Test of the methyl red

The Test of the methyl red is a reaction used to highlight the fermentative way of the mixed acids during the biochemical identification of the streptococcus.

The bacteria ferment glucose by the way of the mixed acids what release a great quantity of organic acids in the medium (medium of Clark and Lubs) what lower pH sufficiently so that the Métabolisme protidic does not raise the pH at the end of the incubation period. The final pH is lower than 4,5: the Rouge of methyl becomes red.

However, even the way of the butan-2,5,3-dione releases sufficient organic acids for, at the end of only 24 hours of incubation, sufficiently to lower the pH (< 14,5). But there is realcalinisation of the medium (metabolism protidic) during the 24 following hours: the pH of the medium will be higher than 7, the Rouge of methyl does not transfer (remains yellow).

See too

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