Determination of metal tolerance levels. Susceptibilities to HgCl2
were determined for each selected strain in 96-well plates by using the
2-fold microdilution method (15) with slight modifications. The bacteria
were precultivated in LB medium for 16 h at 30°C, and a 1/100 dilution
was used to inoculate 100l LB or PB medium supplemented with HgCl2
(2 Mto 1 mM). Each metal concentration was tested in triplicate. Control
wells contained either HgCl2 or the bacterium in the culture medium
considered. After 48 h of incubation at 30°C under shaking (180 rpm),
growth was monitored by measuring the optical density at 595 nm on a
Multiskan FC plate reader (Thermo Fisher Scientific, Illkirch, France) and
subtracting the signals measured for the metal salts alone.