Several enzymes exist that can remove the carboxyl group (COOH) from amino acids. The presence of decarboxylase activity can aid in the identification of Enterobacteriaceae.
Amino Acid
Enzyme
Product
lysine
lysine decarboxylase
cadaverine
ornithine
ornithine decarboxylase
putrescine
arginine
arginine dihydrolase, then ornithine decarboxylase
citrulline to ornithine to putrescine
Moeller decarboxylase medium is used, with the L (levo) form of the amino acid of interest added. The control tube is the medium them without addition of the amino acid.
Both tubes are incubated anaerobically (by overlaying with mineral oil) at 35°C for 18 to 24 hours.
The amine products are alkaline, so the indicator bromcresol purple will be purple when the reaction is complete if decarboxylation occurs. If decarboxylation does not occur then the color indicator will be yellow from acid production; the control tube should be yellow.
Causes of a false positive reaction:
(1) mixed isolate
Causes of a false negative reaction:
(1) reading the tube too soon
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