## 3 Results and Discussion
### Bacterial Strain Tolerance to Hg  
All of the bacteria were cultured for 120 h. The OD\({}_{600}\) values were measured every 2 h. The growth curves of BL21-0, BL21-1, BL21-2 and BL21-7 cultured in liquid LB medium without Hg were S shaped. The growth of these four strains cultured in liquid LB medium containing 5 \(\upmu\)mol/L HgCl\({}_{2}\) also demonstrated an S shape. However, at the stationary phase, the OD\({}_{600}\) values of BL21-0 and BL21-1 cultured in the medium containing 5 \(\upmu\)mol/L HgCl\({}_{2}\) were lower than those corresponding cultures without Hg. The OD\({}_{600}\) values of BL21-7 cultured in medium containing 5 \(\upmu\)mol/L HgCl\({}_{2}\) were similar to those of corresponding cultures without Hg. In the media containing 10 \(\upmu\)mol/L, 20 \(\upmu\)mol/L and 40 \(\upmu\)mol/L HgCl\({}_{2}\), the growth curves of BL21-7 and BL21-2 still showed an S shape. BL21-0 cultured in LB medium without Hg grew much more quickly than that cultured in LB medium containing HgCl\({}_{2}\). The highest concentration of HgCl\({}_{2}\) that BL21-0 could withstand was 5 \(\upmu\)mol/L (Figure 3). The growth of BL21-0 was inhibited completely in LB medium containing a HgCl\({}_{2}\) concentration of 10 \(\upmu\)mol/L or more (Figure 3). There were no significant differences between the OD\({}_{600}\) values of BL21-0 cultured in these media for 16 h and those of BL21-0 cultured in these media for 120 h. BL21-1 was also very sensitive to HgCl\({}_{2}\); it could not grow in LB medium containing a HgCl\({}_{2}\) concentration of 10 \(\upmu\)mol/L or more (Figure 3). After BL21-2 and BL21-7 were cultured in LB liquid medium containing 20 \(\upmu\)mol/L HgCl\({}_{2}\) for 16 h, their OD\({}_{600}\) values were 1.1 \(\pm\) 0.05 and 1.2 \(\pm\) 0.05, respectively. They exhibited good tolerance to 20 \(\upmu\)mol/L HgCl\({}_{2}\) (Figure 3). After being cultured for 120 h, BL21-7 cultured in LB containing 20 \(\upmu\)mol/L HgCl\({}_{2}\) had similar OD\({}_{600}\) values to BL21-7 cultured in LB without Hg. The bacterial strain tolerance to HgCl\({}_{2}\) can be ordered from the strongest to the weakest as follows: BL21-7, BL21-2, BL21-1 and BL21-0. Figure 3: _E. coli_ strains’ tolerance to HgCl\({}_{2}\). The OD\({}_{600}\) values were measured after the strains were cultured in LB liquid medium containing HgCl\({}_{2}\) for 120 h.  
Figure 2: The expression levels of _ppk-rpsT_ in different _E. coli_ strains. Columns 1-4 represent BL21-0, BL21-1, BL21-2 and BL21-7, respectively. To identify whether BL21-7 displayed good tolerance to organic Hg, the engineered bacterial strain was cultured in a series of LB media containing different quantities of MMC or PMC. All of the bacteria were cultured for 120 h. The OD\({}_{600}\) values were measured every 2 h. The growth curves of BL21-0 and BL21-7 cultured in liquid LB medium without Hg showed an S shape. BL21-0 grew well in liquid LB medium containing 5 \(\upmu\)mol/L MMC. However, it did not grow in LB medium containing 5 \(\upmu\)mol/L PMC or 10 \(\upmu\)mol/L MMC. After BL21-7 was cultured in LB liquid medium containing 10 \(\upmu\)mol/L MMC for 16 h, the OD\({}_{600}\) value of the culture was still approximately 1.4 \(\pm\) 0.08. This bacterial strain grew well in LB containing 20 \(\upmu\)mol/L MMC, 40 \(\upmu\)mol/L MMC, 20 \(\upmu\)mol/L PMC or 40 \(\upmu\)mol/L PMC (Figure 4). To further identify the engineered strain's tolerance to Hg, its MICs of HgCl\({}_{2}\), PMC and MMC were measured. A series of conical flasks were filled with 200 mL of fresh LB liquid medium. A stock solution of HgCl\({}_{2}\) (0.1 mol/L) was initially prepared and a series of HgCl\({}_{2}\) stock solutions were added to the medium to final concentrations of 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 \(\upmu\)mol/L. The OD\({}_{600}\) values of the cultures were measured every h. The minimum Hg concentration of LB liquid medium in which the bacteria could not grow was used as the MIC. The bacteria's MICs for MMC and PMC were measured following the same method.