Researchers have discovered and characterized at the atomic level a mechanism that enables bacterial pathogens—including hospital bacteria Acinetobacter baumannii and Pseudomonas aeruginosa—to assemble antibiotic-resistant three-dimensional (3D) biofilms. These findings open a new avenue for developing therapies against multidrug-resistant bacterial infections by targeting the biofilm assembly.

Many pathogenic bacteria form 3D biofilms to protect themselves from the immune system, antibiotic treatments, and drying on environmental surfaces. Some of the most problematic hospital bacteria, such as multidrug-resistant A. baumannii and P. aeruginosa, use specialized hair-like filaments called adhesive pili to attach to tissues or abiotic surfaces. After attaching, the bacteria then grow into thick 3D biofilms consisting of multiple layers of bacteria. This process is also mediated by adhesive pili, but until now it has been unclear how they prevent the growing 3D biofilm from falling apart.

Using a combination of advanced electron microscopy methods, the researchers at the MediCity Research Laboratory of the University of Turku in Finland, led by S. Jusélius Senior Researcher Anton Zavialov, discovered that adhesive Csu pili from neighboring A. baumannii bacteria attach to each other in an antiparallel manner. These pili rapidly assemble into flat sheets that link bacteria together and shield them from hostile environments.

“Impressively, Csu pili can self-assemble into huge, complex networks connecting hundreds of bacterial cells,” says Dr. Zavialov.

The team demonstrated that Csu pili can form at least two types of flat structures and resolved them at a near-atomic resolution.

“Cryo-electron microscopy methods are developing very rapidly. To obtain the first model, I initially developed a manual approach, and only later did we apply computational tools to solve these exceptionally large assemblies in 3D,” explains first author, Doctoral Researcher Henri Malmi.

The researchers also found that the pilus network becomes embedded in a less defined matrix composed of polysaccharides and DNA secreted by the bacteria.

“This final structure somewhat resembles reinforced concrete: the pili act like steel bars, while polysaccharides and DNA form the concrete. In this way, the bacteria effectively hide in a bunker,” adds Dr. Zavialov.

The team is now focused on developing inhibitors that target the connections between pili. Such inhibitors could be used in combination therapies to prevent 3D biofilm assembly and help antibiotics eliminate the pathogens more effectively.