The selenite broth It is a selective liquid culture medium. It was designed by Leifson for the enrichment of samples where the presence of enteropathogenic bacteria of the genus Salmonella is suspected..
This medium complies with the requirements of the American Public Health Association (APHA), and therefore is accepted for the investigation of the presence of Salmonella in samples of stool, urine, liquid or solid food, water, among others..
Its chemical composition favors the recovery of these microorganisms and in turn inhibits the growth of others. It is primarily toxic to most bacteria belonging to the Enterobacteriaceae Family. However, it also allows the recovery of Shigella strains and does not inhibit the growth of Pseudomonas and Proteus..
It is composed of anhydrous sodium hydrogen selenite, anhydrous sodium phosphate, peptones, and lactose. There is also a variant to which cystine is added, hence its name selenite-cystine broth..
Currently, the use of selenite-cystine broth is preferred, since a higher percentage of Salmonella recovery is obtained, equivalent to that observed with other selective media for the same purpose, such as sodium tetrathionate broth..
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The peptones contained in the broth serve as nutrients for the proper development of microorganisms. Salmonella strains use peptones as a source of nitrogen, vitamins and amino acids.
Lactose is the fermentable carbohydrate, while sodium selenite is the inhibitory substance that stops the growth of Gram positive bacteria and most of the bacteria present in the intestinal flora, especially those of the Enterobacteriaceae family. Sodium phosphate is the buffer that stabilizes the pH of the medium.
In the case of the selenite broth variant containing L-cystine, this additional compound is a reducing agent that minimizes selenite toxicity, increasing Salmonella recovery..
If you have the components of the mixture, you can weigh:
4 g of anhydrous sodium hydrogen selenite.
10 g of anhydrous sodium phosphate.
5 grams of peptones.
4 gr of lactose.
Dissolve the compounds in 1 liter of sterile distilled water. It can be heated slightly to dissolve completely.
Some laboratories expose the medium for 10 minutes in flowing steam to sterilize it, since an autoclave should not be used. If the medium is sterilized, it can be stored in the refrigerator until use.
It can also be prepared non-sterile and served direct 10 to 15 ml to sterile test tubes.
In this case, it should be left to rest and use immediately. As the medium is not sterile, it cannot be stored in the refrigerator for later use..
If the commercial medium is available, weigh 23 g of the dehydrated medium and dissolve it in one liter of sterile distilled water. Heat briefly to finish dissolving. Do not autoclave. Aseptically serve 10 or 15 ml in sterile test tubes.
The final pH of the medium must be 7.0 ± 0.2.
It should be noted that the color of the dehydrated medium is beige and the preparation is light and translucent amber..
It contains the same compounds as selenite broth, but 10 mg of cystine is added. The rest of the procedure is exactly the same as described above.
This medium is special to be used in epidemiological studies, in cases where the disease is not in its acute stage, asymptomatic patients or healthy carriers.
Isolation of the genus Salmonella is generally difficult, as they are normally only poorly contaminating samples. Being in few quantities, it is easily overlapped by the growth of other bacterial genera that are found in greater quantity..
On the other hand, the raw material with which processed foods are made are often exposed to heat, dehydration processes, the use of disinfectants, radiation and preservatives, among others..
Therefore, the Salmonellas present in the raw material are abused by subjecting the product to the aforementioned industrial processes. Likewise, in the case of clinical samples such as feces, the strains may be weak if they come from patients who have been treated with antibiotics..
Therefore, any sample in which the presence of Salmonella is suspected should be pre-enriched in lactose broth, and subsequently enriched in selenite broth to optimize its recovery in selective media, such as SS agar, xylose agar, lysine deoxycholate (XLD ), enteric Hektoen agar (HE) and bright green agar, among others.
For stool samples, take 1 g of sample and suspend in a tube with 10 to 15 ml of selenite broth. If the stool is liquid, take 1 ml and suspend it in the broth. For rectal swabs, discharge the swab material into the broth.
In solid food samples take 1 gr and suspend in the selenite broth.
In liquid foods mix in equal parts with the selenite broth at double concentration.
For urine samples, centrifuge, discard the supernatant, take all the sediment and suspend it in the selenite broth..
The broths are incubated at 37 ° C for 24 hours of incubation. Bacterial growth is evidenced by turbidity. An additional tube per sample can also be included for incubation at 42 ° C. Subsequently, seeding selective solid media from the selenite broth.
To control sterility, a selenite broth from each uninoculated batch is incubated at 37 ° C for 24 hours. No turbidity or color change of the medium is expected..
To control the proper functioning of the medium, known strains can be used, such as:
Salmonella enteritidis ATCC 13076, Salmonella typhimurium ATCC 14028, Salmonella choleraesuis ATCC 12011, Escherichia coli ATCC 25922 and Proteus mirabilis ATCC 43071.
The expected results are:
Selenite broth medium is toxic to human skin, therefore direct contact should be avoided..
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