About two decades ago, the Denmark-based pharmaceutical company Novozymes discovered an antimicrobial peptide (a short chain of amino acids) capable of targeting disease-causing bacteria. It was unlike any existing antibiotic.

While most peptide antibiotics work by targeting bacterial membranes in a non-specific manner, Novozymes’ plectasin specifically targets a rare lipid called ‘Lipid II’ in the plasma membrane. 

However, the molecular details of plectasin’s action remained a mystery for years due to technical limitations. Finally, a new study reveals the unique mechanism plectasin employs to kill bacteria.

“By assembling into large structures, plectasin latches onto its target on the bacterial cell surface comparable to how both sides of Velcro form a bond,” the study authors note. 

The study authors suggest that these findings could lead to the development of antibiotics that effectively work against antibiotic-resistant bacteria, which represent a growing and concerning threat.

Decoding the velcro trap of plectasin

Plectasin is naturally produced by the black cup fungus (Pseudoplectania nigrella). It is a ‘host defense peptide’ that protects the fungus against pathogenic gram-positive bacteria. 

To understand its molecular mechanism in detail, the study authors used modern methods such as high-speed atomic force microscopy (HS-AFM) and solid-state nuclear magnetic resonance (NMR). These techniques allowed scientists to visualize and study plectasin’s structure at the molecular and atomic levels. 

Gram-positive bacteria are surrounded by a thick wall of peptidoglycan that is essential for their survival. According to the study authors, plectasin blocks the biosynthesis of peptidoglycan by targeting the building block Lipid II with the help of calcium ions. 

Once Lipid II is unavailable, the bacteria cannot make more peptidoglycan, and eventually die. In more detail, the sequestration of Lipid II in the cell wall of bacteria is supported by calcium ions. These positively charged ions highly specifically bind to a shallow, negatively-charged pocket in plectasin. The way the molecules bind causes plectasin to form into a structure called a supramolecular carpet.

“The binding of calcium allosterically changes the global structure of plectasin, which improves target binding and, most importantly, also the killing activity of plectasin. Moreover, calcium-binding also fosters the formation of a highly-ordered supramolecular carpet of plectasin molecules,” Dr. Markus Weingarth, one of the study authors and an associate professor at Utrecht University, told ZME Science. 

This ‘carpet’ works like velcro, trapping Lipid II molecules. Velcro has a hook and loop interlocking system. One side has many tiny hooks and the other has many loops. If only one of those loops breaks free from its hook, the rest of the velcro still remains well-attached. 

Similarly, bacteria trapped in the supramolecular carpet are attached just like loops to numerous microscopic hooks. Even if one of the Lipid II molecules breaks free from the binding, the bacteria remain trapped as other molecules are still bound. Eventually, the pathogen succumbs to death.

Plectasin remains a mystery 

While researchers have successfully decoded the molecular action of plectasin against bacteria, some questions remain to be answered.

“I hate to say it, but even after eight years of hard work, we do not fully understand the mechanism at the molecular level. For example, we do not know exactly how plectasin oligomerises or how calcium drives the oligomerisation. This would require more sophisticated solid-state NMR methods.” Weingarth said. 

Oligomerization refers to the process by which a few monomer units (smaller molecular units) join together to form a larger molecule called an oligomer. 

The study authors also didn’t confirm whether plectasin is effective as a drug from antimicrobial-resistant bacteria. However, they believe that it is a compelling clinical candidate as it meets several important criteria. It has high stability in serum, low toxicity levels, low resistance rates, and shows good efficacy in animal models.

What’s more interesting is that they have found the supramolecular action of plectasin in other antibiotics as well. For instance, they recently discovered that two antibiotics, teixobactin and clovibactin, also target Lipid II by forming supramolecular structures.

“We observe evidence for some sort of supramolecular action. While this needs further thought and research, it appears that supramolecular mechanisms are widepsread among Lipid II binding drugs. To me, this is the most important finding of our paper,” Weingarth told ZME Science.

These findings indicate that in the future we may witness the rise of not one but many antibiotics that can fight antimicrobial resistance.

The study is published in the journal Nature Microbiology.