Purpose: Purple Broth and Phenol Red Broth
are used to detect bacterial ability to ferment carbohydrates
. fermentation characteristics are useful in differentiating Gram-negative bacilli, especially members of Enterobacteriaceae.
the principle behind purple broth and phenol red broth is the same. each medium consists of a basal recipe to which a single fermentable carbohydrate is added. both media include peptone, a pH indicator, and a specific carbohydrate. purple broth includes bromcresol purple as the pH indicator, which is yellow below pH 6.8 and purple above
; pr broth includes phenol red, which is yellow below pH 6.8, pink above pH 7.4 and red in between
. Acid production from fermentation of the carbohydrate lowers the pH below the neutral range of the indicator and turns the medium yellow. deamination of the peptone amino acids produces ammonia which raises the pH and turns PR broth pink or, in the case of Purple broth, produces no color change. during preparation of the media an inverted durham tube is placed inside the broth to trap gas that may be produced during fermentation. a positive indication of gas production is a bubble or pocket in the durham tube where the broth has been displaced. The ability of these media to detect acid production is largely dependent upon incubation time. if the medium is examined within 18 hours of inoculation, pink color(pr broth) or purple color(purple broth) indicate that the organism has not fermented the substrate and has likely deaminated the peptone amino acids. readings after 18 hours, however are unreliable because of the possibility of a reversion. a reversion is what happens in the medium when an organism switches metabolic modes from fermentation to deamination after the exhaustion of carbohydrate. testing for: ability to ferment sugar:by looking for acid production (A) &/or gas production (G).If pH is high, there was deamination, or ammonia production. This means the test is negative for fermentation, and the broth will be pink or have no color changeif pH is low, there was acid production from fermentation of the carbohydrate and this will turn the medium yellow.