The methyl red (MR) test detects the production of sufficient acid during the fermentation of glucose and the maintenance of conditions such that the pH of an old culture is sustained below a value of about 4.5, as shown by a change in the colour of the methyl red indicator which is added at the end of the period of incubation.
Clark and Lubs developed MR-VP Broth which allowed both the MR and VP tests to be performed from the same inoculated medium by aliquoting portions to different tubes.