## 3 Results and Discussion  
### Minimum Inhibitory Concentration  
Table 2 presents the MIC of two zinc salts against four bacterial strains.  
Zinc chloride was more active than zinc sulfate heptahydrate against most bacterial strains, as evidenced by its lower MIC values. This can be attributed to its greater solubility and dissociation properties, leading to more bioavailable Zn\({}^{2+}\) ions, which have been reported to exhibit antimicrobial activity through interference with bacterial cell membrane disruption and enzymatic processes.  
The antimicrobial properties of zinc salts have been extensively explored in recent research. In one study, zinc sulfate heptahydrate suppressed multidrug-resistant bacteria, including _E. coli_ and _Enterobacter_ spp., with MIC values ranging from 10 mg/mL to 14 mg/mL depending on the bacterial species, in agreement with our finding for _E. coli_ ATCC 14169 [13]. Additionally, the impact of Zn\({}^{2+}\) ions on biofilms was established in a study that found that zinc sulfate caused considerable inhibition against _E. coli_ biofilms at concentrations of only 0.005 M, with further inhibition observed at higher concentrations [14]. This supports the hypothesis that the antimicrobial activity of zinc salts is achieved through interference with biofilms, as well as their bacteriostatic potential.  
In our present research, _E. coli_ ATCC 14169 was the more resistant among the two, with MIC values of 5 mg/mL for zinc sulfate heptahydrate and 10 mg/mL for zinc chloride. The latter is compared with _E. coli_ ATCC 25922, which was comparatively more susceptible. These differential sensitivities towards zinc are consistent with data from past literature, which shows that _E. coli_ strains were similarly resistant due to genetic heterogeneity in efflux pumps and membrane permeability [15].  
The results for _E. faecalis_ indicate that bacterial species-specific factors, such as metal ion transporters and membrane permeability, underlie the demonstrated MIC differences.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Bacterial Strain**} & \multicolumn{2}{c}{**MIC (mg/mL)**} \\ \cline{2-3}  & **ZnSO\({}_{4}\)\(\times\) 7H\({}_{2}\)O** & **ZnCl\({}_{2}\)** \\ \hline _Escherichia coli_ ATCC 14169 & 5 & 10 \\ _Escherichia coli_ ATCC 25922 & 0.625 & 0.312 \\ _Enterococcus faecalis_ ATCC 19433 & 1.25 & 0.625 \\ _Enterococcus faecalis_ ATCC 29212 & 2.5 & 2.5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The minimum inhibitory concentration of zinc salts.  
Studies on _E. faecalis_ have confirmed that ZnO nanoparticles exhibit more antimicrobial action against this species than against other Gram-negative bacteria, again reinforcing the effect of the structure of bacterial cell walls on susceptibility to zinc ions [16].  
The present work also supports previous reports that zinc chloride exhibits stronger antimicrobial activity than zinc sulfate heptahydrate. This has been validated by research on various zinc salts, where zinc chloride exhibited enhanced antimicrobial activity against _Streptococcus pyogenes_ and _E. coli_ due to its more effective ionic dissociation, resulting in the higher bioavailability of zinc [14]. Nonetheless, based on other research, biofilm aging, metal sequestration mechanisms, and inhibition by enzymes may regulate the antibacterial effect of zinc salts [5].