“Laboratory Data”, submit the answers to the following questions.

  1. How might lactose fermentation in the gastrointestinal tract lead to medical problems in some people?
  2. Gram-positive bacteria do not grow on MacConkey agar plates. How could you test those bacteria for lactose fermentation?

Introduction and Background

MacConkey agar is a selective and differential medium used to support the growth of Gram-negative bacteria (selective) and determine whether these bacteria can metabolize the sugar lactose (differential).  It is a growth medium commonly used in characterizing gastrointestinal bacteria, which can live in the gastrointestinal tract as part of the normal flora of the gastrointestinal tract.  MacConkey agar is designed to support the growth of these organisms and their differentiation in the following manner:

  1. Crystal Violet and Bile Salts – Inhibit the growth of Gram-positive bacteria (selective)
  2. Lactose – Allows to differentiate between species of Gram-negative bacteria
  3. Neutral red – a pH indicator that changes color if lactose is fermented.

If you obtain bacterial growth on your MacConkey agar plate, it is most likely a Gram-negative bacterium.  Then you can determine if it is a lactose fermenter by viewing the color of the medium (uninoculated MacConkey agar is light purple in color):

  1. Positive for lactose fermentation – The bacterial colonies turn red to purple
  2. Negative for lactose fermentation – The bacterial colonies remain off-white or their natural color

Examples of bacteria in both categories of lactose fermentation include Escherichia coli (positive) and Proteus vulgaris (negative). 

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