Summary
Numerous peptides and proteins have been found to possess an inherent tendency to convert from native functional states into amyloid aggregates. This phenomenon is associated with a range of common human disorders, including Alzheimer and Parkinson diseases (Chiti & Dobson 2017). Substantial evidence implicates oligomeric intermediates of amyloids as toxic species in the pathogenesis of these chronic disease states. A growing body of data has suggested that these toxic species form ion channels in cellular membranes causing disruption of calcium homeostasis, membrane
depolarization, energy drainage, and in some cases apoptosis (Kagan et al. 2012). Such channels are not only toxic to their native organism but have been found to possess antimicrobial activity. Besides protegrin-1, which kills microbes by a channel forming mechanism (Kagan et al. 2012), a recent study demonstrates that amyloid-β (Aβ) can function as an antimicrobial peptide, and additional data show that bacteria and yeast can seed Aβ deposition into amyloid (Golde 2016). Seeding amyloid formation is a general mechanism used by numerous microbes for their advantage. Recent insights into bacterial biofilm matrix structures, for example, have induced a paradigm shift toward the recognition of amyloid fibres as common building block structures that confer stability to the exopolysaccharide matrix (Taglialegna, Lasa & Valle 2016). Amyloid fibres and therefore oligomeric intermediates have been shown to be present from prokaryotes to eukaryotes and are besides biofilm formation integral to many biological functions such as cell-cell contacts, scaffolding and substrate adhesion (Garcia- Sherman et al. 2014). In fact, microbial amyloids are not only structural components but modulate host-microbe- (Gilchrist et al. 2012; Bois et al. 2013) and likely microbe-microbe- interactions. Many microbes utilize functional amyloids to specifically attach to and colonize the host (Garcia-Sherman et al. 2014).
In summary, current data reveals a bivalent character of amyloid forming proteins as beneficial and as toxic compounds in microbe-microbe-interaction and raises two fundamental questions: (1) How do microbes in complex microbial communities protect themselves from various toxic amyloids they and others produce and (2) how and where is amyloid fibril formation seeded in microbe-microbe and host-microbe interactions?
