Salmonella Shigella Agar
Culturing techniques efficiently identify bacterial strains based on their growth requirements. The Salmonella Shigella agar (SSA) medium restricts certain gram-positive strains while permitting the growth of specific gram-negative strains. It contains peptone, lactose, and essential micro-nutrients. Bile salts and brilliant green dye act as selective agents. The pH indicator, neutral red, turns red when pH drops below 6.8 due to lactose fermentation by Escherichia coli. Salmonella uses peptone as a carbon source and does not ferment sugar.
To aid identification, SSA contains ferric acid and sodium thiosulfate as reducible sulfur sources. Most Salmonella strains produce hydrogen sulfide, turning the medium black. In contrast, Shigella lacks the enzymes for sulfur reduction, maintaining its original color. SSA provides valuable insights into bacterial characteristics concisely and effectively.
Figure 1: Labelled SSA agar plate showing the difference in colony color of four different bacterial species.
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