the MR test -- and while 
some S. marcescens will be positive, not all will be.  So probably you just 
have an MR-negative strain of Serratia.  Try over with E. coli, and I bet it'll 

As for the V.P., often that needs to go for longer than 2 days -- typical 
protocols may specify *5* days.  Try both of your strains after a longer 
incubation.  Also, be sure to put parafilm over the tube and shake it 
vigorously several times over several minutes, before deciding it's negative.  
The reaction needs oxygen.  (Don't expect necessarily an instant result - it 
might take as long as a half an hour.  I've also seen some suggestions that the 
mixture be heated gently, but I've never had to do that.)  Finally, the volumes 
you mention *might* be at fault - you don't say what exact volumes you're using 
- I've had good results with a half ml of each reagent for every ml of medium.

Good luck.

Terry W. Hill
Dept. of Biology
Rhodes College
Memphis, TN  38112

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