Phenol Red Fermentation Tubes


Purpose:  To determine if a bacterium can ferment a particular carbohydrate and determine the end products of that fermentation.

Principle:  Phenol Red Fermentation medium contains peptone, phenol red (a pH indicator) and the carbohydrate to be tested.  Phenol red is yellow at a pH < 6.8 and red at a pH of > 7.4, therefore if a bacterium ferments a sugar to acid a yellow color will develop.  Additionally, phenol red tubes contain an inverted Durham tube.  If a bacterium produces a gas during fermentation this tube will trap that gas a a bubble will be observed in the Durham tube.  Sometimes an orange color will develop in the tube, this represents a pH > 6.8 and should not be considered positive.  Also be catuious of tubes that inititally produce acid then turn red or pink as nitrogenous products are produced.  This type of reaction is characteristic of oxidative, not fermentative metabolism.

Possible Results:


 
Possible phenol red tube results include: (A) Formation of acid and gas (bubble is indicated by arrow), (B) Formation of acid, (C) uninoculated control, (D) alkaline byproducts, and (E) no acid or gas formation.

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