VetBact

Applications:

Purple agar with lactose is often used for the detection and typing of members of the family Enterobacteriaceae, beacuse it can be used for discrimination of lactose fermenters and non-lactose fermenters. MacConkey agar is used in some laboratories instead of purple agar (with lactose).

The medium contains:

• peptone (enzymatic digest of animal tissues or milk, which constitute a source of nutrients)
• beef extract ( source of nutrients)
• lactose (to differentiate between lactose fermenters and non-fermenters)
• NaCl (provides the osmotic balance)
• sodium ammonium phosphate (buffering agent)
• bromcresol purple (pH indicator)
• agar (solidifying agent)
• H₂O

The pH of the medium should be 6.8 and at this pH bromcresol purple is bluish-purple. Lactose fermenting bacteria lower the pH of the medium to acid, which causes the medium to change in colour from blue-purple to yellow. Lactose can be replaced with for instance maltose or trehalose to determine if a certain bacterium is able to ferment these carbohydrates. These modified media are used for other bacteria than members of the family Enterobacteriaceae, for instance members of the genus Staphylococcus.

Other comments:

• Bacteria, which ferment the contained carbohydrate, form colonies, which appear to be yellowish, because the surrounding agar  turns yellow.
• Bacteria, which do not ferment the contained carbohydrate, form colonies, which apper to be light purple, because the surrounding agar remains purple.

Updated: 2018-05-09.