The Salmonella-Shigella agar also known as SS agar, it is a moderately selective and differential medium, specially designed for the isolation of enteropathogenic bacteria of the genera Salmonella and Shigella, both from environmental and clinical samples..
SS agar has a complex composition; It is made up of meat extract, peptone, lactose, bile salts, sodium citrate, sodium thiosulfate, ferric citrate, agar, neutral red, bright green and distilled water. Given its great selectivity, samples with abundant mixed flora can be sown..
In microbiology laboratories, the Salmonella-Shigella medium is widely used to investigate the presence of Salmonella and Shigella in diarrheal stool samples, wastewater, drinking water and food..
Sometimes it is necessary to use pre-enrichment broth (lactose broth) and enrichment broth (selenite cystine broth) to recover strains of Salmonella.
These steps are required when the existence of Salmonella in a very low quantity is suspected, or where the strain may be abused by processes of industrial production, mainly processed foods. It is also advisable to enrich stool samples from patients who have been treated with antibiotics..
Subsequently, the enriched broth can be seeded on Salmonella-Shigella agar and other similar media, such as xylose agar, lysine deoxycholate (XLD) and enteric Hektoen agar (HE)..
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Each component of the Salmonella-Shigella culture medium has a specific function, and the mixture as a whole gives it the properties that characterize it..
The meat extract and peptone (digested with casein and animal tissue) provide the required nutrients (nitrogens, carbon and vitamins) for the development of microorganisms capable of tolerating the rest of the components..
The agar-agar is in charge of providing the solid consistency to the medium.
This medium is highly selective because it contains bile salts, sodium citrate, and bright green. Therefore, it inhibits the growth of all Gram positive bacteria and most Gram negative rods, including some coliforms..
While bacteria of the genus Salmonella and some strains of Shigella do support these compounds.
Mainly, the Salmonella genus is very resistant to bile salts, so much so that they are able to live in the gallbladder of some carrier patients who constantly shed the bacteria in their stools..
Lactose is the fermentable carbohydrate that helps differentiate lactose-fermenting strains from non-fermenting ones. This property is evidenced by the presence of the pH indicator, which in this medium is phenol red..
Lactose fermenting strains give red colonies, while non-fermenting strains are colorless. This characteristic is important, since Salmonella and Shigella do not ferment lactose..
On the other hand, this medium contains sodium thiosulfate as a source of sulfide and ferric citrate as a source of iron. Both compounds are able to differentiate bacteria capable of producing hydrogen sulfide. These react to form a visible, insoluble, black ferric sulfide precipitate..
This property is found in some strains of the genus Salmonella. Normally their colonies are flat colorless with a black dot in the center of it. The rest of the Salmonellas do not produce HtwoS and develop as colorless colonies.
On the other hand, colonies of the genus Shigella are flat colorless without blackening..
This medium is very simple to prepare.
Weigh 63 g of the dehydrated commercial medium and dissolve in a liter of distilled water. Heat the solution and stir. The mixture can boil for up to minutes.
This medium must not be autoclaved. After dissolving, it is served directly on single or double sterile plates..
When they solidify, they are arranged in an inverted way in plaquers and stored in a refrigerator (2-8 ° C) until use..
The medium after preparation should be at pH 7.2 ± 0.2 and with an orange-red color..
It is important to allow the plates to warm up before seeding the samples. The original sample can be sown directly, discharging material in a part of the agar and then streaking from there by exhaustion.
In case of using enriched broths, pass a portion of the selenite broth and sow with a drigalski spatula.
Incubate at 37 ° C for 24 hours in aerobiosis.
It must be taken into account that the number of grams to be weighed and the final pH of the medium may vary from one commercial house to another. The middle base always brings the indications for its preparation.
It is frequently used in stool culture analysis and in the microbiological study of wastewater, drinking water and food samples..
Frequently double plates are prepared, on one side Salmonella-Shigella agar is placed and on the other XLD agar.
-Some Shigella strains do not grow on this medium. Therefore, it is not recommended for primary isolation of this genus..
-Not every transparent colony with a black center is indicative of Salmonella; Biochemical tests must be carried out to make a correct identification, since the colonies of some Proteus strains are indistinguishable from those of Salmonella.
-The dehydrated medium must be taken care of from exposure to the environment, as it is very hygroscopic. Therefore, it must be kept in a dry and well-closed environment. Open for very short periods.
-Over time the bile salts in the medium may precipitate, forming a mat-like image within the agar, but this does not affect the results..
-Some Shigella strains can slowly ferment lactose.
To prove that the medium is working correctly, it is advisable to plant known or certified control strains and observe if the growth meets the expected characteristics..
For this you can use strains of E. coli, Enterobacter sp, Klebsiella pneumoniae, Shigella flexneri, Salmonella typhimurium or Enterococcus faecalis.
The expected results are:
Escherichia coli --pink convex colonies.
Enterobacter and Klebsiella- large colonies and red or pink mucoids.
Shigella flexneri --clear or colorless flat colonies.
Salmonella typhimurium - colorless colonies with black center.
Enterococcus faecalis-- total inhibition.
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