Salmonella and Shigella are major causes of bacterial enteric illness with transmission occurring via the fecal-oral route, person-to-person contact, or by the ingestion of contaminated food or water.

• Ready-to-use – Convenience of a prepared media.
• Performs screening and isolation on the same dish using two agars.
• Ease of handling and less product is used.
• Ease of identification – On SS agar colonies of Salmonella and Shigella are smooth and opaque; some appear to have black centered colonies whereas on XLD agar Salmonella produces red colonies without black center and Shigella colonies are transparent.
• Selective – Due to the inclusion of selective inhibitory substances brilliant green, bile salts, thiosulphate, and citrate in SS agar and desoxycholate in XLD agar, most gram-positive and coliform organisms are inhibited.

The SS agar contains bile salts, brilliant green and citrates which acts as selective agents and inhibit gram-positive bacteria and coliforms. Sodium thiosulfate, a sulfur source and ferric ammonium citrate work as an indicator to enable organisms which produce H₂S to form black-centered colonies, including some strains of Salmonella. Lactose is a differentiator.

The XLD agar contains desoxycholate which acts as a selective agent and inhibits gram-positive organisms. Xylose is fermented by most gram-negative bacilli. Lysine acts as a differentiator for Salmonella spp. from non-pathogenic enteric bacilli. Salmonella produces lysine decarboxylase which causes pH to revert to alkaline after xylose is fermented, producing red colonies. Lactose and sucrose are added in excess to prevent lysine-positive coliforms from reverting to alkaline conditions. Sodium thiosulfate, a sulfur source, and ferric ammonium citrate work as indicators to enable organisms which produce H₂S to form black-centered colonies under alkaline conditions.

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