Identification of the Type Eleven Secretion System (T11SS) and Characterization of T11SS-dependent Effector Proteins

Host-associated microbes live in dangerous environments as a result of host immune killing, nutrient provisioning, and physiological conditions. Bacteria have evolved a host of surface and secreted proteins to help interact with this host environment and overcome nutrient limitation. The studies included within this dissertation describe the identification of a novel bacterial secretion system which has evolved to transport these symbiosis mediating proteins. This system, termed the type eleven secretion system (T11SS), is present throughout the Gram negative phylum Proteobacteria, including many human pathogens such as Neisseria meningitidis, Acinetobacter baumanii, Haemophilus haemolyticus, and Proteus vulgaris. Furthermore, these studies describe how novel cargo proteins of this secretion system were identified and characterized using molecular biology and physicochemical techniques. Chapter 1 establishes the importance of nematode model systems in researching symbiosis, highlighting how research in entomopathogenic nematodes identified the first T11S. Chapters 2 and 3 use a T11SS-dependent hemophore named hemophilin and its transporter protein to demonstrate T11SS secretion and its mechanisms of cargo specificity. Chapter 3 also explores the role of hemophilin within the nematode symbiont X. nematophila in surviving heme starvation and facilitating nematode fitness. Chapter 4 demonstrates that the lipidated symbiosis factor NilC is surface exposed by the T11SS NilB and uses a combination of metabolomics, proteomics, and lectin library analysis to describe the role of NilC in colonization. Chapter 5 describes a protocol for bioinformatically controlling genome co-occurrence analyses and utilizes this technique to demonstrate significant co-occurrence of T11SS with metal uptake pathways, single carbon metabolism, and mobile genetic elements. Additionally, this protocol allowed prediction of 141 T11SS-dependent cargo falling into 10 distinct architectures, including never before seen T11SS-dependent adhesins and glycoproteins. Finally, Chapter 6 summarizes our findings and contextualizes how the T11SS plays essential roles in host-microbe association in mutualistic bacteria and pathogenic bacteria alike.