Mannitol Lysine Agar, 5 kg

Human Salmonella infections are most commonly caused by ingestion of food, water or milk contaminated by human or animal excreta (1). One of the most important criteria in the identification of Salmonella species is the production of hydrogen sulphide. Salmonella Typhi and Salmonella Paratyphi A can be differentiated from the rest of the Salmonella due to their inability to form hydrogen sulphide. Mannitol Lysine Agar, formulated as described by Inoue et al (2) is used for the selective isolation of Salmonella species other than Salmonella Typhi and Salmonella Paratyphi A from different foods and faeces. Mannitol Lysine Agar may be used directly with the specimen or from an enrichment culture (3). Enrichment can be carried out in Modified Semisolid RV Medium (M1482). Mannitol Lysine Agar does not depend upon lactose fermentation and is therefore recommended for investigating lactose fermenting Salmonellae like Salmonella Arizonae . Further tests should be carried out for confirming Salmonella species. Peptic digest of animal tissue, beef extract, yeast extract provide essential nutrients for the growth of Salmonella . Mannitol is the fermentable carbohydrate in the medium while L-lysine is the amino acid. Salmonellae grow as large purple colony with black center because of H₂S production. Mannitol is fermented by organisms and the resulting acid stimulates lysine decarboxylation. This elevates the pH due to production of amines and promotes blackening. Sodium thiosulphate and ferric ammonium citrate help in H₂S production. Atypical Salmonella strains do not produce H2S and form grey colonies. Brilliant green dye in the medium inhibits gram-positive and majority of gram-negative organisms. Mannitol Lysine Medium should be used in conjunction with Brilliant Green Agar, Modified (M016) or Bismuth Sulphite Agar (M027). Mannitol Lysine Medium can be directly inoculated with the specimen or the specimen can be first enriched in Modified Semisolid RV Medium Base (M1482). Atypical Salmonella will form a characteristic bulls eye due to less H₂S production, which gets concentrated in the centre of the colony. Salmonella colonies will form purple black colonies. Presumptive Salmonella should be confirmed by biochemical tests. Storage and Shelf-life: Store below 30°C in tightly closed container and the prepared medium at 2 - 8°C. Use before expiry date on the label. References: 1. Koneman E. W., Allen S. D., Janda W. M., Schreckenberger P. C., Winn W. C. Jr., 1992, Colour Atlas and Textbook of Diagnostic Microbiology, 4th Ed., J. B. Lippinccott Company 2. Takao Inoue et al, 1968, Jap. J. Vet. Sci., 30.>br /> 3. Aspinall S. T., Hindle M. A. and Hutchinson D. N., 1992, Eur. J. Clin. Microbiol. Inf. Dis., 11:936.