Heat-cured acrylic-based resin materials have been used in dentistry for many years for the production of denture bases and hybrid implant overdentures. These materials play an important role in restoring aesthetic and masticatory functions, thus improving the quality of life of edentulous patients. However, a major drawback of these acrylic materials is their susceptibility to bacterial adhesion (Gupta et al. 2017).

The human oral cavity is a diverse microbial ecosystem with more than 1000 species of bacteria, mainly from the phyla Firmicutes, Actinobacteria, Proteobacteria, Fusobacteria, Bacteroidetes, and Spirochaetes (Sterzenbach et al. 2020). These microorganisms, influenced by factors such as oral health status and environmental conditions such as diet and gingival fluid, form complex biofilms both on teeth and mucosal surfaces and on the surfaces of dental materials such as implants, crowns, and prostheses. Bacterial adhesion to these surfaces can lead to tissue inflammation, causing conditions such as mucositis and peri-implantitis, and can also promote biofilm formation around dental restorations, leading to secondary caries (Sterzenbach et al. 2020; Sedghi et al. 2021).

To overcome these challenges, the incorporation of various nanoparticles into acrylic materials has been proposed. This approach aims to reduce cytotoxic effects and improve antifungal and bacterial adhesion properties of prosthetic base materials (Puspitasari et al. 2023). In our study, zirconia nanoparticles were added to heat-cured acrylic-based resins at concentrations of 1% and 3% by weight. The effect of this modification on bacterial adhesion was evaluated using various beverages (such as distilled water, mineral water, almond milk, and water kefir) at different time intervals (baseline, day 1 and day 14).

Our null hypothesis posited that the integration of zirconia nanoparticles into heat-cured acrylic resins, at concentrations of 1% and 3% by weight, and the exposure to different vegan beverages would not significantly alter the bacterial adhesion properties of these dental materials. Contrary to this hypothesis, our findings elucidate a complex interplay between the nanoparticle concentration, the type of beverage, and the resultant bacterial adhesion, thereby necessitating a rejection of the null hypothesis.

Different microorganisms commonly present in various foods and beverages can form biofilms on teeth, gums, and dental materials. The extent of biofilm formation, particularly that driven by Streptococcus mutans, may vary depending on interactions with other bacterial communities. Zirconia nanoparticles exhibit broad-spectrum bactericidal properties, effectively targeting both Gram-positive and Gram-negative oral bacteria, in addition to demonstrating antifungal efficacy against fungal spores (Pérez-Tanoira et al. 2016; Garcia et al. 2021).

Our study demonstrated that the incorporation of zirconia nanoparticles into heat‐cured acrylic resins significantly influenced bacterial adhesion as quantified in mg/mL. In distilled water, the group modified with 1 wt% zirconia exhibited bacterial adhesion levels of approximately 3.05 ± 0.03 log cfu/mL, indicating a reduction in microbial load compared to unmodified specimens. In contrast, in nutrient‐rich environments such as almond milk and water kefir, the incorporation of zirconia was associated with higher bacterial adhesion levels. For example, the Procryla group modified with 3 wt% zirconia showed bacterial adhesion values reaching up to 5.80 ± 0.03 log cfu/mL in water kefir after 14 days—reflecting an increase of nearly 90% relative to the values observed in distilled water. As the zirconia concentration increased in our study, bacterial adhesion correspondingly rose. Notably, the adhesion levels observed in specimens immersed in almond milk were comparable to those in water kefir. This phenomenon is likely attributable to the distinct bacterial communities inherent in these beverages, which can influence biofilm formation by modulating cellular motility, nutrient composition, and microbial load. Additionally, a study incorporating titanium, aluminum, vanadium, and yttria-stabilized zirconia demonstrated that the S. epidermidis strain p33 and S. aureus P4 exhibited significant adhesion on these modified surfaces (Pérez-Tanoira et al. 2016). These numerical findings align with observations from similar studies in the literature. Puspitasari et al. Puspitasari et al. (2023) reported that the integration of ZnO nanoparticles into acrylic resin resulted in a reduction in mucin adhesion from 20.59 ± 0.85 mg/mL in control specimens to 18.07 ± 0.80 mg/mL in optimally modified samples, suggesting an approximately 10% decrease in adhesion at effective nanoparticle concentrations. Similarly, Altarazi et al. (2023) found that incorporating TiO2 nanoparticles at low concentrations reduced the fungal biomass by 20–30% compared to controls, whereas higher nanoparticle concentrations led to an unexpected increase in microbial colonization—likely due to nanoparticle agglomeration and consequent alterations in surface free energy. The surface structure of zirconium oxide diminishes bacterial adherence relative to other materials, attributable to its electrical conductivity (Pérez-Tanoira et al. 2016).

A study investigated the antifungal effect of a nanocomposite against Candida albicans. The fungus was cultured in Sabouraud dextrose broth at 37 °C for 24 h. Afterward, the suspension’s optical density was adjusted for consistency. This suspension was then diluted to a specific cell concentration. Nanocomposite discs were prepared and placed in a 6-well plate. Each well was filled with the fungal suspension and incubated for 24 h. Post-incubation, the samples were washed and ultrasonically treated to detach fungal cells. The fungal solutions were then plated on agar and incubated again. Fungal colonies were counted, with results expressed as colony-forming units per milliliter. The study found that increased TiO2 (0.10, 0.25, and 0.50 wt%) in the nanocomposites significantly reduced fungal growth (Altarazi et al. 2023).

