The Methyl Red Test in Peptone Media

SUMMARY: The methyl red (mr) reactions of eighteen strains of coliform bacteria were investigated. Two different media were used, glucose phosphate peptone broth and a liquid defined medium. In the glucose phosphate peptone medium the MR test was found to vary with the brand of peptone used, but the defined inorganic salt + glucose medium gave constant results throughout. After 5 days of incubation at 30° in a glucose phosphate peptone medium twelve substrains selected from each of three strains showed a maximum pH difference of 0·2 unit. When the test strains were inoculated into two batches of this medium made up on different days, pH differences up to 0·6 unit were obtained between the two batches.