Selenite media is an enrichment media that is used for primary isolation of enteric pathogens such as Salmonella and Shigella species. It is a liquid media containing chemical constituents, which inhibit normal enteric flora and allow pathogens which may be present in very small numbers in the specimen to grow unhampered and thus enriching them. Shown here is uninoculated selenite media on the left, inoculated (but unincubated) media in the middle, and inoculated media that has been incubated for 24 hours to enrich for enteric pathogens. Isolated colonies of these organisms may be obtained by sub-culturing onto solid media, such as XLD media shown below.

Xylose lysine deoxycholate (XLD) agar is a highly selective and differential medium for the isolation and identification of Salmonella and Shigella species. Both Salmonella and Shigella species can decarboxylate the lysine in the media to produce red colonies. Additionally, Salmonella produces a positive hydrogen sulfide production reaction, which colors the centers of the colonies black.

On the slide above, the Shigella species are on the left of the plate (pink colonies), and the Salmonella species are on the right (black colonies).

Normal fecal flora bacteria can also grow on XLD, but usually growth is limited to a few colonies. The colonies are yellow in color as seen with the E. coli colonies on the left. Yellow colonies with black centers indicate normal enteric organisms that produce hydrogen sulfide.