What does phenol red lactose broth test for?

Phenol Red Lactose Broth is used for detection of lactose fermenting bacteria.

What is a positive result from the lactose broth?

Because the same pH indicator (phenol red) is also used in these fermentation tubes, the same results are considered positive (e.g. a lactose broth tube that turns yellow after incubation has been inoculated with an organism that can ferment lactose).

What is the indicator for phenol red lactose broth?

pH indicator
Phenol Red Lactose Broth is a complete medium with lactose. Proteose peptone and beef extract provide nitrogenous nutrients to the organisms. Phenol red is the pH indicator, which turns yellow at acidic pH. Sodium chloride maintains osmotic equilibrium.

What does phenol red test indicate?

Phenol red is a pH indicator which turns yellow below a pH of 6.8 and fuchsia above a pH of 7.4. If the organism is able to utilize the carbohydrate, an acid by-product is created, which turns the media yellow.

What is the purpose of using phenol red?

Phenol red is a water-soluble dye used as a pH indicator, changing from yellow to red over pH 6.6 to 8.0, and then turning a bright pink color above pH 8.1. As such, phenol red can be used as a pH indicator dye in various medical and cell biology tests.

What is the principle of the phenol red broth?

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.

What color will indicate a positive urease test?

Urease production is indicated by a bright pink (fuchsia) color on the slant that may extend into the butt. Note that any degree of pink is considered a positive reaction.

What does a positive result for both parts of a lactose fermentation test look like?

An inoculum from a pure culture is transferred aseptically to a sterile tube of phenol red lactose broth. The inoculated tube is incubated at 35-37 C for 24 hours and the results are determined. A positive test consists of a color change from red to yellow, indicating a pH change to acidic.

What does it mean when phenol red turns orange?

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.

What causes color change in phenol red?

With an increase in pH (pKa=1.2) the excess proton in the ketone group is lost, resulting in a yellow color. Further increases in pH (pKa=7.7) causes a loss of the hydrogen from the hydroxyl group resulting in a red color. The ratio of these three moieties enables phenol red to be used as a pH indicator dye.

How does phenol red work in a media?

Phenol red, also known as phenolsulfonphthalein, is a pH indicator dye that exhibits a gradual transition from yellow to red over a pH range of 6.2 to 8.2 (Figure 2). Above 8.2 the dye turns a bright fuchsia color. This color change is an indication that even in the absence of contamination, the media needs replacing.

Is E coli urease-positive or negative?

Another generally urease negative bacterial species is Escherichia coli. Among E. coli strains, about 1% of urease-positive isolates were found.

What do you need to know about phenol red broth?

Phenol Red Broth. Phenol Red Broth is a general-purpose differential test medium typically used to differentiate gram negative enteric bacteria. It contains peptone, phenol red (a pH indicator), a Durham tube, and one carbohydrate. The tube inoculated with Escherichia coli produces acid and gas. It would be read as A/G.

Why does the P H drop in lactose broth?

If lactose is fermented to produce acid end products, the p H of the medium will drop. A p H indicator in the medium changes color to indicate acid production. What medium is used? Several media are available for this. Most commonly used is phenol red lactose broth.

What does phenol red fermentation test tell you?

It indicates that there is drop in the pH because of the production of the acid by the fermentation of the carbohydrate (sugar) present in the media. Negative: The tube containing medium will remain red, indicating the bacteria cannot ferment that particular carbohydrate source present in the media. II. Gas Production

What happens if you add carbohydrates to phenol red?

The addition of some carbohydrates to the basal medium may result in an acid reaction. To ensure accuracy of interpretation, uninoculated control tubes and/or inoculated Phenol Red Broth Base control tubes should be run in parallel with the fermentation tests.