Bismuth Sulphite Agar Medium is a grow medium recommended for the selective isolation of Salmonellae from faeces, urine, sewage and other materials
This growth medium is prepared in accordance with USP and is employed for the isolation and preliminary identification of Salmonella Typhi and other Salmonellae from pathological materials, sewage, water, food and other products. It is a modification of Wilson and Blair medium
Salmonella Enteritidis and Salmonella Typhimurium typically grow as black colonies having a rabbit eye colonies characteristics with a surrounding metallic sheen.
Salmonella Paratyphi A instead grow as light green colonies. This growth medium also favors use of larger inoculum and heavily contaminated samples as compared to other selective media, as it has unique inhibitory action towards gram-positive and coliform organisms.
The medium may be inhibitory to some strains of Salmonella species and therefore should not be used as the sole selective medium for these organisms.
Shigella species are mostly inhibited on Bismuth sulphite agar and also some Salmonellae like Salmonella Sendai, Salmonella Berta, Salmonella Gallinarum and Salmonella Abortus are equally inhibited. Proteus species are inhibited but few strains give dull green or brown colonies with metallic sheen.
Principle of Bismuth Sulphite Agar Medium
- Brilliant green and bismuth sulphite incorporated into the medium inhibit the intestinal gram-negative and gram-positive bacteria
- Peptic digest of animal tissue, pancreatic digest of casein and beef extract are rich source for supplying essential nutrients for growth of the organism.
- The fermentable source of carbohydrate in this medium is dextrose, which provides energy for enhanced microbial growth.
- Phosphates incorporated in the medium act as a good buffering agent.
- The bismuth ions are reduced to metallic bismuth, which impart the metallic sheen around the colonies.
- Sulphite is reduced to black ferric sulphide giving the black colour with release of H2S.
Composition of Bismuth Sulphite Agar Medium
The following ingredients are used for the composition Bismuth sulphite agar
|Pancreatic digest of casein||5.000|
|Peptic digest of animal tissue||5.000|
|Bismuth sulfite indicator||8.000|
|Final pH at temperature 25°C||7.6±0.2|
How to Prepare Bismuth Sulphite Agar Medium
The preparation of Bismuth sulphite agar depends on each laboratory workload or the numbers of culture plates available. Anyway, note that a standard Petri dish is equivalent to 20ml when completely poured.
- Start by suspending 52.32 grams in 1000 ml purified/ distilled water.
- Heat to boiling to dissolve the medium completely.
DO NOT OVERHEAT OR STERILIZE IN AUTOCLAVE or by fractional sterilization because overheating may destroy the selectivity of the medium.
- Transfer to a water bath maintained at about 50°C .
The sensitivity of the medium depends largely upon uniform dispersion of precipitated bismuth sulphite in the final gel, which should be dispersed before pouring into the sterile Petri plates.
Appearance of Medium after preparation
The medium appears Yellow to greenish yellow opalescent with flocculent precipitate
Type of Inoculums
Plate reading and Colonies morphology
The following cultural characteristics should be observed after incubation at 30-35 °C for 24-48
|Organisms||Colour of Colony|
|Salmonella Typhimurium||black or greenish-grey may have sheen|
|Salmonella Abony||black with metallic sheen|
|Enterobacter aerogenes||brown-green (depends on the inoculum density)|
|Enterococcus faecalis||No growth (Inhibited)|
|Salmonella Enteritidis||black with metallic sheen|
|Salmonella Typhi||black with metallic sheen|
|Escherichia coli||Brown to green, depends on inoculum density|
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- Washington J.A.,1981,Laboratory Procedures in Clinical Microbiology, Springer-Verlag, New York.
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- Bacteriological Analytical Manual, 8th Edition, Revision A, 1998. AOAC, Washington D.C.
- Murray PR, Baren EJ, Jorgensen JH, Pfaller MA, Yolken RH (editors) 2003, Manual of clinical Microbiology, 8th ed., ASM, Washington, D.C
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