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Chapter 19 : Cell-Cell Contact-Induced Gene Regulation in Streptococcus gordonii-Actinomyces oris Communities
Category: Genomics and Bioinformatics
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Microbial interactions involve sensing and responding to chemical signals or cues that are released from cells. In the early stages of plaque accumulation on freshly cleaned teeth, most interactions occur between the initial colonizers such as Streptococcus spp., Actinomyces spp., and Veillonella spp. However, mature dental plaque biofilms may contain up to 200 different phylotypes of bacteria, resulting potentially in almost 20,000 different pairwise interactions. This chapter focuses on just one of these interactions: that between two initial colonizers of oral biofilms, Streptococcus gordonii and Actinomyces oris. Interbacterial binding between oral bacteria has been investigated extensively in vitro using coaggregation assays. In these experiments, pure cultures of genetically distinct bacteria are mixed in test tubes and interactions are scored based on the extent of clumping (coaggregation). Coaggregation is used to model biofilm communities containing S. gordonii and A. oris and investigate gene regulation in mixed-species cultures. A powerful application of postgenomic technologies is the analysis of gene expression using DNA microarrays. The first genome-level analysis of S. gordonii-A. oris interactions employed microarrays to identify genes in S. gordonii that were regulated in response to coaggregation with A. oris. This study demonstrated that S. gordonii specifically activates a set of genes in response to cell-cell contact (coaggregation) with A. oris. In theory, microarrays and other genome-based expression technologies can be used to investigate cell-cell contact-induced gene regulation in any bacterial species for which the genome sequence is known.
Coaggregation between S. gordonii DL1 and Actinomyces spp. S. gordonii polypeptides SspA and SspB play key roles in coaggregation. These molecules are encoded by tandem genes on the S. gordonii chromosome. (Panel A) SspA and SspB proteins are composed of several domains: an N-terminal region including a signal peptide (N), a series of three full and one partial repeat of an alanine-rich sequence (A), a central variable domain (V), three prolinerich repeats (P), and a C-terminal region (C), including an anchoring motif for sortase-mediated cross-linking to the cell wall. The degree of homology between SspA and SspB over different regions of the proteins is indicated. These molecules are highly conserved, with the exception of the V region. (Panel B) S. gordonii DL1 interactions with six different coaggregation groups of actinomyces. Coaggregation with Actinomyces groups A (including A. oris MG1) and E is dependent upon SspB (line with V shape). SspA (line with square bracket) is required for coaggregation with Actinomyces groups C and D. Interactions requiring SspA also involve an accessory adhesin of S. gordonii (dashed line with curve), possibly CshA/B. Adapted from reference 10 .
Pathways of arginine metabolism in S. gordonii. L-Arginine is synthesized from glutamate and aspartate in several enzyme-catalyzed steps. Import of arginine is mediated by ArcD. The arginine dihydrolase system (ArcABC) degrades L-arginine to produce energy (ATP), CO2, and ammonia (NH3). Coaggregation with A. oris leads to changes in expression of many genes involved in arginine metabolism (genes encoding steps that are indicated by solid arrows). Only arginine catabolism genes, encoding ArcABC (reactions represented by dashed arrows), were not regulated by coaggregation. The inset shows regulation of the S. gordonii argC gene during growth in monoculture, coculture without induced coaggregation, or coaggregate culture. In monocultures and cocultures, argC expression increased markedly between 2 and 3 h, following depletion of arginine from the medium. In coaggregate cultures, argC was upregulated within 1 h compared with monocultures and remained relatively stable throughout growth. Data are reproduced from reference 15 .
Generation of gradients within cell clusters. Cell clusters, such as S. gordonii-A. oris coaggregates, are represented by spheres, and high concentrations of signaling molecules or cues for gene regulation are indicated by dark shading. Movement of molecules through the clusters may be limited by diffusion or by adhesion to, or reaction with, bacterial cells. Molecules produced within the cluster become concentrated in the center (A), whereas molecules moving into the clumps from outside concentrate in the outer regions (B). The direction of flow through cell clusters is indicated by arrows.
Model for the regulation of S. gordonii arginine metabolism genes in monocultures and in coaggregate cultures with A. oris. Three ArgR family regulators are encoded in the genome of S. gordonii DL1: ArcR, ArgR, and AhrC. ArgR family regulators in bacteria control gene expression in response to arginine depletion. In arginine-replete mono-cultures, ArgR regulators are bound to arginine and repress expression of arginine biosynthesis genes. During growth, arginine is depleted from the medium. Arginine dissociates from ArgR family regulators, and arginine biosynthesis genes are expressed. In coaggregate cultures, S. gordonii may acquire arginine from juxtaposed A. oris cells. This could occur, for example, by active secretion of arginine or an arginine-containing peptide by A. oris, or by proteolytic processing of the A. oris cell surface by S. gordonii proteases. As a consequence of coaggregation, there is a stable supply of arginine to S. gordonii cells.
Model to describe the role of S. gordonii-A. oris interactions in the formation of dental plaque biofilms. (Panel 1) Coaggregation between S. gordonii and A. oris promotes surface colonization and growth of S. gordonii. A. oris cells are incorporated into the biofilm. Production of H2O2 by S. gordonii inhibits growth of A. oris. However, A. oris cells remain viable in the biofilm. (Panel 2) Over time, other organisms (e.g., S. oralis) attach to the biofilm. (Panel 3) Coaggregation between S. oralis and A. oris results in mutualistic growth of these organisms.