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Carbapenems are among the antibiotics of last resort and can fight infections for which other drugs have long lost their effectiveness. However, even carbapenem-resistant pathogenic strains have emerged over the last decades. To find out whether a pathogen contains carbapenem-cleaving enzymes, the carbapenemases, Chinese scientists have developed a simple and fast assay based on a fluorescent probe and optical detection. They introduce their approach in the journal Angewandte Chemie.
Carbapenems are a class of β-lactam antibiotics similar to cephalosporins and penicillins. Although some bacterial strains have found powerful strategies to resist β-lactam antibiotics by producing a class of cleaving enzymes, the β-lactamases, most β-lactamases cannot affect the carbapenems. Therefore, these substances are the drug of choice for several diseases such as urinary tract and abdominal infections as well as hospital-acquired pneumonia, if they are caused by multidrug-resistant bacteria. But there is growing evidence of even carbapenem resistance, and some pathogens were found to produce carbapenem-cleaving enzymes, the carbapenemases. Now, Hexin Xie at East China University of Science and Technology and his team have set up a strategy to identify those pathogens that carry the carbapenemases.
The researchers developed a molecule that has the same structure as the carbapenems but has a fluorogenic dye attached. If this carbapenem-mimicking compound, CVB-1, is recognized by a carbapenemase, for example, in an bacteria extract, CVB-1 is cleaved and undergoes spontaneous degradation. As this destroys the electronic interaction of the attached dye with the carbapenem compound, the dye turns into a green fluorescent molecule, which means, if it is irradiated with light of a certain wavelength, it emits intense green light. Thus, the assay in principle works as follows: If there is an active carbapenemase present, for example, in a bacteria extract, a couple of minutes later the sample glows green upon excitation. Xie and his colleagues said: "CVB-1 [...] is essentially non-fluorescent [...], and the addition of [the carbapenemase] triggers the turn-on of the fluorescent signal upon excitation [...] with over 200-fold enhancement ratio."
This technique allows the detection of antibiotic resistance activity by fluorescence. Thus, using this fluorescence-based assay system, it would be possible to find out in very short time whether carbapenem-resistant bacteria (such as certain Enterobacteriaceae and Klebsiella pneumoniae strains) are indeed present during an infection. More specific treatment strategies could be designed and an overuse of non-effective drugs could be avoided. The scientists have performed several tests to prove that their CVB-1 assay is specific, that the detection limit is low, and that it can indeed be used in live systems. This fast and simple fluorescence-based assay is certainly a remarkable approach in the ongoing and urgent fight against the fast spread of antibiotic resistance.