Come 2050, the world may see as many as 10 million fatalities a year due to antibiotic-resistant bacterial infections. Contributing to this worrying projection are major challenges in developing effective treatment strategies, including ever-rising resistance to existing antibiotics and a dearth in the discovery and design of next-generation antibiotics.

Antibiotic resistance is acquired in bacteria as an evolutionary pressure to survive. With most of the existing antibiotics targeting a single bacterial gene or functional component, bacteria readily adapt to and evade antibiotic effects over time. The growing urgency of this global health issue has spurred scientists to redesign antibiotics with complex mechanisms of action that can fight pathogens on multiple fronts.

While innovative therapeutic solutions may partly address antibiotic resistance, there is yet another fundamental hurdle to overcome – discerning bacterial infections from other conditions like inflammation and cancers, which can cause similar symptoms. In the absence of tools that can precisely pinpoint bacterial infections, antibiotics will continue to be used as a cautionary measure. Over time, bacteria will adapt to antibiotic effects and patients will stop responding to medications.

Recently, a team of scientists from Singapore and China, led by Professor Bin Liu from NUS Chemical and Biomolecular Engineering, addressed these glaring gaps in the antibiotic development pipeline by synthesizing an antibiotic compound with unique properties that offer it an edge over existing antibiotics. This work was published in Science Advances.

The compound, a fluorescent molecule named TPA2PyBu, can both target and destroy bacteria, including several multidrug-resistant strains, with remarkable precision. In the study, the agent prevented the growth of a wide range of gram-positive and gram-negative bacterial strains, including methicillin-resistant Staphyloccus aureus (MRSA), multidrug-resistant Escherichia coli (MDR E. coli), Acinetobacter baumannii and Enterococcus faecium. Importantly, it achieved this at concentrations of 1-10 μM, a dose low enough to minimise collateral damage to other healthy cells. Similar antibiotic candidates tested were effective against gram-negative bacteria only at doses greater than 50 μM.

The high bactericidal activity of TPA2PyBu can be linked to its dual mechanism of action: where a long alkyl chain strongly binds to the bacterial membrane and disrupts its structure and function, before the drug binds to bacterial DNA, causing it to aggregate into a dysfunctional state. This multi-pronged attack not only neutralises the bacteria but also creates substantial challenges for the bacteria to adapt and acquire resistance.

“Due to the dual mechanism of action of TPA2PyBu, we believe that the development of resistance would be significantly delayed compared to conventional antibiotics. Therefore, TPA2PyBu and similar agents serve not only as powerful therapeutics, but also as time-buying innovations that slow the arms race between drugs and pathogens,” commented Prof Liu.

Its ability to bind selectively to the bacterial membrane also opens up the potential for TPA2PyBu to be used as a diagnostic tool, addressing a critical issue in the treatment of bacterial infections. For this, the researchers tapped a key property of the drug – it fluoresces when in an aggregated state or when bound to a target molecule. Whereas traditional fluorophores are limited by their inability to penetrate gram-negative bacteria, TPA2PyBu exhibited a multi-fold increase in fluorescence in S. aureus-infected in vivo models. This sharp rise was noted solely at the site of infection, even when the models also included inflammation and cancer conditions. The ability of the drug to precisely differentiate bacterial infections from cancer and inflammation showcases its potential as a reliable diagnostic probe.

Although TPA2PyBu is a long way from being used in the clinic, the innovative strategy adopted by the team is expected to set new trends not just in the antibiotic development pipeline, but also in designing potent antifungal and antiviral therapies. As deadlier and seemingly invincible superbugs continue to emerge, millions of patients worldwide will benefit from tools that can enable real-time imaging-based and diagnosis-guided treatment of pathogens.

“This integration of targeted therapy with diagnostic imaging, known as theranostics, marks a major step toward intelligent and traceable antibiotics,” summed up Prof Liu.