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dc.contributor.authorWilliamson, Kurt Elliott-
dc.description.abstractThe importance of viruses in aquatic ecosystems has been established over the past decade. By contrast, the abundance and distribution of viruses in soils is almost completely unknown. An essential first step in any investigation of viruses in soil is evaluation of viral recovery methods suitable for subsequent cultivation-independent analyses. A comparison of four common elution buffers indicated that potassium citrate eluted the highest numbers of viruses from soils (1.5 X 108 gram dry weight-1 grand mean), but specific soil-eluant combinations posed significant problems to enumeration by epifluorescence microscopy. Thus, the measurement of phage abundance in soils varied with both the extraction methodology. Triplicate sequential extractions of viruses from six Delaware soils indicated that initial extractions captured approximately 70% of total viral abundance across al soils. Thus, single extractions represent slight underestimations of natural viral abundance in these soils. More importantly, however, the results suggest that differences in abundance across sample sites appear to be biogeographical in nature, and are not simply artifact. In terms of diversity, extracted soil virus communities were dominated by bacteriophages with a wide range of capsid diameters and morphologies. Land form and management paterns were significantly correlated to both bacterial and viral abundance, as wel as to soil organic mater and water content. Land management practices have also been shown to impact bacterial abundance and diversity. Colectively, these results suggest linkages between viral and bacterial communities in soils. It has been proposed that lysogeny would be favored in soils, as this relationship provides for the long-term maintenance of viruses within host populations. However, litle experimental data are available to support this. Induction assays were performed to test the prevalence of lysogenic bacterial hosts within soil bacterial communities. Thirty percent (6 of 20) of bacterial isolates from Delaware soil samples harbored mitomycin C (MC)-inducible prophages, while only one non-inducible isolate spontaneously produced detectable quantities of phage particles. The magnitude of induction responses was dependent upon incubation conditions; removal of MC from the culture media after 30 minutes resulted in a marked increase in induction responses for three of the six lysogens. Furthermore, two of the isolates were polylysogens that produced more than one distinct type of phage particle, raising questions about the distribution of this phenotype among soil bacteria. These experiments suggest that lysogeny is common among cultivable soil bacteria. However, given the variability in induction responses, care should be taken in using induction assays as a measure of lysogeny within microbial communities. Cultivation-independent induction assays were also developed to assess the prevalence of lysogenic bacteria in a variety of soil samples, including six from Antarctica, and four from Delaware. Induction experiments were carried out either by extracting bacteria from soil and subsequently exposing extracts to MC, or by exposing bacteria to MC through direct addition to soil slurries. The former extraction-induction approach generated significantly higher prophage induction than al other treatments and was highly reproducible. Although inducible fraction was higher in temperate Delaware soils than Antarctic soils, no clear correlations were found between lysogeny and soil physical properties. For the Delaware soil samples, the inducible fraction of extractable soil bacteria based on this cultivation-independent assay was comparable to the frequency of inducible lysogeny among cultivable isolates (approximately 30%). Colectively, these data indicate that lysogeny is common among soil bacteria, but the specific factors that promote lysogeny remain unclear.
dc.publisherUniversity of Delaware
dc.relation.ispartofseriesDoctor of Philosophy
dc.subjectVi khuẩn đất -- Khía cạnh sinh thái
dc.titleEcological aspects of viruses in soils
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