Mannitol Salt Agar (MSA) & Glucose broth

Mannitol Salt Agar (MSA)

This type of medium is both selective and differential. The MSA will select for organisms such as Staphylococcus species which can live in areas of high salt concentration (plate on the left in the picture below). This is in contrast to Streptococcus species, whose growth is selected against by this high salt agar (plate on the right in the picture below).

The differential ingredient in MSA is the sugar mannitol. Organisms capable of using mannitol as a food source will produce acidic byproducts of fermentation that will lower the pH of the media. The acidity of the media will cause the pH indicator, phenol red, to turn yellow. Staphylococcus aureus is capable of fermenting mannitol (left side of left plate) while Staphylococcus epidermidis is not (right side of left plate).

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Glucose broth with Durham tubes

This is a differential medium. It tests an organism's ability to ferment the sugar glucose as well as its ability to convert the end product of glycolysis, pyruvic acid into gaseous byproducts. This is a test commonly used when trying to identify Gram-negative enteric bacteria, all of which are glucose fermenters but only some of which produce gas.
Like MSA, this medium also contains the pH indicator, phenol red. If an organism is capable of fermenting the sugar glucose, then acidic byproducts are formed and the pH indicator turns yellow. Escherichia coli is capable of fermenting glucose as are Proteus mirabilis (far right) and Shigella dysenteriae (far left).  Pseudomonas aeruginosa (center) is a nonfermenter.
The end product of glycolysis is pyruvate. Organisms that are capable of converting pyruvate to formic acid and formic acid to H2 (g) and CO2 (g), via the action of the enzyme formic hydrogen lyase, emit gas. This gas is trapped in the Durham tube and appears as a bubble at the top of the tube. Escherichia coli and Proteus mirabilis (far right) are both gas producers. Notice that Shigella dysenteriae (far left) ferments glucose but does not produce gas.
*Note - broth tubes can be made containing sugars other than glucose (e.g. lactose and mannitol).  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).