7.3.1 Isolation of Salmonella

Salmonella comprise fast-growing facultatively anaerobic species that grow well in basal media such as nutrient agar or nutrient broth.

Activity 11: Isolating Salmonella in your workplace

Timing: Allow about 10 minutes

Think about your workplace activities and consider these questions.

  1. What types of specimens are commonly submitted for Salmonella culture?
  2. What types of primary media are used, and why?
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Answer

  1. Faeces and visceral organs are the most common specimen types submitted to diagnostic laboratories for Salmonella culture. Sometimes, feed may be submitted to identify possible feed-borne outbreaks of salmonellosis.
  2. Basal media are not commonly used as the primary isolation medium for the isolation of Salmonella from faeces, as they will promote the growth of many organisms and the salmonellae will remain undifferentiated.

Clinical specimens obtained from sterile-sites can be inoculated on primary solid selective differential media suitable for Salmonella, such as MacConkey agar, Hektoen enteric medium, xylose-lysine-deoxycholate (XLD) agar, or brilliant green agar. Faeces or other non-sterile site specimens often require prior inoculation into selective enrichment broths such as Rappaport-Vassiliadis broth or Tetrathionate broth.

A common isolation protocol for Salmonella from clinical specimens is shown in Figure 14. Different Salmonella serovars may have different recovery characteristics in selective enrichment media. Therefore, some isolation protocols include two different enrichment broths inoculated in parallel. Note that media are incubated at 35–37°C for 24h in aerobic atmosphere, but some laboratories incubate one broth at 42°C. This provides an additional selection barrier, as many serovars grow well at such a restrictive temperature.

Described image
Figure 14 Isolation and identification scheme for Salmonella suitable for sterile site specimens, using parallel inoculation of two primary media types. The scheme is also suitable for inoculation of two specimens from the same animal. A sterile site specimen (usually visceral organ tissue) is inoculated on a solid medium and faeces into enrichment broths
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A flowchart depicting the isolation and identification scheme for Salmonella suitable for sterile site specimens, using parallel inoculation of two primary media types.

Figure 14 Isolation and identification scheme for Salmonella suitable for sterile site specimens, using parallel inoculation of two ...

A complete identification scheme for Salmonella spp. requires an agglutination test using an anti-Salmonella polyvalent serum. Further identification would require the definition of the serovar, as different serovars have different epidemiology. However, whilst some laboratories may offer partial serotyping limited to the identification of the most common serogroups in their region, most laboratories submit the isolates for serotyping at reference laboratories. Hence, a typical interim laboratory report will be ‘Salmonella spp. (genus) – awaiting serotyping’.

Salmonella isolates can be stored frozen, freeze dried or refrigerated, and may remain viable for several weeks on storage media slants in refrigeration, or even at room temperature.

7.3 Salmonella species

7.4 Staphylococcus aureus