Lab 10 - Phenol Red Carbohydrate Broths

What is observed to assess phenol red broth results?

Color of the broth and presence of a bubble are observed in phenol red broths

You have grown an obligately aerobic culture in phenol red glucose broth, and some of the media has turned yellow. How would you explain this?

Obligate aerobes cannot ferment carbohydrates. However, the metabolic pathways found in obligate aerobes can produce acids (Krebs cycle, for example). It is the presence of these acids that turn the phenol red yellow (the broth will often appear orange in these instances).

What type of molecule is used as the final electron acceptor in fermentation?

Organic molecules are used as the final electron acceptor in fermentation

Phenol red turns what color in an acidic solution?

Phenol red becomes yellow in the presence of acid (pH<6.8)

What is the function of phenol red broth?

Phenol red broths are used to determine if a bacterium can ferment a particular sugar, and if it does ferment, if acid or acid and gas are produced.

What pH indicator is used in phenol red broth?

Phenol red is the pH indicator in phenol red broths.

You have an unknown culture and do not have Gram stain results. You are allowed one medium to determine if it can ferment lactose. Would you use phenol red lactose broth or MacConkey agar?

Phenol red lactose broth should be used to test for lactose fermentation. MAC will not give any results regarding fermentation if you are working with a Gram-positive organism. Phenol red lactose broth works with both Gram+ve and Gram-ve bacteria.

What is the function of the Durham tube?

The Durham tube collects gases produced by fermentation.

What is the purpose of the Durham tube?

The Durham tube is used to capture gases produced via fermentation.

You check your fermentation broth and find that the top is red and the bottom is yellow. What can you conclude?

The tube has been incubated too long. As bacteria continue to grow, the carbohydrate in the tube will run out and they will metabolize the proteins in the medium - this leads to the release of amino groups and will raise the pH, turning the top of the tube red.

What are the differential ingredient in phenol red broth?

The type of sugar and phenol red broth are the differential ingredients

What are your observations and conclusions for these phenol red broths? (see picture)

Tube A, uninoculated control Organism B - red broth, no bubble; no fermentation c. Organism C - yellow broth, bubble present; acid and gas fermentation

A culture ferments mannitol and glucose to acid, but not lactose and sucrose. Is this possible? Why?

Yes, it is possible for an organism to ferment some sugars, but not others. The genetics of a given bacterium may allow it to produce the enzymes to metabolize some sugars but not all sugars.

What are the end products in each of the following types of bacterial fermentation? a. homolactic fermentation b. heterolactic fermentation c. mixed acid fermentation d. 2,3 butanediol fermentation

a. homolactic fermentation Lactic acid b. heterolactic fermentation Lactic acid, ethanol, CO2 c. mixed acid fermentation Lactic acid, acetic acid, succinic acid, formic acid, ethanol, CO2, H2 d. 2,3 butanediol fermentation 2,3 butanediol, lactic acid, formic acid, ethanol, CO2, and H2

What observations would you expect to see in Phenol Red Glucose Broth for each of the following types of fermentation? a. homolactic fermentation b. heterolactic fermentation c. mixed acid fermentation d. 2,3 butanediol fermentation

a. homolactic fermentation Yellow broth, no bubble in Durham tube b. heterolactic fermentation Yellow broth, bubble in Durham tube c. mixed acid fermentation Yellow broth, bubble in Durham tube d. 2,3 butanediol fermentation Yellow broth, bubble in Durham tube