Bacterial Biofilms as a Reservoir of Amyloids Formed Through Specific and Nonspecific Mechanisms

Amyloids are protein aggregates with fibrillar morphology and a characteristic spatial structure called “cross-”. Amyloids have been known for over 150 years, and many of them are associated with the development of predominantly incurable human diseases called amyloidoses, some of which including Alzheimer’s disease are of high social significance. At the turn of the 21st century, it was established that amyloids not only result from protein folding disorders but are involved in performing biological functions in of all three domains of the living world: archaea, bacteria, and eukaryotes including humans. The greatest diversity of functional amyloids has been described in bacteria in which these protein aggregates are involved mainly in the processes of biofilm formation that play an important role in the development of bacterial infections and antibiotic resistance. Amyloid fibrils represent an important structural component of the biofilm matrix in various groups of bacteria. Although some bacterial amyloids are formed by specific secretion and assembly systems, the mechanisms of formation of another group of bacterial amyloids, including amyloid states of outer membrane proteins and components of the translation apparatus, are unclear. Current evidence suggests that more general, “non-specific” mechanisms, including regulated cell death during biofilm development, may be involved in the formation of such amyloids.