The STS system that was devised and tested consisted of 4 layers within a single tube. The 4 layers from the top to the bottom are as follows: (1) lysine, glucose, and peptone–containing medium with the ingredients necessary to detect H2S production; this layer was slanted; (2) lysine, glucose, and peptone–containing media without the ingredients for H2S production; (3) sterilized petrolatum; and (4) standard urea agar.
     An uninoculated STS tube (A) shows that the sterilized petrolatum layer clearly separates the urea agar at the bottom of the tube from the lysine-containing media in the top half of the tube. The STS tube inoculated with Salmonella (B) demonstrates hydrogen sulfide production, which does not obscure the positive lysine decarboxylation reaction, and a negative urea reaction. The STS tube inoculated with Shigella (C) demonstrates glucose fermentation (no gas was produced) and negative lysine decarboxylation and urease reactions. The STS tube inoculated with Proteus (D) demonstrates a positive urea reaction, deamination of amino acids, and hydrogen sulfide production
Source: Medscape
A Single-Tube Screen (STS) for Salmonella and Shigella