TABLE 2.

Exogenous isolation of mercury resistance plasmids in three recipient strains from bulk soil and wheat rhizosphere soil portions 10 days after different amounts of HgCl₂ had been added

Recipient strain	Exogenous isolation frequency obtained with bacterial populations from soil treated witha:
	No HgCl2		4 μg of HgCl2 g of dry soil−1		28 μg of HgCl2 g of dry soil−1		55 μg of HgCl2 g of dry soil−1
	Bulk soil	Rhizosphere soil	Bulk soil	Rhizosphere soil	Bulk soil	Rhizosphere soil	Bulk soil	Rhizosphere soil
None	0b	0	0	0	0	0	0	0
Pseudomonas putida UWC1 Rpr	0	1 × 10−7	0	3 × 10−7	0	5 × 10−7	2 × 10−8	1 × 10−6
Pseudomonas fluorescens R2f Rpr	2 × 10−8	0	1 × 10−7	5 × 10−8	6 × 10−7	3 × 10−8	1 × 10−6	3 × 10−7
Enterobacter cloacae BE1 Rpr	0	7 × 10−9	2 × 10−7	2 × 10−7	4 × 10−7	4 × 10−7	5 × 10−7	NDc

^aCFU of recipients resistant to mercury (20 μg ml⁻¹)/total CFU of recipients. 

^b0, below the limit of detection (less than 10⁻⁹ transconjugant per recipient). 

^cND, not determined.