Fluorescent peptide probe tracks lung infections

24 Oct 2018 Belle Dumé

Identifying Gram-negative bacteria in the lung

A new fluorescent imaging probe that can image Gram-negative bacteria in the human lung in real time could be used to find out whether a patient is suffering from a bacterial infection. The device, which has been tested in a small clinical study, opens up a whole new field in medicine and could make for a new diagnostic platform for pneumonia and other infections, say the researchers at the University of Edinburgh who developed it.

Patients with respiratory disease can contract Gram-negative pulmonary infections such as pneumonia in hospital. The problem is unfortunately even worse for patients who are on mechanical ventilators in intensive care units.

It is, however, extremely difficult to diagnose these infections, but doing this is crucial if they are to be treated before they become potentially fatal. Indeed, existing techniques like lung biopsies are not without risk, and routinely employed molecular sequencing approaches like the polymerase chain reaction (PCR) that “amplify” potentially dangerous microbes in samples taken from the patient are oversensitive and often produce false positives. This means that the patient might be given massive doses of an antibiotic that she or he doesn’t need. 

Direct imaging

Researchers of the PROTEUS Hub and the Centre for Inflammation Research at Queen’s Medical Research Institute at the University of Edinburgh say they have now overcome these problems with their new water-soluble fluorescent imaging probe that can directly image Gram-negative bacteria in human lungs. The device contains two components, explains team leader Kev Dhaliwal: “a targeting ligand based on a modified antibiotic (an antimicrobial cyclic peptide) called polymyxin (PMX); and a reporter fluorophore, made of 7-nitrobenz-2-oxa-1,3-diazole (NBD), whose fluorescence increases within the increasingly polar environment of the bacterial membrane. This fluorophore thus ‘lights up’ brightly when it specifically binds with a lipid (lipid A) of a lipopolysaccharide on the membrane surface.”

The researchers tested their probe on six patients with bronchiectasis, a chronic pulmonary disease, and were able to successfully identify Gram-negative bacteria, such as K. pneumoniae and H. influenzae. They administered a microdose of the NBD-PMX using an intrapulmonary catheter during a bronchoscopy procedure and imaged lung airways and alveoli immediately afterwards using a technique called optical endomicroscopy.

Safe but more work needed

Although the NBD-PMX is safe for biological cells, Dhaliwal and colleagues say they will have to do more research to make sure that the technology is safe for long-term, repeated use. The probe is not able to detect an important Gram-negative bacterium called P. mirabilis either since this pathogen does not bind to PMX. P. mirabilis causes nosocomial pneumonia and is particularly dangerous for patients whose immune systems are weak. The cost of the probe will also have to be evaluated, add the researchers.

Finally, although Gram-negative bacteria are the main culprits for causing infection during lung transplants and infections in intensive care units, Gram-positive bacteria or multiple bacterial species may also be responsible for diseases like pneumonia, they warn. “We thus need to develop a matching Gram-positive imaging agent such as fluorescently-labelled vancomycin and also multiplex with inflammation probes, all of which we are currently developing,” Dhaliwal tells Physics World. “Our platform is nonetheless a proof-of-concept that shows much promise as a new diagnostic and monitoring tool for lung infections. It is an exciting development in molecular imaging and significantly adds to current technologies,” he says.

The probe is detailed in Science Translational Medicine 10.1126/scitranslmed.aal0033.

25/10/2018 from physicsworld.com