In our study, parallel to the research as mentioned above, we observed that adding 1% by weight zirconia to the specimens reduced bacterial adhesion in groups immersed in distilled water and 3 wt% zirconia in those immersed in mineral water. However, in groups exposed to other beverages, the incorporation of 1 wt% or 3 wt% zirconia increased bacterial adhesion.

Contrary to expectations, an increase in zirconia concentration was not associated with a further reduction in bacterial colonization. This phenomenon may be attributed to the complex interaction of the surface properties of the materials, in particular their surface free energy and the acidity of the beverages consumed. The changing surface free energy of dental materials with varying zirconia concentrations affects bacterial adhesion patterns. In addition, the acidity of beverages, an important environmental factor, plays an important role in modulating bacterial adhesion to these surfaces. Thus, while the incorporation of zirconium at low concentrations proved beneficial in reducing bacterial adhesion in distilled water media, its effectiveness did not increase linearly with increasing concentrations, underlining the complex nature of bacterial interactions with modified dental material surfaces (Sterzenbach et al. 2020).

There are studies identifying the addition of titanium dioxide nanoparticles in the range of 0.2–2.5 wt% as the most effective concentration for antibacterial and antifungal properties. However, it has been shown that exceeding a certain titanium dioxide concentration threshold can negatively affect some mechanical properties of the nanocomposite material. This negative effect is assumed to be due to the fact that higher nanoparticle content alters the intrinsic molecular structure of the polymerized material, acting as an impurity and potentially affecting the polymerization reaction. Consequently, an excessive amount of nanoparticles may lead to further leaching of unreacted monomers and TiO2NPs (titanium dioxide nanoparticles), resulting in instability of the printed samples and less desirable properties. Therefore, while TiO2NPs are effective in reducing bacterial adhesion, their concentration needs to be carefully balanced to maintain the mechanical integrity of the material (Karci et al. 2019; Altarazi et al. 2023).

In a recent study, zirconia nanoparticles (ZrO2NP) (1%, 2.5%, and 5%) modified groups showed significant antifungal effects and reduced bacterial adhesion compared to the control, especially as ZrO2NP concentration increased. Notable exceptions in antifungal effectiveness were observed between the 2.5% and 5% groups on day 14 (T1), and initially and after 5000 thermal cycles (T0 and T2) in the 1% group. Additionally, no significant effect was seen initially and on day 14 (T0 and T1) in the 5% group. While disk diffusion and filtration paper methods indicated no inhibition zones for ZrO2NP groups, the study confirmed ZrO2NPs’ antifungal properties and their sustained activity in heat-polymerized acrylic resin denture base materials after simulating 1 year of clinical use, suggesting long-term efficacy (Hamid et al. 2021).

Various atomic planes exhibit different surface energies and the surface energy of nanostructures is primarily influenced by the density of dangling bonds present on their surface. The difference in surface energy is crucial in determining the antifungal and antibacterial activity of nanoparticles. It can also be hypothesized that ZrO2NPs have identical surface geometries, but with various shapes, various active planes may exhibit different antimicrobial effects. Another explanation for the antifungal and antibacterial activity of ZrO2NPs is the presence of some nanoparticles on the surfaces of the samples, which may lead to direct contact between the nanoparticles and C. albicans and bacteria. This could interrupt cell function and actively inhibit the growth of fungal and bacterial strains (Hamid et al. 2021).

The findings of this study highlight the significance of beverage selection in relation to bacterial adhesion on heat-polymerized acrylic resins reinforced with zirconia. Specifically, the study draws attention to the consumption of water, mineral water, almond milk, and water kefir and their potential interactions with denture materials. From a bacterial adhesion standpoint, these interactions are crucial (Ebida et al. 2019).

Water, generally considered safe, does not typically contribute to increased bacterial adhesion or denture deterioration. However, mineral water’s acidic nature can enhance bacterial colonization on the denture surface, leading to increased risk of oral infections and necessitating strict denture hygiene post-consumption to mitigate these risks. Almond milk, while a healthier option, may still leave residues that could promote bacterial growth on dentures if not thoroughly cleaned afterward. This necessitates careful cleaning practices to prevent residue buildup, which can serve as a breeding ground for bacteria over time (Sutula et al. 2012).

In our study, the samples were immersed in distilled water, mineral water, almond milk, and water kefir without the use of artificial saliva for 14 days. At the end of this period, bacterial adhesion was measured, but the specific bacterial species present were not identified. Furthermore, aspects such as surface angle, surface roughness, and hydrophobicity of the surface were not investigated in the zirconia (ZrO2) added groups. Future research could usefully incorporate these parameters to provide a more comprehensive understanding of the interaction of the material in various environments. This would offer a deeper insight into the potential of ZrO2 modified materials in dental applications, especially in simulating conditions closer to the natural oral environment (Alla et al. 2017; Ebida et al. 2019).

Within the limitations of this study, incorporating zirconia nanoparticles into heat-cured acrylic denture base resins demonstrated significant effects on bacterial adhesion, depending on zirconia concentration and the immersion medium. In neutral environments like distilled water, a 1 wt% zirconia addition effectively reduced bacterial colonization, while higher concentrations (3 wt%) showed diminishing antibacterial benefits. In mineral water, the 3 wt% zirconia group exhibited enhanced long-term antibacterial properties after 14 days. However, in nutrient-rich environments such as almond milk and water kefir, the addition of zirconia appeared to promote bacterial growth, with the highest adhesion observed in the 3 wt% zirconia group.

In conclusion, a 1 wt% zirconia concentration may be optimal for reducing bacterial adhesion in neutral or mineral-rich environments without compromising resin integrity. However, in nutrient-rich conditions, zirconia may facilitate bacterial growth, underscoring the need for careful consideration of zirconia content based on specific oral conditions.