How do Enterobacter species appear in MacConkey agar?

Updated: Jun 18, 2019
  • Author: Susan L Fraser, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Answer

MacConkey agar is a lactose-containing medium that is selective for nonfastidious gram-negative bacilli such as Enterobacteriaceae. Using the enzymes beta-galactosidase and beta-galactoside permeases, the most frequently encountered species of Enterobacter strains activate the pH indicator (neutral red) included in MacConkey agar, giving a pink or red stain to the growing colonies. Klebsiella and Enterobacter species may appear similar as mucoid colonies but can be differentiated with a few specific tests. In contrast to Klebsiella species, Enterobacter organisms are motile, usually ornithine decarboxylase-positive, and urease-negative. E sakazakii (reclassified into the Cronobacter genus in 2007) produces a characteristic yellow pigment.


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