Curli are proteins made by E. coli, first identified in strains that cause bovine mastitis. Curli are part of the protein matrix that allows bacteria to attach to host cells, a trait that allows them to colonize and invade host tissues. One interesting notable factoid: curli are amyloid fibers. Bacterial curli are structurally similar to the amyloids found in Alzheimer’s and other neurodegenerative diseases, such as Parkinson’s and Huntinton’s diseases. Is there any evidence that E. coli curli are responsible for Alzheimer’s and Parkinson’s?

E. coli and Salmonella curli had been shown previously to promote biofilms that allow Salmonella to stick to Teflon and stainless steel, which contributes to food borne contamination. In people, curli are important in the attachment to and invasion of host cells. Curli also bind to plasminogen and t-PA (tissue plasminogen activator), catalyzing plasminogen activation to plasmin. This has the effect of degrading soft tissue and interfering with clotting. As we have discussed before in this blog, bacterial interference with clotting gives invasive bacteria an advantage in an infection.

Curli also seem to aggravate amyloid formation in neurodegenerative diseases. Previous work has shown that gut bacteria play a role in Alzheimer’s amyloid. Researchers recently demonstrated that curli can produce Parkinson’s-like motor symptoms in roundworms. The paper by Wang et al is titled, “Genome-wide screen identifies curli amyloid fibril as a bacterial component promoting host neurodegeneration”. These researchers identified curli genes that are responsible for Parkinson’s symptoms in the roundworm C. elegans. Roundworms that were fed E. coli expressing curli caused the worms to become stiff and unable to move. Deleting those genes reversed Parkinson’s like stiffness and locomotion disturbance: “deleting the curli genes csgA and csgB in the E. coli genome reduced α-syn–induced cell death, restored mitochondrial health, and improved neuronal functions.”

These findings are interesting because stiffness and motor slowing are hallmarks of human Parkinson’s disease. The idea that gut microbes might cause Parkinson’s gets my attention because my father died this month of complications from Parkinson’s disease. Read his obituary here. For the last three years he had progressive difficulty walking and with balance.

If gut microbes are involved in the causal pathway of Parkinson’s and other neurodegenerative diseases, then treatments targeting microbial virulence might prevent those diseases. Intriguingly, Wang et al. also showed that motor slowing in roundworms could be reversed with a plant compound found in green tea, epigallocatechin gallate (EGCG). Green tea catechins have previously been shown to promote longer lifespan in roundworms. This study suggests that green tea EGCG can prevent the motor symptoms caused by E. coli curli. As I write this, I am drinking matcha tea, a habit I acquired after a visit to Japan in 2019. As it happens, matcha is particularly high in EGCG, the compound that counteracts E. coli curli and prevents motor disease in roundworms.

It is a huge leap to extrapolate a neuroprotective effect from green tea to humans from a roundworm study. But caffeine and tea intake has been shown to protect against neurodegenerative diseases in observational studies, so an effect of tea is at least bio-plausible. The work by Wang et al. is a starting point for work on role for the microbiome in neurodegenerative diseases. We can only hope that this line of research will lead to microbiome-focused treatment strategies for these diseases.

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Joe Alcock

Emergency Physician, Educator, Researcher, interested in the microbiome, evolution, and medicine