Dihydrolase Broth Base

Principle And Interpretation

Vibrios are fairly easy to isolate from both clinical and environmental material, though some species may require growth factors and/or vitamins. Vibrio parahaemolyticus is the leading cause of bacterial diarrhoea associated with the consumption of contaminated food products (1). Dihydrolase Broth Base is formulated as per APHA (2) and is used for studying dihydrolase reaction of V. parahaemolyticus.

Dextrose is utilized by Vibrio species where there is drop in pH indicated by Bromocresol purple resulting in yellow colour. The medium is supplemented with L-Arginine as a substrate for dihydrolase reaction (3,4). L-Arginine is converted to putrescine by the dihydrolase enzyme; however putrescine is also formed from arginine by the decarboxylase system as well. In the decarboxylase system, L-Arginine undergoes decarboxylation to yield agmatine. Agmatine is then catabolized by the enzyme agmatine dihydrolase to putrescine, CO₂ and ammonia by way of an intermediate compound monocarbaminyl putrescine (5). Thus, because of production of amine like putrescine in the medium the pH is elevated (6) changing the colour of the indicator from yellow to purple. Bromocresol purple is the pH indicator in the medium, which turns purple from yellow at alkaline pH. For confirmation, it is suggested to inoculate a basal medium tube, which does not contain L-Arginine. Alkalinization of the surface of the medium may be caused by exposure to air, so a dihydrolase negative organism may be misidentified as positive. It is therefore recommended to protect the inoculated tubes from air with overlay of sterile mineral oil.

Peptone and yeast extract provide nitrogenous nutrients to support bacterial growth. Dextrose is the fermentable carbohydrate. Sodium chloride maintains osmotic equilibrium.