5.1 Using multiple phenotypic tests to identify the taxon

Experienced bacteriologists can often tentatively identify bacteria on the primary medium based on the type of specimen from which it originated, the colony size, colour and even smell. These traits, along with the Gram-stain appearance and a small number of confirmatory tests, can sometimes confirm the identification.

However, most commonly the identification requires obtaining a clinical isolate, as no identification method works on a mixture. Many bacterial groups comprise species with overlapping phenotypes, requiring multiple tests for the identification of the isolate’s species. By selecting the most parsimonious set of differential tests needed for identification, microbiologists can help to make significant savings in both labour and reagents.

Activity 8: Making savings

Timing: Allow about 15 minutes
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  1. Let us hypothesise that Streptococcus agalactiae, S. dysgalactiae and S. uberis account for 99% of all streptococcal infections in veterinary practice and infections caused by other streptococci are extremely rare. The laboratory manager wants to implement a cost-effective testing scheme to identify these three streptococci to species level.

    Which test is the least useful to distinguish between the three species, and why is this?

Answer

The lactose fermentation test does not differentiate between these species and is the least useful.

  1. Is there any value/advantage in including the lactose fermentation test in the identification panel?

Answer

The lactose fermentation test may still be useful for differentiation from other streptococci, but we said that the latter are extremely rare. Moreover, the added value of this test for the identification of rarely occurring species will probably be offset by the false negative and false positive results that will inevitably occur. This shows you the limitations of phenotypic testing for bacterial identification, and why you should use the smallest set of tests that offer the best discrimination in your own area. When you add a test, you also add a certain amount of error associated with it that must be accounted for.

5 Bacterial identification

5.2 Identification to genus and species level