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Title: Investigation of phagosomal compartments and prey digestion in protozoan predators
Author: Dixon, Claire Marie
ISNI:       0000 0004 2721 042X
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2010
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Protozoan grazing is commonly recognised as a major factor in shaping the structure and composition of bacterial communities. However, the complexity in the relationship between these predators and their prey means that ingestion of bacteria is not always coupled with complete digestion. A large number of factors are thought to be involved in determining the outcome of an ingestion event, and is highlighted by the fact that there is a large amount of inconsistency in the literature regarding the digestibility of prey types. Protozoan phagosome processing is poorly understood - which in turn makes it difficult to explain why and how differential digestion of prey occurs. Summation of the research into phagocytosis highlights two main implications for protozoan research. Firstly, in comparison to other phagocytic cell types, particularly macrophage and Dictyostelium discoideum, research in protozoan cells is out-dated, and therefore limited. Secondly, this lag in research means that gaps in the knowledge of protozoan phagosome processing are often filled with information from other model phagocytes; but are they comparable? This study firstly aimed to increase our understanding of phagosome processing by profiling changes in phagosome pH and size over time. Initially attempts were made to update research focusing on the pH profile in protozoa. However, work with both direct and indirect methods, previously utilised in other phagocytic cell types, failed to produce an effective assay for ,pH profiling in ciliates. Therefore, the main conclusions of this work explored the methodological considerations required when working with ciliated protozoa and the transferability of techniques between evolutionary distant phagocytic organisms. This research then progressed to assaying phagosome size over time as a measure of phagosomal processing. Results challenged the current beliefs regarding phagosome processing and produced a new model of phagosome sizing over time. Furthermore, utilising the sizing profile, as a relatively simple measure of phagosome processing events, factors that could potentially lead to an altered outcome were investigated. In this way, the pre-culture conditions of both predator and prey were shown to be significant in affecting phagosome processing. The next study examined the experimental variability that may be affecting the perceived outcome of digestion; rather than the events at the processing level. Specifically, by focusing on the digestibility of prey types as a function of the number of prey cells per phagosome it was shown that this latter parameter could alter the probability of prey digestion, with complete digestion of phagosomal contents being more likely if <6 prey cell contained. Finally, an in silico investigation aimed to take a holistic view to get a measure of how relevant and valid comparisons between phagocytic cell types are. Specifically, by analysing the proteins of a known phagosomal component, the V-ATPase enzyme. This work suggested that care must be taken when drawing general comparisons regarding phagocytosis and future work should strive to incorporate a larger number of protozoan and prey types. Overall the results of this study add further support to the idea that the relationship between prey and protozoan predators is one which has many layers of complexity which in turn has implications for nutrient transfer in environmental models.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available