Principles and uses
Lysine Decarboxylase Broth is used to detect and differentiate Enterobacteria from other microorganisms, based on lysine decarboxylation.
Gelatin peptone provides nitrogen, vitamins, minerals and amino acids essential for growth.Yeast extract is a source of vitamins, particularly of the B-group. Dextrose is the fermentable carbohydrate. Bromocresol purple is the pH indicator. Lysine is added to detect the production of the specific enzyme.
When the medium is inoculated with a bacterium that is able to ferment dextrose, the acid produced lowers the pH of the medium and changes the color of the indicator from purple to yellow. The acidic condition also stimulates decarboxylase activity. The bacteria that decarboxylate the L-Lysine to cadaverine are identified by the presence of a purple-red color. The production of these amines elevates the pH of the medium. A yellow color after 24 hours indicates a negative result. By substituting L-Lysine with Arginine or Ornithine, the new resulting medium (Falkow Broth Base) can be used to study the decarboxylation of these amino acids.
Preparation
Suspend 14 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense quantities of 5 ml into screw-capped tubes. Sterilize in autoclave at 121 ºC for 15 minutes. Leave caps loose to allow gas exchange. Close well after sterilization.
Instructions for use
- Gently insert the inoculation loop with the sample into the tube and move the loop back and forth several times to inoculate the media.
- Incubate at 35±2 °C for 24 hours
- Color change to purple: positive decarboxylation reaction (Escherichia, Klebsiella, Salmonella, except S.parathyphi, Arizona, Alkalescens Dispar, Serratia).
- Yellow color: negative decarboxylation reaction (Proteus, Providencia, S.parathypi A,Shigella, Aeromonas, Citrobacter).