Selenite broth is an enrichment medium with selective properties, designed to isolate pathogenic bacteria of the genus Salmonella (lat. Salmonella). It is very effective if it is necessary to detect this particular pathogen among other representatives of the intestinal microflora. The medium is used for both clinical diagnostics and sanitary purposes in food testing.
General characteristics
Selenite broth is prepared on the basis of a powder, which is a homogenized mixture of various components of the composition. The color of the dry material may be light yellow or cream, depending on the type of medium. The powder has free flowing properties.
Ready selenite broth is a liquid transparent medium of light yellow color. This mass is poured into containers where sowing is carried out for further incubation.
There are three main types of selenite broth:
- pure medium - suitable for the isolation of Salmonella from both clinical and sanitary material;
- with the addition of mannitol (two-component broth) - designed to work with clinical material only;
- cysteine-selenite medium - can be used to isolate salmonella from pathological patient biomaterial (feces, urine, etc.).
These types are slightly different in composition and action features.
The main task of selenite broth is to promote the accumulation of salmonella, inhibiting the growth of accompanying microflora. This makes it possible not only to detect the pathogen in the material, but also subsequently to transfer it to agar. The medium was developed by Leifler, who first discovered the selective effect of selenite against Salmonella.
Composition
Selenite broth contains:
- casein hydrolyzate;
- lactose;
- sodium phosphate;
- sodium hydroselenite.
Selenite-cysteine medium, in addition to these components, contains L-cysteine and sodium hydrogen phosphate. The composition of mannitol broth differs in that instead of casein hydrolyzate, it includes peptic digest of animal tissue. There is no lactose in such an environment, but there is mannitol. The latter acts as a fermentable substrate and provides the buffering properties of the broth.
All components of the powder, except for sodium hydroselenite, are conditionally called part A, and the selective substance is called part B.
The principle of operation and properties of the environment
Selenite broth works as a storage mediumwith selective action. The selective effect is provided due to the toxicity of selenite, which inhibits the growth of most microorganisms. At the same time, bacteria of the genus Salmonella are able to restore this compound, thereby eliminating the toxic effect on their own cells. However, as a result of the reaction, an alkali is formed, which reduces the detrimental effect of selenite on the growth of associated microflora, and therefore it becomes necessary to stabilize the pH. This function is performed by bacteria that ferment lactose with the formation of acid. The buffering of the medium is provided by phosphate.
In two-component broth, the pH is stabilized by mannitol. The cysteine medium improves the selective effect against Salmonella. Such a broth is effective for controlling the infectious background in patients with a non-acute stage of the disease or convalescents.
The manifestation of growth characteristics of reference strains in the medium can be observed after 12-24 hours from the start of incubation. In this case, Salmonella form colorless colonies.
Cooking Features
Preparation of the environment is carried out in two stages. First, a solution is made with a selective component at the rate of 4 grams of sodium selenite per liter of distilled water. Then add 19 grams of the main powder (part A). The solution is thoroughly mixed and heated until the particles are completely dissolved, after which it is poured into sterile test tubes.
When cooking, it is important to followtemperature regime, since the medium is unstable to overheating. Sterilization of the broth can be carried out only in a water bath or use a jet of steam. Autoclaving of the medium is strictly prohibited.
