## 3 Results and Discussion
### The Effect on the Biofilm Formation  
The action of zinc sulfate heptabhydrate (Figure 3) and zinc chloride (Figure 4) on _E. coli_ and _E. faecalis_ biofilm growth is relevant to the antimicrobial activity and biofilm-modulating efficacy of zinc salts. It was found that whereas biofilm formation was inhibited in high concentrations of zinc (\(\geq\)2.5 mg/mL), sub-inhibitory concentrations (0.156-0.019 mg/mL) increased the yield of the biofilm, suggesting bacterial stress response. Our findings are consistent with previous work that has found that high concentrations of zinc salts inhibit biofilm formation. [PERSON] et al. found that the biofilm formation of Streptococcus mutans was inhibited by zinc acetate, which increased the permeability of biofilms at doses above 0.3 mg/mL, thereby validating the assumption that Zn\({}^{2+}\) ions can  
Figure 3: The effect of zinc sulfate heptabhydrate on the biofilm formation of _E. coli_ ATCC 14169 (blue), _E. coli_ ATCC 25922 (red), _E. faecalis_ ATCC 19433 (yellow), and _E. faecalis_ ATCC 29212 (green). PC—positive control; NC—negative control. The data shown represent a single biological replicate per strain. disrupt the integrity of the biofilm at bactericidal doses [5]. However, under sub-inhibitory concentrations (0.156-0.019 mg/mL), biofilm development was enhanced in _E. coli_ ATCC 14169, _E. faecalis_ ATCC 19433, and _E. faecalis_ ATCC 29212, indicating that biofilm development is a survival mechanism against low zinc levels. The same has been observed in _P. aeruginosa_ and _S. aureus_, where sub-lethal zinc concentrations triggered biofilm development under metal stress [1]. Similarly, [PERSON] et al. demonstrated that exposure to low doses of zinc promotes biofilm development in bacteria associated with medical devices, highlighting the bimodal nature of zinc's modulation of biofilms [22]. Zinc salts exhibit antimicrobial action through the disruption of membrane integrity, inhibition of enzymes, and inhibition of oxidative stress by Zn\({}^{2+}\) ions. At high concentrations, low inhibitory concentrations may facilitate the development of biofilms and resistance. Antimicrobial peptides (AMPs) possess broad-spectrum and often targeted action, with less potential for resistance due to their rapid, membrane-targeted mechanism. A recent survey identified over 300 novel AMPs from marine biofilm bacteria, with most exhibiting vigorous activity against drug-resistant microorganisms [23]. The discoveries suggest that AMPs can be promising alternatives or adjuncts to zinc, especially in applications where specificity is required and little resistance potential exists. Zinc chloride inhibited biofilm development more effectively at higher concentrations (10-2.5 mg/mL) but elicited stronger biofilm reactions at low concentrations. _E. coli_ ATCC 14169 produced more biofilm at 0.156 mg/mL and attained a peak at 0.078 mg/mL, demonstrating that zinc chloride stress at low concentrations triggers biofilm production. _E. coli_ ATCC 25922 produced the most significant increase at 0.312 mg/mL, supporting enhanced protective biofilm production under zinc chloride stress. These findings align with those of [PERSON] et al., who indicated that zinc sulfide and zinc oxide nanoparticles significantly inhibited the growth of _Klebsiella oxytoca_ and _P. aeruginosa_ biofilms and induced protective biofilm responses at sub-lethal concentrations [24]. Zinc sulfate hepthydrate induced a slower-growing biofilm response in sub-inhibitory concentrations, where _E. faecalis_ ATCC 19433 and _E. faecalis_ ATCC 29212 showed greater biofilm production at 0.019 mg/mL. This means that Gram-positive _E. faecalis_ strains employ biofilm development as a survival strategy for low Zn\({}^{2+}\) stress. [PERSON] et al. demonstrated that  
Figure 4: The effect of zinc chloride on the biofilm formation of _E. coli_ ATCC 14169 (blue), _E. coli_ ATCC 25922 (red), _E. faecalis_ ATCC 19433 (yellow), and _E. faecalis_ ATCC 29212 (green). PC—positive control; NC—negative control.