Moeller Decarboxylase Broth w/ Ornithine Hydrochloride

Principle And Interpretation

Moeller Decarboxylase Broth with Ornithine hydrochloride is used for differentiating gram-negative enteric bacilli on the basis of their ability to decarboxylate L-Ornithine hydrochloride. Decarboxylase Broth was introduced by Moeller for detecting the production of lysine and ornithine decarboxylase and arginine dihydrolase (1). Prior to Moellers work, bacterial amino acid decarboxylases were studied by Gale (2) and Gale and Epps (3). Production of ornithine decarboxylase is helpful criterion in differentiating Klebsiella and Enterobacter species. Klebsiella are nonmotile and do not produce ornithine decarboxylase while Enterobacter are motile and produce ornithine decarboxylase except Enterobacter agglomerans (4). Decarboxylase media are also recommended by standard methods for identification of bacteria (5-8).

This medium contains HM peptone B and peptone which provide nitrogenous nutrients for the growth of bacteria. Dextrose is the fermentable carbohydrate and pyridoxal is the co-factor for the decarboxylase enzyme. Bromo cresol purple and cresol red are the pH indicators in this medium. When the medium is inoculated with the dextrose fermenting bacteria, the pH is lowered due to acid production which changes the colour of the indicator from purple to yellow. Acid produced stimulates decarboxylase enzyme. Putrescine is produced due to ornithine decarboxylation. Formation of amine putrescine increases the pH of the medium, changing the colour of the indicator from yellow to purple. If the organisms do not produce the appropriate enzyme, the medium remains acidic, yellow in colour. Each isolate to be tested should also be inoculated into the basal medium tube lacking the amino acid. After incubation, a decarboxylase test may show two layers of different colours, yellow and purple. Shake the tube gently before interpreting the results (4).