## 4 Discussion  
This research illustrates that both copper sulfate and copper nitrate present antimicrobial effects against _E. coli_ and _E. faecalis_, which are significant indicators of fecal pollution in aquatic environments, in a dose-dependent manner. At elevated concentrations, these copper compounds effectively inhibited bacterial proliferation and diminished biofilm development. Conversely, sublethal doses resulted in unexpected outcomes, enhancing biofilm resilience and modifying antibiotic resistance profiles, especially in _E. faecalis_. Figure 5: Growth curves of bacterial strains exposed to copper salts over 48 h, measured as average absorbance at 595 nm. **(a)** Growth of E. coli strains (EC1 and EC2) under different conditions: alone (blue and green, respectively), with copper sulfate (red and orange, respectively), and with copper nitrate (yellow and black, respectively). **(b)** Growth of E. faecalis strains (EF1 and EF2) under different conditions: alone (blue and green, respectively), with copper sulfate (red and orange, respectively), and with copper nitrate (yellow and black, respectively). Figure 6: **(a)** Resazurin assay results of _E. coli_ ATCC 14169 with copper sulfate (blue), _E. coli_ ATCC 25923 with copper sulfate (red), _E. coli_ ATCC 14169 with copper nitrate (yellow), and _E. coli_ ATCC 25923 with copper nitrate (green). **(b)** Resazurin assay results of _E. faecalis_ ATCC 19433 with copper sulfate (blue), _E. faecalis_ ATCC 29212 with copper sulfate (red), _E. faecalis_ ATCC 19433 with copper nitrate (yellow), and _E. faecalis_ ATCC 29212 with copper nitrate (green). The agar-well diffusion tests validated the strong antibacterial characteristics of the copper salts, showing that inhibition zones expanded in proportion to the concentration applied. These findings are consistent with earlier research indicating that copper can disrupt cellular membranes, produce reactive oxygen species, and interfere with critical enzymatic functions [12,20]. The minimum inhibitory concentration (MIC) for all examined strains consistently measured at 1.56 mg/mL, highlighting copper's broad-spectrum effectiveness across various bacterial types. This suggests that a single well-defined concentration could be useful against multiple microbial contaminants in pipeline systems, potentially streamlining dosage approaches in industrial contexts. However, biofilm assays indicated a more complex interaction. While elevated concentrations were effective in hindering biofilm formation, sublethal levels, particularly in the range of 0.39 to 0.78 mg/mL, led to enhanced biofilm strength in _E. faecalis_. This phenomenon may represent an adaptive mechanism involving stress-induced increases in extracellular polymeric substances or the activation of genes associated with biofilm survival [20,24]. These insights are particularly pertinent given the ongoing challenge of biofouling in water systems, where inadequate antimicrobial dosing might worsen contamination issues instead of mitigating them. The profiles of antibiotic susceptibility offered additional insights regarding the risks associated with copper exposure. While descriptive analysis indicated a noteworthy emergence of resistance--most notably, the appearance of ceftazidime resistance in E. faecalis ATCC 29212--statistical evaluation through one-way ANOVA did not establish significance for any antibiotic (\(p>0.05\)). This inconsistency may stem from substantial variability between groups or a lack of statistical power, suggesting the need for further research with larger sample sizes. Nonetheless, the qualitative shift in resistance patterns aligns with previous studies indicating that metal stress can co-select for antibiotic resistance through shared regulatory mechanisms or closely situated resistance genes [7]. These results underscore the dual function of copper salts in managing microbial populations.