4.3.2 Enteric pathogens

Your laboratory will almost certainly identify a wide variety of enteric pathogens from stool samples. For GLASS, the focus is on Salmonella and Shigella.

The first step in identification is the appearance of colonies on solid media. For example, on XLDShigella colonies are red, while Salmonella are pink with a black middle (Figure 8). Both organisms have pale colonies on MacConkey agar as they are non-lactose fermenters (NLF). They are also urease negative.

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Figure 8 Salmonella colonies in a mixed culture on XLD agar.
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The black colour is caused by hydrogen sulfide production and the colony is otherwise colourless as the organism does not ferment the lactose which is present in the medium – in contrast to most non-pathogenic, enteric coliforms. The other colonies are other enteric flora, such as E. coli.

Figure 8 Salmonella colonies in a mixed culture on XLD agar.

  • Why is it important to do a urease test?

  • NLF enteric organisms, such as Proteus and related species (Providentia, Serratia and Morganella) have a similar appearance on solid media. It is therefore important to do a urease test to prevent mis-identifying these as enteric pathogens.

    Confirmatory tests for Salmonella and Shigella include:

    • a negative oxidase test – to differentiate from other NLF organisms like Pseudomonas
    • Kligler’s Iron Agar (KIA) test
    • biochemical tests
    • antisera agglutination tests

    For details of all tests, see the UK SMI guidance for Salmonella (PHE, 2015a) and Shigella (PHE, 2015b)

4.3.1 Enterobacterales species

4.3.3 Acinetobacter