The suspensions were transferred to a microplate, covered with a sensitive film used for TEM photography (SO-163; Kodak), and cultivated for 4 h at 30°C in the dark. The foggy areas on the film, due to reduction of the Ag emulsion of the film by the mercury vapor, were interpreted as Hg(0) volatilization. Escherichia coli strain DU1040, which carries the mer plasmid R831 (46), was used as a positive control. Sterile LB medium, alone or with HgCl2 at concentrations up to 100M, was used as a control to assess the level of mercury volatilization due to the culture medium.