Lipidomic Analysis of the Streptococcal Cellular Membrane
Joyce, Luke R
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Streptococci are Gram-positive bacteria that natively colonize niches throughout the human body. While carriage of streptococci is mostly asymptomatic, they can be major human pathogens causing a variety of diseases across all age groups. Research on streptococcal mechanisms of pathogenicity, colonization, and transmission have identified virulence factors such as proteins, extracellular polysaccharides, intercellular communication pathways, and gene regulation pathways. These virulence factors have been demonstrated to aid in adherence to and invasion of human tissues and evasion of the immune system. A major component of the streptococcal cell is poorly understood: the cellular membrane. The cellular membrane is a critical site in host-pathogen interactions, providing an anchor for extracellular polysaccharides, protecting the internal workings of the bacterial cell, aiding transport of nutrients, and promoting survival in harsh conditions. The overarching goal of my dissertation work was to elucidate the biosynthetic pathway of phosphatidylcholine (PC) in Streptococcus mitis and S. oralis, members of the Mitis group streptococci, and to characterize the streptococcal cellular membrane using a culture medium that mimics the human host. In my research, I utilized lipidomics coupled with isotope tracing to discover that the glycerophosphocholine (GPC) pathway is required for PC biosynthesis in the Mitis group streptococcal species S. mitis, S. oralis, and S. pneumoniae. Briefly, the GPC pathway scavenges the major human metabolites GPC and lysophosphatidylcholine (lysoPC) and acylates them to form PC. Additionally, I determined that S. pneumoniae, S. pyogenes, and S. agalactiae synthesize PC when cultured in the presence of human serum. I demonstrate that lysoPC is the major substrate scavenged for an abbreviated GPC pathway in S. pyogenes and S. agalactiae. Furthermore, I characterized the structure of the novel aminoacylated glycolipid lysylglucosyl-diacylglcyerol (Lys-Glc-DAG) in S. agalactiae. I experimentally confirmed that the enzyme multiple peptide resistance factor (MprF) is the biosynthetic enzyme responsible for the lysine modification of Glc-DAG, establishing a novel glycolipid substrate for the S. agalactiae MprF. The MprF of S. agalactiae also catalyzes the addition of lysine to phosphatidylglycerol, forming lysyl-phosphatidylglycerol (Lys-PG), as expected based on prior knowledge of MprF in other bacteria. Using in vitro assays, I show that the lysine lipids, Lys-PG and Lys-Glc-DAG, impact the cellular membrane physiology such that S. agalactiae lacking the two lysine lipids are unable to survive in acidic conditions, have a more net negative outer surface charge, and exhibit significantly reduced human cell adherence and invasion. Taken together, my research provides critical insight into the cellular membrane of streptococci, evidence of cellular membrane remodeling through scavenging of major human metabolites, and the characterization of a novel aminoacylated glycolipid which impacts S. agalactiae colonization and invasion potential.