Summary: Populations of and were monitored in natural soil amended with nutrients. The fates of a multicopy plasmid pIJ673, and an actinophage, KC301, were determined and the extent of gene transfer was estimated. The soil was incubated for 60 d during which time ‘spent’ soil was periodically removed followed by addition of fresh, uninoculated soil. Maximum numbers of bacteria and phage inoculants occurred at 15 d; this correlated with a peak in the amount of plasmid DNA detected and total numbers of transconjugants recovered. A KC301 lysogen of was also recovered at this time. Plasmid DNA was monitored by two methods, bead-beating and SDS/heat lysis; the latter was specific for mycelium while the former released DNA from spores and mycelium. Southern blots of soil DNA only showed the presence of plasmid DNA in SDS/heat lysis extracts from 15 d and 17 d samples, whereas positive signals were obtained throughout the experiment from bead-beaten extracts. The results confirmed that a well-developed mycelium was necessary for conjugation, phage infection, multiplication and lysogeny in soil.


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