Lighting up antibiotic resistance

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, which are called "antibiotics of last resort," 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 noneffective 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.