Salmonellae are the most complex taxonomically diverse group of bacteria in Enterobacteriaceae. Salmonella infections in humans are usually caused by the consumption of food, milk, and water contaminated with animal or human excreta. S. Typhi is only found in humans. There are four types of Salmonella infection that can be distinguished: enteric fever (bacteremia or septicemia), gastroenteritis (gastroenteritis), and a carrier. Bismuth Sulphite Agar is the most effective media for the isolation and preliminary identification Salmonellae. This is a modified version of the original Wilson & Blair Medium for isolation of Salmonella Typhi, other salmonellae, and it is especially useful for isolation of lactose fermenting salmonellae.
Principle of Bismuth Sulphite Agar (BSA)
Peptone and HM Peptone A are sources of carbon, nitrogen and long-chain amino acid, vitamins and other essential growth factors. Dextrose is the main carbon source. The osmotic equilibrium is maintained by disodium phosphate. Brilliant green and the bismuth sulfite marker both inhibit intestinal gram positive and gram negative bacteria. The detection of hydrogen sulfuride production can be assisted by ferrous sulfate. You can use clinical samples to inoculate Bismuth sulphite agar. Pre-enrichment is required for food samples before inoculation.
S. Typhimurium, S. Typhi and S. Enteritidis typically grow in black colonies. This is due to hydrogen sulfide reduction and production of hydrogen sulfide. Salmonella Paratyphia A can grow as light-colored colonies.
Composition of Bismuth Sulphite Agar (BSA)
|HM Peptone B #||5.000|
|Bismuth sulfite indicator||8.000|
Final pH: 7.7±0.2
Preparation and Method of Use of Bismuth Sulphite Agar (BSA)
- Take 52.33 grams and mix it with 1000 ml of distilled water.
- To dissolve the medium completely, heat to boiling
- Do not sterilize in AUTOCLAVE or fractional sterilization as overheating can cause the medium to lose its selectivity.
- Mix well and pour onto thick plates (25ml medium per plate). Let the medium settle with the dish covered.
- Be sure to dry the plates well before you use them.
- Properly prepared plates should be smooth and creamy-like in appearance with pale straw colors. The indicator should not be smudged.
- Innoculate the plates together with the specimen.
- For typical colonies, incubate the media at 35o for 48 hours. Incubate for an additional 18-24 hour if plates don’t grow after 48 hours.
Notice: The medium’s sensitivity is dependent on the uniform dispersion and distribution of precipitated bismuth sulfur sulfite in its final gel. This should be done before pouring into sterilized Petri plates.
Result Interpretation of Bismuth Sulphite Agar (BSA)
|Enterobacter aerogenes||Poor growth; Brown-green (depends on the inoculum density)|
|Escherichia coli||Poor growth; Brown-green (depends on the inoculum density)|
|Salmonella Enteritidis||Good-luxuriant growth; Black with a metallic sheen|
|Salmonella Typhi||Good-luxuriant growth; Black with a metallic sheen|
|Salmonella Typhimurium||Good-luxuriant growth; Black with a metallic sheen|
|Salmonella Abony||Good-luxuriant growth; Black with a metallic sheen|
Uses of Bismuth Sulphite Agar (BSA)
- Bismuth Sulphite Aggar is recommended to isolate and identify Salmonella Typhi from pathological materials, sewage and food.
Limitations of Bismuth Sulphite Agar (BSA)
- Bismuth Sulphite agar can be inhibitive to certain Salmonella strains and should not be used solely as a selective medium.
- This medium is more effective than other selective media in allowing for larger inoculums, because it has a unique inhibitory effect on gram-positive organisms as well as coliforms.
- Certain Salmonella species may have black colonies with metallic sheen that can be seen near heavy inoculation. However, isolated colonies might show green colonies.
- This medium inhibits most Shigella species; S. flexneri, S. sonnei are exceptions.
- Some Salmonella like S. Sendai, S. Berta, S. Gallinarum, S. Abortus-equi are also inhibited.