Best Culture Media for Stool or Faeces.

SALM Chrom agar for stool culture
SALM Chrom agar for stool culture

As a medical scientist, I have used chrom/chromogenic agar, which seems to be the best culture media for stool to detect and isolate Salmonella species like Salmonella Typhi and Salmonella Paratyphi.

HOW DOES CHROMAGAR WORK

Chrom agar works by identifying bacteria by specific colour.

This colour is as a result of complex interaction between the bacteria and the constituent of the media that leads to the production of a specific colour. In this case, purple/mauve.

SALM Chrom Agar

For stool culture, there is a particular chrom agar that is used to identify salmonella species. Salmonella species appears purple in colour on this media. Other bacteria have different colours like blue or white etc.

This chrom agar can be called SALM. Short form of the bacteria SALMonella.

However, different companies that produce these agars can have different names given to SALM agar.

CHROMID® Salmonella ELITE agar

This is chrom agar produced by Biomerieux. Biomerieux is a company that deals in a lot of things about medical and research laboratories, ranging from equipment to reagents and consumables. If you need to buy this media, you can visit their website here.

Below is how Salmonella species look on chrom agar. The media is milk-coloured, while the bacteria colonies are purple or mauve.

Chrom agar for Salmonella culture

CHROMagar™ Salmonella

This is the same chrom agar for Salmonella but produced by another company called CHROMagar.

The media colour is still the same milky and Salmonella colonies are purple in colour as well. Below is a SALM chrom from CHROMagar company. You can visit the company’s website here

Chrom agar for Salmonella Identification

Non Salmonella Species on Chrom Agar

Any other bacterial species look white,blue etc. on chrom agar as seen below

From the colour above, it shows the bacteria colonies are white and not purple, hence the patient does not have salmonella species. Therefore, the processing of the sample for salmonella will be discontinued.

Shigella Species on SALM Chrom Agar

Though shigella species are pathogenic bacteria in stool samples, they cannot be isolated from SALM chrom agar because SALM chrom is specifically for selective identification of Salmonella species.

So, on this media, the Shigella species will be white or any other colour, but not purple. If Shigella species must be screened for, other culture media like XLD (Xylose-Lysine-Deoxycholate)or MacConkey agar must be used in conjunction with SALM chromogenic agar. Fortunately, there is always the need!

How to Use Salmonella Chrom Agar in The Lab

Laboratories differ in their policies and procedures which determine how Salm chrom agar can be used.

There are two ways to use this media.

The first one is to culture stool samples directly on the Salm chrom agar. Incubate for 24 hours in air(aerobic incubator) at 35-37°C and read the culture plate.

The second option is to culture stool samples first in selenite-F broth (liquid solution). After overnight incubation, the sample from the Selenite-F is subcultured onto Salm chrom agar incubated for 24 hours again.

Let me explain how Selenite F broth help in identification of Salmonella in SALM chrom agar.

How Selenite F broth Help in Identification of Salmonella

Selenite-F broth is used to enhance the detection of Salmonella species in patients with low Salmonella counts in their stool samples.

This is usually the case in the non-acute stages of Salmonella infection when the organism occurs in low numbers in the stool/faeces; as seen in asymptomatic or convalescent patients.

Selenite-F works by inhibiting the growth of other bacteria especially E. coli by being toxic to them. It selectively allows Salmonella species to grow if it is present.

So this makes it possible for stool samples that have very low salmonella species to be isolated in the lab, because selenite-F gives them the chance to flourish. From this selenite-F, a subculture is made on SALM Chrom which finally helps in colour identification and differentiation.

WHAT IS THE COMPOSITION OF SALM CHROMOGENIC AGAR

The composition below is from the CHROMagar powder base.

Chromagar composition

This means you can prepare yours if you buy the powder base. You just need to weigh 34.9g of the powder base and mix in 1 liters of water , then follow manufacturer’s instruction.

SALMONELLA INFECTION

Salmonella infection is world wide in distribution and more common in countries with poor sanitary conditions especially in the developing countries.

From CDC, and I quote, ‘Salmonella bacteria cause about 1.35 million infections, 26,500 hospitalisations, and 420 deaths in the United States every year.’

You can imagine the data for poor countries!!

Have you used chrom agar for stool cultures? Share your opinion!!

Nevertheless, to read more on Salmonella, click here

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