Aggregatibacter actinomycetemcomitans (Aa) Under the Radar: Myths and Misunderstandings of Aa and Its Role in Aggressive Periodontitis

Aggregatibacter actinomycetemcomitans (Aa) is a low-abundance Gram-negative oral pathobiont that is highly associated with a silent but aggressive orphan disease that results in periodontitis and tooth loss in adolescents of African heritage. For the most part Aa conducts its business by utilizing strategies allowing it to conceal itself below the radar of the host mucosal immune defense system. A great deal of misinformation has been conveyed with respect to Aa biology in health and disease. The purpose of this review is to present misconceptions about Aa and the strategies that it uses to colonize, survive, and evade the host. In the process Aa manages to undermine host mucosal defenses and contribute to disease initiation. This review will present clinical observational, molecular, and interventional studies that illustrate genetic, phenotypic, and biogeographical tactics that have been recently clarified and demonstrate how Aa survives and suppresses host mucosal defenses to take part in disease pathogenesis. At one point in time Aa was considered to be the causative agent of Localized Aggressive Periodontitis. Currently, it is most accurate to look at Aa as a community activist and necessary partner of a pathogenic consortium that suppresses the initial host response so as to encourage overgrowth of its partners. The data for Aa's activist role stems from molecular genetic studies complemented by experimental animal investigations that demonstrate how Aa establishes a habitat (housing), nutritional sustenance in that habitat (food), and biogeographical mobilization and/or relocation from its initial habitat (transportation). In this manner Aa can transfer to a protected but vulnerable domain (pocket or sulcus) where its community activism is most useful. Aa's “strategy” includes obtaining housing, food, and transportation at no cost to its partners challenging the economic theory that “there ain't no such thing as a free lunch.” This “strategy” illustrates how co-evolution can promote Aa's survival, on one hand, and overgrowth of community members, on the other, which can result in local host dysbiosis and susceptibility to infection

Diversity and drivers of coral reef cryptofauna communities

Tropical coral reefs are exceptionally biodiverse ecosystems, and many species within reefs remain undiscovered, undescribed or under-studied. This is especially true of organisms living hidden within the cavities and crevices of the reef matrix, the cryptofauna. Cryptofauna, and the cryptobenthic communities they form, make up a significant portion of animal diversity within coral reefs and play functional roles vital for reef productivity and trophodynamics. Globally, coral reefs are experiencing degradation due to climate-change and direct human impacts, and face mass biodiversity loss by the end of the 21st century. To predict the impacts of projected climate change on coral reefs, we must have a comprehensive understanding of the baseline diversity and composition of the communities they harbour. This thesis explores the diversity and drivers of cryptobenthic communities across the remote and protected Chagos Archipelago Marine Protected Area (MPA)(Central Indian Ocean), an important scientific reference site for the wider Indian Ocean due to its near-pristine status and minimal local human impacts. Standardised artificial substrates named Autonomous Reef Monitoring Structures (ARMS) were deployed across three reefs in 2018 and retrieved in 2019 and 2021 to collect image and genetic data on cryptobenthic communities. Little research has been conducted on cryptobenthic communities in the Chagos Archipelago, and we have a poor understanding of their biodiversity across the MPA in comparison to other reefs of the Indo-Pacific. Here, a rich and highly diverse community of coral reef organisms is recovered using multi-marker (COI, 18S) metabarcoding, and significant spatial and temporal variability in the metazoan community composition of three reefs is observed. However, only 3-4% of the 18,436 COI sequence variants identified are assigned to species level. With high confidence I identify 168 fully described species, but 95% of these are not represented in online museum records which hold specimen collected from the archipelago. This indicates that the archipelago’s reef biodiversity is likely far higher than presently understood. However, less than half of the species known to inhabit the archipelago have representative sequences in this study’s classifier, which is used to assign taxonomy to sequences , highlighting how the current paucity of references databases hinders metabarcoding studies of cryptofauna. Studies integrating biological and physical components of reef habitats are needed to understand how cryptobenthic communities may respond to shifts in environmental conditions. I show how ocean-facing and lagoonal reefs differ in environmental conditions, using seven in-situ environmental variables, and how this is reflected in the community composition of sessile cryptobenthic organisms. Internal waves are detected across ocean-facing reefs and found to maintain lower temperature conditions, and findings demonstrate the importance of using in-situ data, rather than ex-situ satellite-based data. Calcifying organism abundance significantly correlates with lower temperature profiles of ocean-facing reefs, but fleshy macroalgal abundance is associated with more variable profiles of pH and dissolved oxygen in a lagoonal habitat. Overall, results highlight habitat preferences of sessile invertebrate and algal groups within year-old communities and suggest which may be more resilient to climate-change induced increases in temperature and pH conditions. All artificial substrates used to study coral reefs have inherent biases, and it is important to determine what those biases are when using them to study natural reefs. I compare sessile communities recovered on ARMS against those on dead tabular Acropora sp. coral, a common species in the archipelago, to help understand these biases. I find similar abundances of sponges, soft-tube worms and ascidians on both substrates, but that ARMS may significantly overestimate natural abundances of calcifying invertebrates and under-estimate those of hard corals, turf and crustose coralline algae. I also compare communities recovered, using metabarcoding, on ARMS versus those from filtered water samples (environmental DNA) collected in the vicinity. I find eDNA sampling may be a poor proxy for studying motile invertebrates, but valuable for studying sponges, as it detects 17 out of 20 most abundant sponge species as well as an additional 9 species not detected on ARMS. Finally, I explore the use of fluorescence imaging to study ARMS sessile communities, and record 35 hard coral colonies on average per m2 across study sites. I find overall coral abundance is equal between lagoonal and ocean- facing reefs, but that coral abundance in the lagoonal reef is higher on the undersides of ARMS plates than on top of them, a pattern not observed across ocean-facing reefs and potentially driven by higher sedimentation occurring in the lagoon. This study highlights how fluorescence imaging may be a valuable additional method for the study of cryptobenthic assemblages on ARMS and I recommend its integration within the ARMS toolkit. In summary, this thesis significantly advances the knowledge of cryptobenthic communities of the Chagos Archipelago MPA, highlights its importance as a biodiverse scientific reference site, and furthers our understanding of the ARMS methodology. Whilst further work will be needed to fully characterise cryptofauna biodiversity across these reefs, findings presented in this thesis provide a baseline for future studies of these complex communities across the Indian Ocean region