Exchange of a Conjugative Plasmid At Different Soil Moisture Levels Between Streptomyces Species Colonizing Artificial Soil Aggregates

Spores of a Streptomyces lividans strain (donor) bearing the recombinant conjugative plasmid pIJ303, which codes for thiostrepton resistance, and spores of a plasmid-free Streptomyces parvulus strain (recipient) were added together to finelysieved, sterile silt loam soil. This bulk soil was adjusted to either 20%, 35% or 45% of the soil’s water holding capacity. To this inoculated bulk soil were added sterile, nutrientamended, artificial soil aggregates. After incubation to allow streptomycete growth and aggregate colonization, both the sieved bulk soil and the aggregates were assayed for numbers of transconjugants by spreadplanting on a thiostrepton – agar selective medium. This allowed estimation of parental ability to colonize nutrient – rich soil sites at different soil moisture levels, and comparison of plasmid exchange frequency in the nonamended bulk soil versus the nutrient – rich soil aggregates. Plasmid exchange was detected only on aggregates of about 20% water holding capacity or less (4.2% wt/wt H20). The soil moisture threshold for heterotrophic streptomycete activity appeared to lie between 2.87% and 4.2% wt/wt H20 for the nutrientamended soil aggregates.