Selenite broth

Selenite Broth is a selective enrichment medium used in the isolation of Salmonella spp. from clinical and non-clinical samples. It was first introduced by Leifson in 1936, and its primary purpose is to enhance the growth of Salmonella species while inhibiting the growth of competing bacteria. Selenite Broth is particularly useful in the isolation of Salmonella from fecal specimens, food samples, and environmental swabs.

Composition

The broth contains sodium selenite as the selective agent, which is toxic to most Gram-positive and many Gram-negative bacteria at the concentration used in the medium. However, Salmonella spp. can tolerate sodium selenite, allowing them to proliferate. The medium also contains lactose, peptone, and sodium phosphate for nutritional support of the target organisms. The pH of the medium is typically neutral to slightly alkaline, which favors the growth of Salmonella over other enteric pathogens.

Mechanism

The selective inhibition of non-target bacteria by sodium selenite is believed to be due to the toxic effects of selenite on bacterial enzymes. This selective pressure allows Salmonella spp., which are more resistant to selenite, to grow preferentially. The enrichment process increases the likelihood of detecting Salmonella in samples containing a low number of organisms among a high background of competing flora.

Usage

Selenite Broth is used in microbiology laboratories for the enrichment of Salmonella spp. from specimens prior to plating on selective agar media for isolation and identification. The sample is inoculated into the broth and incubated at 35-37°C for 12-24 hours. After incubation, a subculture is made onto solid media such as Xylose Lysine Deoxycholate (XLD) agar, Hektoen enteric agar, or MacConkey agar, which allow for the differentiation and identification of Salmonella colonies.

Limitations

While Selenite Broth is effective for the enrichment of Salmonella, it does not completely inhibit the growth of all non-target organisms. Some non-Salmonella enteric pathogens may survive and grow in the medium, potentially complicating the isolation of Salmonella from the subculture. Additionally, the broth does not provide definitive identification of Salmonella spp., and further biochemical and serological testing is required for confirmation.

Safety

Handling of Selenite Broth, as with all culture media containing toxic compounds, should be done with care. Sodium selenite is a hazardous substance, and appropriate safety measures, including the use of personal protective equipment (PPE), should be employed to avoid exposure.

Selenite broth Resources
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