Kdo derivatives as tools to metabolically label bacterial lipopolysaccharides

Carbohydrates are versatile molecules that not only provide energy but also play essential structural and functional roles both within and on the surface of living cells. They form complex structures on cell surfaces, where they support communication, defense, and interactions with the environment. In bacteria, these
sugar structures are integral to their survival, acting as a barrier against harmful substances and contributing to environmental adaptation. Because of their structural diversity and biological relevance, bacterial surface carbohydrates are valuable tools for studying how bacterial cells build and maintain their membranes.
In Gram-negative bacteria, lipopolysaccharides (LPS) are essential components of the outer membrane, providing structural stability and acting as a selective barrier against the external environment. The projects described in this Thesis focused on Kdo, a key sugar in LPS, investigating whether modified forms of Kdo can serve as efficient tools to visualize the cell surfaces of Escherichia coli strains, and how the structural changes influence LPS biosynthesis and membrane integrity.
It was discovered that the efficiency of Kdo-based LPS labeling varies widely depending on the bacterial strain and growth conditions, emphasizing the importance of optimizing the method for each specific study. Interestingly, some modified Kdo derivatives were successfully accepted by the LPS biosynthesis
pathway and result in subtle changes in membrane integrity under specific stress conditions. In contrast, other derivatives were not. These results suggested that bacteria are very selective about the sugars they use, and even small changes can affect whether those sugars are accepted or not. Altogether, the results suggested that Kdo modifications could serve as tools to investigate LPS assembly and test the limits of membrane integrity. These findings offer new strategies for studying LPS assembly and targeting the outer membrane of Gram-negative bacteria.