Interactions between actinophage and streptomycetes in soil
A method was developed based upon soil dispersion using an ion exchange resin and differential centrifugation that allowed the selective isolation of Streptomyces spores rather than mycelia; this allowed the characterisation of a germination/sporulation cycle in situ. The method was able to detect relatively low numbers of streptomycetes in soil, through its concentrating action. The ecology of a temperate actinophage derived from 4'C31, containing the thiostrepton-resistance gene, was studied in conjunction with strains of Streptomyces lividans. In sterile amended soil, phage numbers showed initial increases due to a corresponding germination of the host spore inoculum; subsequently phage numbers declined when the host streptomycete was no longer in the mycelial state and hence receptive to phage infection. Lysogens were readily obtained in sterile amended soil; use of the spore extraction method and another method that isolated both spores and mycelia enabled lysogenic mycelia to be first detected after 2 days and lysogenic spores after 5 days. Phagemediated gene transfer of the thiostrepton-resistance gene from a lysogenic donor to a non-lysogenic recipient was also demonstrated in sterile amended soil. In nonsterile soil, host growth was considerably retarded with respect to sterile amended soil, although phage numbers showed a similar pattern with an initial burst of activity followed by subsequent decline. Lysogens were only rarely found in nonsterile soil and were also found to be less fit in situ than the parent strain. No difference was found in sterile amended soil.