The following picture shows the isolated black colonies of Salmonella on Salmonella-Shigella (S-S) Agar. S-S Agar is used as a selective and differential medium for the isolation of Salmonella and some Shigella species. The inclusion of bile salts is to inhibit gram-positive organisms. Ferric citrate is added to the medium as an indicator for hydrogen sulfide production. Salmonella and Shigella ( Non-lactose-fermenting) appear as transparent or translucent colorless colonies on SS Agar, however Salmonella colonies may also appear with a black center due to hydrogen sulfide production. This is submitted by the team of Microbiology & Immunology department at the Faculty of Pharmacy, Modern Sciences & Arts University (MSA) , Giza , Egypt.