Prepared Plated Media PPM

XLD AGAR (XYLOSE LYSINE DESOXYCHOLATE AGAR)

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Catalog Number : 503

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Description

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XLD AGAR was developed principally for isolating and diff erentiating Gram-negative enteric bacilli, particularly Shigellaand Salmonella. It has been shown to be more eff ective than other enteric diff erential media. The reactions that take place are the degradation of the three fermentable carbohydrates: xylose, lactose and sucrose, with the production of acid, manifested in the color change from red to yellow. Sodium thiosulfate serves as a reactive substance, with Ferric ammonium citrate as an indicator of the formation of hydrogen sulfi de under alkaline conditions. Lysine allows the Salmonella group to be diff erentiated from the non-pathogens since, without it, salmonellae would quickly ferment the xylose and be indistinguishable from non-pathogenic species. Once the salmonellae consume the xylose, lysine is attacked via the enzyme, lysine decarboxylase, with a reversion to an alkaline pH which is similar to the Shigella reaction. The bacteria that decarboxylate the L-Lysine to cadaverine are indentifi ed by the presence of a purple- red color around the colonies due to the elevation of the pH. Phenol red is the pH indicator. Yeast extract is the source of vitamins, particularly of the B-group essential for bacterial growth. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Sodium desoxycholate is the selective agent inhibiting Gram-positive microorganisms. Bacteriological Agar is the solidying agent.

The European Pharmacopoeia recommends to inoculate and incubate Escherichia coli (indicative) as well as Salmonella at 30 - 35ºC during 18 - 48 hours.It also recommends in Paragraph 2.6.13 “Microbiological examination of non-Sterile products: test for specifi ed microorganisms” to subculture in this medium after incubation in Rappaport Vassiliadis Salmonella Enrichment Broth (Cat. 1414), at 30 - 35ºC for 18 - 24 hours and incubate this medium at 30 - 35ºC for 18 - 48 hours.Interpretation: The possible presence of Salmonella is indicated by the growth of well-developed red colonies, with or without black centers. This is confi rmed by identifi cation tests.The product complies with the test if colonies of the types described are not present or if the confi rmatory identifi cation tests are negative.

MICROBIOLOGICAL TEST
The following results were obtained in the performance of the medium from type cultures after incubation at a temperature of 30 - 35ºC and observed after 18 - 48 hours.