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Understanding antibiotic resistance
Understanding antibiotic resistance

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4.3.4 The Nordmann/Dortet/Poirel test

The Nordmann/Dortet/Poirel (NDP) test can detect the presence of ESBL-producing bacteria in blood samples in less than two hours (Nordmann et al., 2012). As you might remember from Week 4, ESBLs are a major determinant of resistance to cephalosporin antibiotics. The presence of ESBLs in a bacterial strain can be used to diagnose a cephalosporin-resistant infection. The NDP test detects the enzyme activity of ESBLs in the sample.

  • What enzymatic reaction do ESBLs catalyse?

  • Recall from Week 3 that β-lactamases destroy β-lactam antibiotics by hydrolysing the β-lactam ring.

When cefotaxime (a third-generation cephalosporin) is hydrolysed by ESBLs, it results in acidification; that is, the sample becomes more acidic. In the NDP test, this acidification can be measured as a colour change in the sample using a pH indicator. The colour of a sample gives a visible indication of the presence of ESBL-producing bacteria (Figure 13).

The findings of an experiment to identify ESBL-producing bacteria using the NDP test.
Figure 13 The NDP test to identify ESBL-producing bacteria. Samples containing ESBL-producing bacteria (E. coli (10.16) CTX-M-15 and K. pneumoniae (09.200) TEM-3) hydrolyse cefotaxime, leading to acidification which is detected as a change from red to orange using a pH indicator (Nordmann et al., 2012). You do not need to study the details of this figure.