Moeller Decarboxylase Broth w/ Lysine Hydrochloride

Principle And Interpretation

Many species of bacteria possess enzymes capable of decarboxylating specific amino acids in the test medium releasing alkaline-reacting amines and carbon dioxide as byproducts. The decarboxylase activity of Enterobacteriaceae is most commonly measured with Moeller Decarboxylase Broth (1). This medium was formulated by Moeller for detecting the production of lysine and ornithine decarboxylase and arginine dihydrolase (2). Prior to Moellers work, bacterial amino acid decarboxylases were studied by Gale (3) and Gale and Epps (4).

Decarboxylase media are also recommended by standard methods for identification of bacteria (5-8). Moeller Decarboxylase Broth w/lysine hydrochloride is used for differentiating bacteria on their ability to decarboxylate lysine hydrochloride. 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. Decarboxylation of lysine yields cadaverine. Formation of the amine cadaverine 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).