Date Published: February 28, 2019
Publisher: Springer Berlin Heidelberg
Author(s): Dongru Chen, Yina Cao, Lixia Yu, Ye Tao, Yan Zhou, Qinghui Zhi, Huancai Lin.
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.
It is revealed that among all microbial and chronic infections, 65% and 80% are associated with biofilm formation, respectively (Jamal et al. 2018). Recently, 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, thus be called functional amyloid fibers (Erskine et al. 2018). In bacteria, amyloid fibers often mediate cell–cell and cell–surface interactions. Besides, amyloid fibers can act as protection barrier or interfere with the function of specific proteins, and amyloid fibers may thus represent novel targets for antibacterial drugs (Blanco et al. 2012).
In this study, we firstly show the clear morphology of amyloid fibers emanating from S. mutans, intertwined into net-like. The net formed by amyloid fibers around S. mutans might be helpful in gathering other matrix or cells. There are mainly two morphologies, different in width. The aggregated amyloid fibers by purified truncated protein C123 also follow two aggregation types, forming into rigid amyloid fibers. Various morphologies and properties of amyloid fibers produced by the same protein can also be found when exposed to differing environmental conditions during growth, including temperature, salt concentration and shear forces (Bekard and Dunstan 2009; Lee et al. 2007; Petkova et al. 2005; Yoshimura et al. 2012). Variations in fibril morphology with subtle changes in growth conditions can change the cytotoxicity of amyloid fiber (Petkova et al. 2005). However, the different roles of the two morphological different amyloid fibers in S. mutans are still unknown, which needs to be further explored.