[Microbiology] Salmonella–Shigella Agar

Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory, Salmonella–Shigella Agar

Purpose

Salmonella-Shigella (SS) Agar is a selective medium originally used for the isolation of Salmonella and many Shigella species (i.e., lactose nonfermenting enteric bacteria) from the lactose fermenting enterics (the coliforms). It is no longer recommended for isolation of Shigella, since Hektoen and XLD agars are more effective, but is still of use in isolating Salmonella species.

Principle

Salmonella-Shigella Agar is an undefined, differential, and selective medium with bile salts and brilliant green dye acting as the selective agents against Gram-positives and many Gram-negatives. Lactose is included as a fermentable carbohydrate and sodium thiosulfate provides a source of reduceable sulfur. Neutral red is the pH indicator and ferric citrate reacts with H2S to form a black precipitate, thus indicating sulfur reduction. Lactose fermenters will produce reddish colonies as neutral red changes from colorless to red in the low pH. Salmonella and Shigella species will be their natural color due to their inability to ferment lactose. Salmonella and Proteus species typically reduce

sulfur, which is indicated by colonies with black centers. Refer to Figures 2-26 and 2-27.

2-26 SALMONELLA-SHIGELLA AGAR SS Agar

inoculated with (clockwise from top) Escherichia

coli, Shigella flexneri, Salmonella typhimurium, and

Enterococcus faecalis. Note the pink color of E. coli

due to acid production during lactose fermentation. 

Note also the black precipitate in the

Salmonella growth due to the reaction of ferric

citrate in the medium with H2S produced from

sulfur reduction. This phenomenon produces

black colonies when the organism is streaked

for isolation (Figure 2-27). Growth of E. faecalis

is inhibited by bile salts and brilliant green dye.  

2-27 SALMONELLA-SHIGELLA AGAR

STREAKED FOR ISOLATION SS Agar inoculated

with Salmonella typhimurium and Shigella

flexneri. Note the colonies with black centers

and clear edges characteristic of Salmonella on

this medium.