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Review
. 2017 Apr;32(2):89-106.
doi: 10.1111/omi.12158. Epub 2016 Apr 25.

Collagen-binding proteins of Streptococcus mutans and related streptococci

A Avilés-Reyes  1 , J H Miller  2 , J A Lemos  1 , J Abranches  1
Affiliations
Review

Collagen-binding proteins of Streptococcus mutans and related streptococci

A Avilés-Reyes et al. Mol Oral Microbiol. 2017 Apr.

Abstract

The ability of Streptococcus mutans to interact with collagen through the expression of collagen-binding proteins (CBPs) bestows this oral pathogen with an alternative to the sucrose-dependent mechanism of colonization classically attributed to caries development. Based on the abundance and distribution of collagen throughout the human body, stringent adherence to this molecule grants S. mutans with the opportunity to establish infection at different host sites. Surface proteins, such as SpaP, WapA, Cnm and Cbm, have been shown to bind collagen in vitro, and it has been suggested that these molecules play a role in colonization of oral and extra-oral tissues. However, robust collagen binding is not achieved by all strains of S. mutans, particularly those that lack Cnm or Cbm. These observations merit careful dissection of the contribution from these different CBPs towards tissue colonization and virulence. In this review, we will discuss the current understanding of mechanisms used by S. mutans and related streptococci to colonize collagenous tissues, and the possible contribution of CBPs to infections in different sites of the host.

Keywords: adhesins; collagen binding; streptococcus.

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Figures

Figure 1
Figure 1. Amino acid sequence analysis of CBPs identified in S. mutans
(A) Conserved predicted domains identified in SpaP, WapA, Cnm, Cbm, CnaB and CbpA. (SS) secretion signal, (Ala) alanine-rich repeats, (V) variable region, (P) proline-rich repeats, (A) collagen binding A domain, (B) threonine-rich repeats (B domain), LPXTG cell-wall anchor motif. (B) Sequence alignment between the collagen-binding region of SpaP, WapA, Cnm and Cbm with the prototypical Cna from S. aureus. Amino acids in the shaded box represent the linker region between sub-domains N1 and N2 known to play a crucial role in the collagen-binding affinity. (C) Ribbon diagrams of the collagen-binding domains of Cnm, Cbm and WapA based on available crystal structures from Cna (swissmodel.expasy.org). Areas of high homology are represented in blue low homology regions appear in orange.
Figure 2
Figure 2. Proposed mechanisms of virulence mediated by S. mutans CBPs
(A) The presence of CBPs on the surface of S. mutans mediate direct attachment to collagen exposed after endothelial injury. This promotes bacterial adherence preventing the activity of host immune factors involved in tissue repair and homeostasis. (B) Systemic infection by S. mutans during atherosclerotic plaque growth may lead to an exacerbated immune response through: (i) recruitment of monocytes, and (ii) subsequent lipid digestion leading to formation of foamy macrophages that contribute to the development of atherosclerosis. (iii) Foamy macrophages and (iv) bacterial invasion of endothelial cells increase pro-inflammatory responses while smooth muscle cells produce ECM proteins (e.g. collagen), leading to (v) platelet deposition and activation. This, in turn, results in the development of a fibrin clot which provides the substrates recognized by circulating bacteria, leading to pathogen colonization as well as further immune activation. (C) CBP+ S. mutans can employ two distinct strategies to evade immune surveillance and antimicrobial challenges: i) through direct binding to the collagen-like domain of C1q, thus preventing complement activation, and ii) through invasion of endothelial cells that serve as a reservoir where bacteria can escape host-derived, antibacterial factors (e.g. antimicrobial peptides and complement) and antibiotic treatments. BM, basement membrane; ox-LDL, oxidized Low Density Lipoprotein; MΦ, macrophage.
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