Bismuth Sulphite Agar (BSA)- Composition, Principle, Preparation, Results, Uses

Bismuth Sulphite Agar (BSA) is a highly selective medium used for isolating Salmonella spp., particularly Salmonella Typhi, from food and clinical specimens. Salmonella Typhi is the causative agent of typhoid fever, a serious systemic infection that can be fatal if left untreated. Salmonella spp. are gram-negative bacilli that belong to the Enterobacteriaceae family and are widely distributed in nature. They can cause various types of infections in humans and animals, ranging from gastroenteritis to septicemia.

Bismuth Sulphite Agar (BSA) is a selective and differential medium for the isolation and presumptive identification of Salmonella spp, especially Salmonella Typhi. The composition of BSA varies slightly depending on the manufacturer, but the typical ingredients and their quantities are as follows:

• Peptone: 10.000 gms/liter
• Beef extract: 5.000 gms/liter
• Dextrose (Glucose): 5.000 gms/liter
• Disodium phosphate: 4.000 gms/liter
• Ferrous sulfate: 0.300 gms/liter
• Bismuth sulfite indicator: 8.000 gms/liter
• Brilliant green: 0.025 gms/liter
• Agar: 20.000 gms/liter

Bismuth Sulphite Agar (BSA) is a selective and differential medium that inhibits the growth of most gram-positive and gram-negative bacteria except Salmonella species. It also allows the differentiation of Salmonella species based on their ability to produce hydrogen sulfide gas from ferrous sulfate.

To prepare Bismuth Sulphite Agar (BSA), you will need the following ingredients and quantities:

• Peptone: 10 g
• HM Peptone B: 5 g
• Dextrose (Glucose): 5 g
• Disodium phosphate: 4 g
• Ferrous sulfate: 0.3 g
• Bismuth sulfite indicator: 8 g
• Brilliant green: 0.025 g
• Agar: 20 g
• Distilled water: 1000 ml

The growth and appearance of colonies on Bismuth Sulphite Agar (BSA) depend on the type and number of organisms inoculated, the incubation time and temperature, and the amount of hydrogen sulfide (H2S) produced by the bacteria. The following are some common results observed on BSA:

• Salmonella Typhi typically produces black colonies with a metallic sheen due to H2S production and reduction of sulfite to black ferric sulfide. The surrounding medium may also turn green due to the oxidation of brilliant green by H2S.
• Salmonella Paratyphi A usually produces light green colonies without H2S production.
• Other Salmonella species may also produce black colonies with or without a metallic sheen, depending on their ability to produce H2S. Some examples are Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Choleraesuis.
• Shigella species are mostly inhibited on BSA, except for Shigella flexneri and Shigella sonnei, which may grow as colorless or green colonies without H2S production.
• Other gram-negative bacteria such as Escherichia coli, Proteus species, and Pseudomonas species are usually inhibited or suppressed on BSA. However, some strains may grow as colorless or green colonies without H2S production.
• Gram-positive bacteria such as Staphylococcus species and Streptococcus species are completely inhibited on BSA.

Bismuth Sulphite Agar (BSA) is a highly selective medium that is recommended for the isolation and preliminary identification of Salmonella spp., especially Salmonella Typhi, from various sources. Some of the fields where Bismuth Sulphite Agar (BSA) can be used are:

• Clinical microbiology: Bismuth Sulphite Agar (BSA) can be used to isolate Salmonella spp. from pathological materials, such as feces, urine, blood, pus, sputum, etc. This can help in the diagnosis and treatment of salmonellosis in humans and animals.
• Food microbiology: Bismuth Sulphite Agar (BSA) can be used to isolate Salmonella spp. from food samples, such as meat, poultry, eggs, dairy products, vegetables, fruits, etc. This can help in the prevention and control of foodborne outbreaks of salmonellosis.
• Environmental microbiology: Bismuth Sulphite Agar (BSA) can be used to isolate Salmonella spp. from environmental samples, such as water supplies, sewage, soil, etc. This can help in the assessment and management of environmental health risks associated with salmonellosis.

Bismuth Sulphite Agar is a highly selective medium that can effectively inhibit the growth of most gram-positive and gram-negative bacteria other than Salmonella spp. However, it also has some limitations that should be considered when using it for isolation and identification of Salmonella spp.

• Bismuth Sulphite Agar can be inhibitory to some strains of Salmonella spp., especially those that do not produce hydrogen sulfide or have low sulfite reductase activity. Therefore, it should not be used as the sole selective medium for these organisms.
• Bismuth Sulphite Agar favors the use of larger inoculum as compared to other selective media, as it has a unique inhibitory action towards gram-positive organisms and coliforms. This may result in overgrowth of some Salmonella spp. near heavy inoculation and undergrowth of others away from the inoculation site.
• Bismuth Sulphite Agar may not produce typical black colonies with a metallic sheen for some Salmonella spp., such as S. Paratyphi A, S. Sendai, S. Berta, S. Gallinarum, and S. Abortus-equi. These organisms may grow as light green, brown, or colorless colonies on this medium. Therefore, biochemical and serological tests are necessary to confirm the identity of these organisms.
• Bismuth Sulphite Agar may also allow the growth of some non-Salmonella organisms that can produce hydrogen sulfide or reduce sulfite, such as Proteus spp., Citrobacter spp., Edwardsiella spp., and Shigella flexneri and Shigella sonnei. These organisms may grow as black or green colonies on this medium and may be confused with Salmonella spp. Therefore, additional tests, such as indole production, lysine decarboxylation, urease activity, and agglutination with specific antisera, are required to differentiate these organisms from Salmonella spp.

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