The majority of experiments on curli biogenesis and functions were carried out on E.coli and Salmonella spp. Curli homologues were discovered among the representatives of Bacteroidetes, Firmicutes and Thermodesulfobacteria genera by bioinformatic analysis [48]. CsgEFG operons were found in the majority of the bacteria mentioned above with potential CsgA and CsgB homologues, while CsgC and CsgD proteins were often absent. In spite of the fact that many bioinformatic assays are awaiting the experimental confirmation, there are grounds to suppose that structures similar to curli can be more widely spread in biofilm-forming bacteria than it was thought before [49].

Adhesin Р1 located on the cell surface of Streptococcus mutans that causes dental caries is an amyloid protein [50]. This adhesin induced a shift in the CR dye absorption spectrum, green birefringence in the CR stained sample and a specific TT fluorescence. Using microscopic methods, fibrils were detected in the sample of this adhesin; this, coupled with spectrophotometric assay results, confirmed its amyloid nature [50]. The obtained data indicate that Р1 is not the only protein of S. mutans cell surface capable of forming amyloids, because the colonies of the bacteria deprived of this adhesin still induced green birefringence after CR staining [50].

Mycobacterium tuberculosis pili are another example of how amyloids of microbial extracellular matrix can possibly contribute to pathology. This microorganism causes tuberculosis that leads to 3 million deaths every year worldwide [2]. Pili on gram-positive M. tuberculosis surfaces are not soluble in the chloroform/methanol mix (2 : 1) and in the sodium dodecyl sulfate-containing buffer (SDS); they also interact with amyloid-specific CR dye, which suggests their amyloid nature [2]. Pilus protein deletion mutants of M. tuberculosis exhibited reduced virulence [2]. The researchers explain that pili are capable of binding to laminin, the extracellular matrix protein, thus contributing to the firm adhesion of a microorganism to host tissues. Thus, M. tuberculosis uses these amyloid proteins to successfully colonize the host [2]. In the serum of patients with tuberculosis, high titers of antibodies interacting with M. tuberculosis pili are found [2].

Other gram-negative microorganisms that can colonize different human organs and tissues, such as cocci Staphylococcus aureus, cause various diseases, from minor skin infections to bacteremia and sepsis. Many of these diseases are associated with biofilm formation in the host [20]. Extracellular amyloid fibrils have been identified in S. aureus biofilms. They consist of short peptides called phenol-soluble modulins (PSM) [12].

S. aureus or S. epidermidis PSMs have many functions [51, 52, 53, 54]. It has been shown that in their fibrillar form PSMs are necessary for S. aureus to provide biofilm stability against various dispersing (biofilm degrading) agents and physical impact [12]. The authors of that work believe that the inhibition of phenol-soluble modulins export is a promising research area that can contribute to preventing diseases induced by pathogenic staphylococci. The search for minor molecules — amyloid polymerization inhibitors — is one of the ways that can lead to the development of drugs for staphylococci elimination on the stage of biofilm formation [49].

Bacillus subtilis pili are an important component of biofilm extracellular matrix formed by the bacteria on hard surfaces and at water–air interface [55]. This microorganism is not pathogenic, however, it is widely spread and can be found in soil, air, water and food. The main protein subunit of B. subtilis pili is TasA protein [22, 56]. Fibrils formed by TasA in vitro are very similar to B. subtilis pili morphologically [22]; at the same time they interact with amyloid-specific dyes such as CR and TT, are rich in β-sheets, as suggested by CD-spectroscopy, and can be depolymerized only after the incubation in the presence of formic acid [22]. It should be noted that TasA was first identified as a secreted protein and a protein of B. subtilis spore surfaces with distinct antibacterial properties [57, 58]. Antibodies used in the diagnosis of neurodegenerative diseases recognize both metastable intermediates generated in the course of amyloid fibril formation and TasA oligomers, which suggests a possible structural similarity of these two oligomer types [22, 59, 60]. Antibodies used in the diagnosis of neurodegenerative diseases in humans recognize TasA oligomers [22, 59, 60], which suggests their immunological similarity.

Amyloids forming amphypathic membranes on microbial cell surfaces

Hyphae, spores and fruiting bodies of many fungi are covered with amphypathic (i. e., having both hydrophilic and hydrophobic areas) rodlet layers that form a mosaic of parallel fibrils 5 to 12 nm wide [18]. Those amphypathic layers do not dissolve when boiled in the presence of 2 % SDS and 1 M NaOH, and dissociate into monomers only when treated with formic or trifluoroacetic acids [9]. The main and probably the only component of fungal rodlet layers is class I hydrophobins [61, 62]. The polymerization of hydrophobins is most effective at interfaces with high surface tension, such as liquid-air interface; agents reducing surface tension also reduce the rate of hydrophobin polymerization in vitro [63].

Hydrophobins are a large family of low molecular weight proteins (7–9 kDa) found in fungi [61]. This family got its name due to being rich in hydrophobic amino acid residues [9]. Hydrophobin encoding genes are present in many fungi. Class I hydrophobins are typical functional amyloids because they have a role in spore and fruiting body formation; they are also important for adhesion to the host cell surface and protection against the host immune system [18, 64]. Thus, in the infection caused by filamentous fungi Arthroderma benhamiae (dermatophytes, i. e., surface mycosis pathogens in humans and animals), hydrophobin HypA has a masking function and protects the microorganism from the host immune system. Deletion of the hydrophobin gene leads to a rapid wetting of fungal filaments and conidia, which induces increased activation of granulocytes, neutrophils and dendritic cells and is accompanied by elevated titers of interleukins IL-6, -8, -10 and tumor necrosis factor TNF-α [65]. RodA hydrophobin, a component of the rodlet layer that covers pathogen spores, contributes to the development of the infection induced by another filament fungus Aspergillus fumigatus that can lead to invasive aspergillosis. In the experiments on animals the spores of the mutant strain with deleted RodA or ∆laeA mutant containing 60 % less hydrophobins, were susceptible to macrophage phagocytosis [66].

Amyloids as a part of yeast cell walls: adhesins and glucantranspherase Bgl2p

The development of systemic amyloidosis in mice injected with Candida sp. lyophilized cells is well known, but is not widely discussed [67]. The authors of that article emphasized that amyloid depositions could occur in experimental animals as a response to casein, albumin, bacteria or E. coli endotoxin administration [68, 69, 70, 71]; but after the injections had been cancelled, amyloid depositions started to reduce gradually or disappeared [72, 73]. Laboratory mice injected with Candida sp. lyophilized cells died of systemic amyloidosis within 400 days after the last injection [67]. Separate experiments showed that injecting mice with Candida sp. intracellular matter did not cause amyloidosis. The authors concluded that amyloidosis development was stimulated by cell walls components [67].