Vanacore, Adele (2025) Structure, function and properties of bacterial cell envelope components. [Tesi di dottorato]

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Abstract

Bacterial membranes are highly dynamic structures, exhibiting significant variability in both lipid and protein composition. This flexibility allows the molecules to rearrange, greatly affecting the membrane's fluidity, stability, and permeability. These characteristics are important because they directly influence cellular functions and interactions with the environment. One of the main components, lipopolysaccharides (LPS), plays a crucial role in the outer membrane of Gram-negative bacteria, serving as an essential interface with the environment. Along with capsular polysaccharides (CPS) and exopolysaccharides (EPS), LPS supports bacterial interactions with their hosts and the surrounding microbiota, playing a significant role in adhesion, biofilm development, and immune responses. In addition, hopanoids, structurally and functionally resembling eukaryotic steroids, are essential for stabilizing membranes, ensuring they remain intact and resilient even in tough environmental conditions. This PhD research investigates the many roles that bacterial glycans play, especially their impact on the immune system and how they interact with immune functions. By carefully characterizing the chemical structures of these glycans, we discovered new insights into how they help shape immune responses in the host and maintain the delicate equilibrium between tolerance and immunity. To achieve this, we isolated and analyzed key components from the bacterial cell envelope, including LPS, CPS and EPS, from specific strains known to interact with both human and plant hosts, such as Herbaspirillum Root189, Methylobacterium extorquens PA1 and Paenalcaligenes hominis. We then analyzed the immune recognition patterns of these glycans to identify the specific structures that trigger immune responses and activate receptors. This approach has enabled us to unveil the molecular pathways through which glycans modulate immune activity and mediate host-microbe interactions. Glycans play a crucial role in detailed communication between hosts and microbes, influencing health in different ways that can be either beneficial or harmful, depending on the context. In humans and plants, glycans activate the immune system, triggering defense mechanisms against harmful bacteria and enhancing innate immune responses. Conversely, in symbiotic and commensal relationships, glycans promote beneficial interactions, improve the host's tolerance and regulate immune responses, which is essential for maintaining a balanced microbiota that supports health. In plants, microbial glycans have been shown to enhance symbiotic relationships, promoting growth, improving nutrient uptake, and increasing resistance to environmental stresses. Similarly, in human-associated bacteria, glycans can significantly impact on immune responses, emphasizing their important roles in both maintaining health and contributing to disease development. The main aim of this thesis is to elucidate the complex structure-function relationships of microbial glycans that come from various bacteria, including symbiotic, commensal, and pathogenic strains. This research provides important insights into the molecular mechanisms underlying host-microbe interactions and immune responses. It highlights new structural features and biological roles of bacterial glycans, making an important contribution to the field of glycobiology. By clarifying the complex relationships between microbial life and their hosts, it simplifies aspects that remain poorly understood. These results increase our understanding of how glycans assist communication and recognition between microbes and hosts. This suggests innovative implications, such as the development of new therapeutic strategies that can modulate the host-microbe interface, refining interventions designed to re-establish a healthy microbial equilibrium, and utilizing beneficial bacteria for applications in agriculture and biomedicine. Moreover, this work emphasizes the potential of microbial glycans in sustainable and green applications. Understanding these molecular interactions not only advances biomedical research but also sets the stage for environmentally conscious agricultural practices. Promoting beneficial microbial interactions in crops, for example, can naturally enhance growth and resilience, reducing the dependency on chemical fertilizers and pesticides. Similarly, harnessing these mechanisms could lead to greener biotechnological solutions aimed at preserving ecological balance and supporting environmental health. Furthermore, the ability to modulate microbial communities responsibly aligns with global sustainability goals, offering pathways toward less invasive medical treatments and sustainable agricultural practices that reduce environmental impact and support long-term ecosystem health.

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