Gluconate Test HiVeg™ Medium

Principle and Interpretation

Gluconate Test HiVeg Medium is a modification of Gluconate Test Medium prepared by replacing Peptic digest of animal tissue with HiVeg peptone. Therefore this medium is free of TSE/BSE risk. Besides HiVeg peptone, yeast extract in the medium also serves as supplier of nitrogen, vitamins and other essential growth nutrients. Dipotassium hydrogen phosphate helps in buffering the medium. Potassium salt of gluconate in media serves as a readily available sole carbon source for the organism to be tested for gluconate metabolism.

This medium is used to check the ability of an organism to oxidize gluconates to alpha keto-gluconate which subsequently accumulates in the medium (1). Pseudomonas aeruginosa is known to accumulate atleast 50% of ketogluconate after 48 hrs of incubation (2). The basis of the Gluconate test is the change from gluconate, a non-reducing compound to alpha keto-gluconate which is a reducing compound (1, 3). The alpha keto-gluconate formed when tested with a suitable reagent like Benedicts reagent, reduces copper sulphate (blue colour) to an insoluble cuprous oxide which is precipitated out. A yellow to orange to orange red precipitate is formed. The colour depends on the amount of reducing substance accumulated. Greater the amount of alpha keto-gluconate in the medium more orange to orange - red colour develops. However colours ranging from slight green to deep orange indicates oxidation. If the medium remains blue or bluish green after addition of Bendicts solution, it is considered as negative reaction which means no reducing substance is present or formed and potassium gluconate from the media is not metabolized.