Transfer and loss of naturally-occuring plasmids among isolates of Rhizobium leguminosarum bv. viciae in heavy metal contaminated soils

Plasmid transfer among isolates of Rhizobium leguminosarum bv. viciae in heavy metal contaminated soils from a long-term experiment in Braunschweig, Germany, was investigated under laboratory conditions. Three replicate samples each of four sterilized soils with total Zn contents of 54, 104, 208 and 340 mg kg¿1 were inoculated with an equal number (1×105 cells g¿1 soil) of seven different, well-characterized isolates of R. leguminosarum bv. viciae. Four of the isolates were from an uncontaminated control plot (total Zn 54 mg kg¿1) and three were from a metal-contaminated plot (total Zn 340 mg kg¿1). After 1 year the population size was between 106 and 107 g¿1 soil, and remained at this level in all but the most contaminated soil. In the soil from the most contaminated plot no initial increase in rhizobial numbers was seen, and the population declined after 1 year to