5.6 Colorimetric tests

Colorimetric tests work on the principle that a substrate changes colour when broken down by an enzyme. This principle is used in chromogenic media (e.g. the commercially available CHROMagarTM agars developed for detection of carbapenemase producers, Acinetobacter, MRSA etc.). As well as having substrates for the enzymes, some of them are also selective, that is they inhibit the growth of susceptible organisms and species which are not of interest and only support those to be isolated. Some also change colour for a particular species so can be used to identify the species as well as the resistance mechanism.

Another example is nitrocefin – a chromogenic cephalosporin – which can be used to screen for β-lactamases (Video 1). Nitrocefin is broken down by all known β-lactamases leading to a colour change from colourless to red and so works as an indicator antimicrobial. Although it can be used to test whether a β-lactamase is present it will not discriminate between types of β-lactamase. It is routinely used to look for β-lactamase in N. gonorrhoeae and H. influenzae.

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Video 1 Nitrocefin test for β-lactamases
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This is a commercially produced test. The sticks contain the colourless β-lactam, nitrocefin. The end of the stick is dipped in the culture and incubated in moist conditions for five minutes. If any β-lactamase is present it will break down the nitrocefin, producing a red colour.

Video 1 Nitrocefin test for β-lactamases
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5.5 Antigen detection tests

6 Genotypic tests