Salmonella are ubiquitous in the environment. These organisms are usually present in small numbers compared to coliforms; therefore it is necessary to examine a relatively large sample to isolate the organisms (1). Salmonella present in food samples may be sublethally damaged during various stages of food processing where they may be exposed to low temperatures, heat drying, radiations, various chemicals (2). These damaged cells are able to cause spoilage, and if ingested cause diseases under favourable conditions. Therefore it is important to resuscitate these damaged bacteria before enumeration. Fluid Tetrathionate Medium (with added iodine and brilliant green) is recommended for the selective enrichment of Salmonella including Salmonella Typhi from faeces, urine, food and other material of sanitary importance. The medium, originally formulated by Mueller (3) is recommended by APHA (4-6) for enrichment of Salmonella.
Due to the addition of iodine and potassium iodide, tetrathionate is formed in the medium. Organisms possessing the enzyme tetrathionate reductase grow in this medium.
Casein enzymic hydrolysate and peptic digest of animal tissue are the sources of carbon, nitrogen, vitamins and minerals. Bile salts inhibit accompanying gram-positive microorganisms. The selectivity depends on the ability of thiosulphate and tetrathionate in combination to suppress commensal coliform organism (7, 8). Calcium carbonate neutralizes the acidic tetrathionate decomposition products. Brilliant green also helps to select Salmonella by inhibiting the accompanying bacteria. For further confirmation, streak the enriched cultures after incubation, on plates of Brilliant Green Agar (M016), MacConkey Agar (M081) and Bismuth Sulphite Agar (M027).
Aseptically inoculate test specimen into Fluid Tetrathionate medium (with added iodine and brilliant green) and incubate at 35-37°C for 18-24 hours. Following the incubation, isolate onto selective media plates. Refer standard procedures for enrichment and isolation. (4-6)
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. Cherry et al, 1972, Appl. Microbiol., 24:334
2. Hartman and Minich, 1981, J. Food and Prot., 44:385
3. Mueller, 1923, Compt. Rend. Sco. Biol., 89:434.
4. Downes F. P. and Ito K., (Eds.), 2001, Compendium of Methods For The Microbiological Examination of Foods, 4th Ed.,APHA, Washington, D.C.
5. Eaton A. D., Clesceri L. S. and Greenberg A. W., (Eds.), 2005, Standard Methods for the Examination of Water and Wastewater, 21st Ed., APHA, Washington, D.C.
6. FDA Bacteriological Analytical Manual, 2005, 18th Ed., AOAC, Washington, DC.
7. Pollock M. R. and Knor R., 1943, Biochem J., 37:476.
8. MacFaddin J. F., 1985, Media for Isolation-Cultivation-Identification-Maintenance of Medical Bacteria, Vol. 1, Williams and Wilkins, Baltimore.