Understanding How Bacteria Construct Protective Biofilms
Researchers have identified, at near-atomic resolution, a crucial mechanism that enables dangerous hospital pathogens such as Acinetobacter baumannii and Pseudomonas aeruginosa to assemble robust, antibiotic-resistant three-dimensional (3D) biofilms. This breakthrough opens new possibilities for therapies that directly disrupt biofilm formation and enhance antibiotic effectiveness.
Many pathogenic bacteria defend themselves by forming 3D biofilms. These structures shield them from immune attack, antibiotic treatment, and environmental stress, including drying on surfaces. In healthcare settings, multidrug-resistant strains of Acinetobacter baumannii and Pseudomonas aeruginosa pose serious risks because they readily form such biofilms on tissues and medical devices.
Initially, the bacteria use hair-like filaments called adhesive pili to attach to tissues or abiotic surfaces. Subsequently, they proliferate and organize into thick, multi-layered 3D biofilms. However, until now, scientists did not fully understand how these pili stabilized the growing biofilm and prevented it from collapsing.
Atomic-Level Insights into Pilus Assembly
To address this question, researchers at the MediCity Research Laboratory, University of Turku, led by Senior Researcher Anton Zavialov, applied advanced electron microscopy techniques.
As per the press release, they discovered that adhesive Csu pili from neighboring A. baumannii bacteria attach to one another in an antiparallel orientation. As a result, the pili rapidly self-assemble into flat sheets that interconnect bacterial cells and reinforce the biofilm structure.
“Impressively, Csu pili can self-assemble into huge, complex networks connecting hundreds of bacterial cells,” explained Dr. Zavialov.
Resolving Complex Structures with Cryo-Electron Microscopy
Furthermore, the team demonstrated that Csu pili form at least two distinct types of flat assemblies. Using cryo-electron microscopy, they resolved these exceptionally large structures at near-atomic resolution.
First author Henri Malmi described the process: “Cryo-electron microscopy methods are advancing rapidly. To obtain the first model, I initially developed a manual approach. Later, we applied computational tools to solve these large assemblies in 3D.”
A Reinforced ‘Bunker’ That Protects Bacteria
In addition to pilus networking, the researchers observed that the pilus sheets embed within a surrounding matrix composed of polysaccharides and extracellular DNA secreted by the bacteria.
Consequently, the final biofilm resembles reinforced concrete. The pili act like steel bars, while polysaccharides and DNA serve as the concrete. Together, they create a fortified structure that allows bacteria to effectively hide within a protective “bunker.”
Toward Novel Combination Therapies
Importantly, the team is now developing inhibitors that specifically disrupt the connections between pili. By targeting these structural interactions, such inhibitors could prevent 3D biofilm assembly.
Ultimately, combining these inhibitors with existing antibiotics may improve treatment outcomes and offer a promising strategy against multidrug-resistant hospital infections.