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 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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