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A joint study in Japan provides new insights into how Porphyromonas gingivalis, the bacterium responsible for gum disease, causes plaque formation.
Using cryo-electron microscopy (cryo-EM), researchers at the Okinawa Institute of Science and Technology (OIST), Tottori University, Hiroshima University and Nagasaki University revealed the 3D structure of Mfa pili, an arm-like filament that enables the bacteria to stick to host tissues and other microbes, publishing their findings in Communications Biology.
“By understanding how P. gingivalis attaches to host tissues, establishes infection, and participates in biofilm formation, we can inform the development of future therapeutic strategies,” first author Dr Satoshi Shibata said.
“Our detailed structural information may serve as a drug-design template for identifying compounds that block attachment and infection.”
To attach to hosts or other microbes, P. gingivalis uses two different types of filaments (pili): Fim and Mfa. Both are made of multiple protein subunits, which join together to form long arm-like structures and can bind to different types of bacteria and human tissues. In Mfa, most of these subunits are Mfa1 proteins.
Using cryo-EM, the team visualised the 3D structure of polymerized Mfa1, with four protein subunits of the filament.
The authors have long been interested in the structure and function of these filaments, previously describing the structure of FimA, a key component of Fim pili. With this new publication, they provide a fuller picture, elucidating the structure of Mfa1 chains central to Mfa pili and describing how these assemble and bind to other bacteria.
To understand filament formation, the researchers first polymerised the Mfa1 protein in vitro and analysed it by cryo-EM, determining the structure at a near-atomic resolution of 3.0 Å.
By modifying the proteins, the team explored the role and importance of particular sites in filament assembly. They demonstrated how Mfa proteins come together in a process called strand exchange, dependent on interactions within one particular region of the protein (the C-terminus) for structural stability.
The cryo-EM mapping revealed metal ions within the Mfa filament, which, through further analysis, were identified as calcium.
“Our tests suggest that this calcium binding can help the bacterium avoid immune recognition,” Dr Shibata said.
Using computer simulations, the researchers were also able to visualise the interaction of the Mfa filaments with Streptococcus gordonii, another bacterium that commonly binds to P. gingivalis in dental plaques. By identifying how these bacteria interact, scientists may more easily identify compounds to block these interactions, inhibiting plaque formation.