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Bdellovibrio bacteriovorus are small, predatory bacteria that grow within the periplasmic space of a host bacterium. Bdellovibrio has a biphasic life-cycle switching from a uni-nucleoid, growth-senescent ‘attack-phase’ to a novel, multi-nucleoid filamentous ‘growth-phase’, which elongates and divides, growing saprophytically within the periplasmic space of their prey. Little is known to date about Bdellovibrio developmental processes and cell division within this periplasmic niche. Recent publications have demonstrated that bacterial cytoplasms house highly organised matrices of protein structures, called the bacterial cytoskeleton. The Bdellovibrio processes of prey-cell entry, filamentous cell growth and division coordination brings cellular morphological changes and challenges that could be coordinated by cytoskeletal elements. Green Fluorescent protein (GFP)-tagging and gene knock out approaches were used to gain insights into the function of these elements including: an Intermediate filament like protein Ccrp, which has a role in the maintenance of cell morphology; two actin homologues, which appear to function at different points in the predatory cycle, MreB1 and MreB2; and a new type of cytoskeletal element designated ‘bactofilin’, which may have a role in cell division control. Recent advances in GFP technologies have led to the development of optimised GFP variants, such as mTFP1 and mCherry. These have been used to reveal previously unseen detail of Bdellovibrio development within prey. Bdellovibrio do not follow the familiar pattern of bacterial cell division by binary fission, instead divide synchronously at multiple sites along their length, once prey resources are depleted. This yields both odd and even numbers of progeny Bdellovibrio.

The peptidoglycan layer surrounding S. pneumoniae is essential for maintaining the shape of the cell and resisting osmotic lysis. This structure is the target for many antimicrobial approaches including direct digestion by lysozyme. This enzyme is widely produced to resist bacterial colonisation on host mucosal surfaces. For S. pneumoniae, post-synthetic modifications of the peptidoglycan structure protect cells from chemical insults arising from the host. For post-synthesis approaches to be effective, peptidoglycan modification must be linked to synthesis for the cell to be completely resistant. As part of a study into the regulation of PBP (penicillin binding protein) enzymes, which build the peptidoglycan layer, a link between peptidoglycan modification and the activity of PBP2A has been identified. In a Δpbp1aΔmacP background there is loss of the essential class A PBP activity due to the essential pairing of pbp1a and macP (Fenton et al., 2018). Mutations in the key virulence gene pgdA (peptidoglycan N-acetylglucosamine deacetylase) have been found to suppress this lethal genotype. Utilising a green fluorescent protein (GFP) N-terminally tagged PgdA, localisation patterns of PgdA have been uncovered by this study. Also the establishment of an optimised lysozyme sensitivity assay found sensitivity phenotypes in cells lacking members of the peptidoglycan biosynthesis machinery. Using a genetic approach, this work sought to characterise PgdA within the context of peptidoglycan synthesis and to investigate the link between cell wall biosynthesis and its post-synthetic modifications.

Within mammals and other vertebrates, males are often found to be the more infected sex and are more likely to suffer from mortality or adverse effects due to parasite infections compared to females. This is certainly true in humans, as mortality from infectious diseases is heavily male-biased. Sex-differences in both susceptibility and parasite exposure are two suggested mechanisms underlying the observed male bias. Studies from wild vertebrates that report sex-biased parasitism however, are usually cross-sectional and conducted in a single parasite-host system at a specific time point or even within a specific population. There is increasing evidence to suggest however, that sex-differences in the wild might be more dynamic and context-dependent. Understanding the contexts within which sex-differences are observed might not only provide a better understanding of the causes and consequences underlying sex-biased parasitism, but might also provide better insight into the role of host sex in host-parasite dynamics and epidemiology. This thesis aims to study sex-differences in parasite infection at the individual as well as a population level using the wild system of the wood mouse (Apodemus sylvaticus) and its natural parasite community. To address this key aim, I conducted paired field and laboratory experiments to investigate sex-differences in susceptibility, infection risk and immune responses in the wood mice system. Through experimental immunisations and diet manipulations, I addressed how the sexes differ in their vaccine-induced immune responses and the role of nutrition in driving these sex differences. I then investigated sex-differences in parasite infection at the population level by using a 6 year-long longitudinal dataset of multiple populations of wood mice and a community of ~14 parasite and pathogen species. Specifically, I tested whether there were consistent sex-differences in parasite prevalence and intensity across different parasite species, years, woodland field sites, and host condition. Finally, using the same long-term dataset, I addressed sex-differences in the fitness and survival consequences of infection and treatment. I analysed data from large-scale, multi-year helminth removal experiments to test whether anthelmintic treatment reduces helminths in wild wood mice, if this reduction impacts mouse condition and survival, and importantly, if this effect is sex-specific. To conclude, I discuss my findings and their implications both for applied translational research as well as fundamental research in disease ecology.