This is a bacterial staining technique used to differentiate between organisms based on their cell wall properties. The staining procedure takes up to 15 minutes. The organisms can then be observed for characteristic shapes which will add to the information needed to identify them. HE Hektoen enteric agar is a differential medium which is used for isolating enteric pathogens. These plates are green and contain sugars to differentiate between organisms that are able to ferment sugars and those that cannot.
Fermentation is determined by the color of the colonies.
Fermenting colonies may also give a yellow color to the agar around them. Some organisms like Salmonella will have a black center caused by hydrogen sulfide production. This test is used to determine if the amino acid tryptophane is hydrolyzed to produce indole. A colony is picked off of a BAP with a cotton swab and rubbed onto a piece of filter paper with Kovac's reagent. This procedure takes up to 15 minutes.