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 precisely this pathogenic organism among other representatives of the intestinal microflora. The medium is used both for clinical diagnostics and for sanitary purposes when checking food products.
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 bulk properties.
Ready selenite broth is a liquid transparent medium of light yellow color. This mass is poured into containers, where sowing is done 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 only with clinical material;
- cysteine-selenitic medium - can be used to isolate salmonella from a patient's pathological biomaterial (feces, urine, etc.).
These types differ slightly in the composition and characteristics of the action.
The main task of selenite broth is to promote the accumulation of salmonella, inhibiting the growth of concomitant 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 was the first to discover the selective effect of selenite on salmonella.
Composition
The composition of selenite broth includes:
- 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 the broth with mannitol is different in that instead of casein hydrolyzate, peptic digestion of animal tissue is included. There is no lactose in such an environment, but there is mannitol. The latter acts as a fermentable substrate and provides buffering properties of the broth.
All components of the powder, except sodium hydroselenite, are conventionally called part A, and the selective substance is called part B.
The principle of operation and properties of the environment
Selenite broth acts as a selective storage medium. The selective effect is 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 for their own cells. However, as a result of the reaction, alkali is formed, which reduces the detrimental effect of selenite on the growth of concomitant microflora, and therefore it becomes necessary to stabilize the pH. This function is performed by bacteria that ferment lactose to form acid. The buffering medium is provided by phosphate.
In a two-component broth, the pH is stabilized by mannitol. A cysteine environment improves the selective effect on salmonella. Such a broth is effective for controlling the infectious background in patients with an acute stage of the disease or convalescents.
The manifestation of the 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
The preparation of the medium is carried out in two stages. First, make a solution 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 tubes.
When cooking, it is important to monitor the temperature regime, since the environment is unstable to overheating. Sterilization of the broth can only be done in a water bath or use a steam jet. Autoclaving the medium is categorically unacceptable.