The effects of varying moisture and nutrient levels on the transfer of a conjugative plasmid between Streptomyces species in soil

Abstract

Biparental crosses were established in sterile silt loam between a plasmid-free Streptomyces parvulus strain (recipient) and a recombinant Streptomyces lividans strain (donor) bearing the recombinant, conjugational plasmid pIJ303 which codes for thiostrepton resistance. The crosses were established in autoclaved portions of soil adjusted to approximately 60, 40, and 20% water-holding capacity. The soil was amended with either CaCO3 alone (limed), or CaCO3 along with cottonseed flour, chitin, and cellulose (nutrient amended). After a month or longer incubation at 30 °C, the number of transconjugants in each soil treatment was determined by spread-plating on thiostrepton–agar selective medium. The heterotrophic microbial activity of each soil was also assayed with fluorescein diacetate. Nutrient amendment resulted in two to three times more fluorescein diacetate activity than for the unamended, limed treatments of equivalent moisture content. The nutrient-amended, low moisture treatment resulted in the greatest frequency of plasmid transfer. In comparison, plasmid transfer frequencies for the nutrient-amended, higher moisture treatments and for the limed treatments were approximately 5 to 73 times lower. The results suggest that nutrient-amended, relatively dry soils possess frequent microsites where mycelial growth and conjugationally mediated plasmid exchange between streptomycetes occur readily.Key words: Streptomyces, plasmid, conjugation, soil, gene transfer.