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Title: Interactions between actinophage and streptomycetes in soil and the fate of phage-borne genes
Author: Marsh, Peter
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1993
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Host activity is essential for bacteriophage survival. Bacterial growth in soil is sporadic and discontinuous in time and space. Phages must therefore adopt strategies to overcome periods of host inactivity, such as the lysogenic life-cycle, for long-term survival in soil. KC301 (a derivative of oC31) which contains the thiostrepton resistance gene tsr was inoculated into soil microcosms as a lysate or via lysogenic hosts, together with combinations of Streptomyces spp. populations. Phage and host populations were monitored over periods of incubation of between 12 and 47 days. The possibility of KC301 lysogenizing indigenous streptomycetes in nonsterile soil was investigated by selection for thiostrepton resistance and molecular analysis of lysogen DNA. The effects of selection for phage-borne tsr was examined by adding thiostrepton to soil microcosms before inoculation of soil with resistant lysogenic and sensitive uninfected strains of Streptomyces lividans. Correlation between metabolic activity of S. lividans and infection by KC301 in soil was also assessed. KC301 lysogenized indigenous streptomycetes in nonsterile soil when added as a lysate, but not as a prophage in lysogenic spores. A sample of indigenous lysogens proved to contain integrated KC301 DNA. Most of the isolates were stable lysogens, retaining thiostrepton resistance after several rounds of sub-culturing. Interactions between S. lividans and KC301 in soil were limited by the densities of both host and phage populations. In most cases, host population densities in soil were not reduced by lytic KC301 infections. When reduction by lytic KC301 infections of the mycelial stage occurred in soil, sporulation compensated for loss of total viable propagules. KC301 infected hosts in soil, and established itself as a prophage in lysogenic spores in soil. Turnover of soil at defined times stimulated a small proportion of lysogenic spores to germinate and release KC301 into the soil. KC301 released from germinating lysogenic spores could infect and lysogenize previously uninfected soil inoculants of the same or different Streptomyces spp. Lysogenic populations reached lower levels than those of uninfected counterparts, whether growing in the presence or absence of those counterparts. Thiostrepton in soil caused reduction in the mycelial stage during vegetative growth of sensitive strains following inoculation, but as with phage infections, this was compensated by sporulation. The KC301 lysogen were not affected by thiostrepton in soil, although no selective advantage was conferred on the lysogen population by the presence of the antibiotic. The majority of the KC301 population in soil existed in the prophage state when soil was inoculated with lysogenic hosts. Although lysogenic hosts were competitively compromised, inputs into the free phage pool could occur without further detriment to host density. Sporulation of hosts compensated for killing of mycelium in soil, and facilitated long-term persistence of KC301.
Supervisor: Not available Sponsor: Natural Environment Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QH Natural history ; QR Microbiology