Abstract
There are signs that amyloid fibers exist in Streptococcus mutans biofilm recently. However, the characteristics of amyloid fibers and fibrillation influencing factors are unknown. In this study, we firstly used transmission electron microscopy (TEM) and atomic force microscopy (AFM) to observe the morphology of amyloid fibers in S. mutans. Then the extracted amyloid fibers from biofilm were studied for their characteristics. Further, the influencing factors, PH, temperature and eDNA, were investigated. Results showed there were mainly two morphologies of amyloid fibers in S. mutans, different in width. Amyloid fibers inhibitor-EGCG obviously destroyed biofilm at different stages, which is dose-dependent. The amount of amyloid fibers positively correlated with biofilm biomass in clinical isolates. Acidic pH and high temperature obviously accelerated amyloid fibrillation. During amyloid fibrillation, amyloid growth morphologies were observed by TEM and results showed two growth morphologies. Amyloid fibers formed complex with eDNA, which we call (a)eDNA. The molecular weight of (a)eDNA was similar to genomic DNA, greatly larger than that of eDNA in matrix. Combined use of DNase I and EGCG was more efficiently in inhibiting amyloid fibers and biofilm biomass. In conclusion, amyloid fibers are the crucial structures for S. mutans biofilm formation, showing two types of morphology. Acidic pH and temperature can obviously accelerate amyloid fibrillation. Amyloid fibers form complex with (a)eDNA and combined use of DNase and amyloid fiber inhibitor is more efficiently in inhibiting S. mutans biofilm formation.
Highlights
It is revealed that among all microbial and chronic infections, 65% and 80% are associated with biofilm formation, respectively (Jamal et al 2018)
We aim to investigate the important role of amyloid fibers at different stages during S. mutans biofilm formation, and to verify whether amyloid fibers are the universal structure in clinical isolates in biofilm formation and whether amyloid fibers appear in planktonic state
Morphology of amyloid fibers in S. mutans When observed by transmission electron microscopy (TEM), amyloid fibers emanating from the S. mutans could be observed, which showed various length, from 50 nm to several microns (Fig. 1a)
Summary
It is revealed that among all microbial and chronic infections, 65% and 80% are associated with biofilm formation, respectively (Jamal et al 2018). Amyloid fibers are found in the surface of bacteria and play important role in biofilm formation. Unlike misfolded toxic aggregates in human tissues, amyloid fibers formed on the surface of bacteria can provide biological functions, be called functional amyloid fibers (Erskine et al 2018). E. coli has a dedicated secretion system, chaperone proteins and inhibitors. More bacteria with dedicated secretion systems forming amyloid fibers have been described, e.g. various species of Pseudomonas (Dueholm et al 2013). Unlike E. coli with dedicated secretion systems, cell surface proteins on some bacteria have been discovered to form amyloid fibers. Studies indicate that biofilm matrix components, extracellular DNA (eDNA), may accelerate amyloid fibrillation and form complex with amyloid fibers
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