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The purpose of this test is to determine whether an organism has the enzyme beta-galactosidase by using the compound o-nitrophenyl-beta-D-galactopyranoside (ONPG). This test is used to differentiate between lactose-negative and lactose-delayed organisms.
ONPG is hydrolysed by the enzyme to o-nitrophenol, which is yellow, and galactose.
1. Uninoculated medium.
2. Positive reaction: Citrobacter.
3. Negative reaction: Salmonella.
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© Department of Veterinary Disease Biology 2011
Faculty of Health and Medical Sciences - University of Copenhagen
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