**4. Discussion**

The current study was conducted to evaluate the cumulative effect of microwave disinfection and a denture cleaning tablet (sodium perborate) with reduced time and power on the elimination of bacterial biofilm from PMMA denture base resin. It was based on the hypothesis that there will be no significant difference in cleaning efficacy of DC, MW and their combination on the disinfection of *E. coli*, *S. aureus* and *S. mutans* incubated on the PMMA denture base. Therefore, the postulated hypothesis was rejected, as all the tested groups displayed variable results of denture disinfection as compared to the control.

The possibility that a dental prosthesis can be a cause of infection and cross-contamination between patients and dental staff additionally stresses the need for disinfection [17]. Dental appliance disinfection poses a significant problem for clinicians, particularly with regards to removable dentures [18]. The results of the present study revealed that zero mean CFU was obtained after exposing PMMA to MW irradiation in combination with DC within 2 min (MW-DW-DC). In comparison, zero mean CFU for samples exposed to microwave alone (MW + DW) was obtained at 3 min. The one-minute exposure of acrylic specimen

to MW irradiation showed 50% reduction in CFU for *E. coli*, 24% for *S. aureus* and 96% for *S. mutans*. Conversely, the combination of MW and DC resulted in 97% reduction of *E. coli* count, 96% for *S. aureus* and 100% for *S. mutans* after 1 min of exposure. Thus, a combination of MW irradiation with DC showed higher disinfection percentage of bacterial species in disinfection of PMMA polymer denture bases compared to MW and DC alone.

Considering the effectiveness of microwave disinfection, its role for denture decontamination has gained much attention in the past [19]. Multiple previous pieces of research have used variable time and power to ascertain its efficacy for denture disinfection [20]. Webb et al. first identified that an unmodified domestic microwave can perform denture disinfection [10]. In the present study, the efficacy of microwave irradiation on acrylic resin contaminated with bacterial biofilm at different growth hours (24–36–48–96 h) was investigated keeping the power 450 W and irradiation time 1, 2 and 3 min. From the results, it was found that exposure to microwave irradiation took 3 min to achieve complete CFU count zero, i.e., complete removal of *S. aureus*, *S. mutans* and *E. coli* from acrylic slides. Moreover, comparative analysis revealed that the increase in incubation period to 36 h resulted in an increased CFU count. However, this increase in count has not shown any impact on the disinfection capabilities of microwave radiation. These results are in line with the outcomes of the studies conducted by Baysan et al. and Dixon et al. [21,22]. These findings may be due to the fact that microwave irradiation alters structural integrity and cell membrane permeability [23]. It also has detrimental effects on cell metabolism, thus leading to bacterial cell death. In contrast to microwave disinfection of PMMA polymer bases, a combination of chemical disinfection (denture cleaning tablets) and microwaving effectively and efficaciously disinfects the material faster than microwaving alone, within 2 min of exposure time [24]. It is also important to note that the dentures in the present study were immersed in distilled water during microwave irradiation, a practice which has been a technique for eliminating microorganisms [25].

It was also observed that chemical disinfection using DC displayed lower antimicrobial disinfection percentage than the combined use of MW and DC and MW alone at 1 min or 2 min disinfection time for all three bacteria. In the present study, the denture cleaning tablet used was Fittydent (Fittydent international, GmbH, Wien, Austria) According to the manufacturer information, the tablets containing sodium perborate not only clean removable dentures but also have antibacterial ability against bad odor microbes, dissolve tough stains and help in plaque removal [26]. In case of bacterial biofilm, as the exposure time to the cleaning tablet increases the CFU count decreases, demonstrating the antimicrobial efficacy of the cleaning tablet used. There was 9% to 61% reduction in CFU count as the exposure increased from 1 to 3 min, and 96% to 100% of removal of CFU counts when it was exposed between 4 to 5 min. Thus, the cleaning tablet had more efficacy against bacterial biofilm when specimens were exposed for 4 to 5 min. The present result was in agreement with Silva et al., who evaluated the efficacy of commercial denture cleanser containing sodium perborate on the disinfection of acrylic specimens contaminated with *C. albicans* and *S. mutans* [11]. They reported that denture cleaning tablets have effective antimicrobial efficacy against streptococcus species as compared to *C. albicans* [27].

It is worth mentioning that the current study was conducted in vitro, with inherent limitations when simulating in vivo environment. A more clinically relevant study protocol could be an ex vivo model, including the disinfection of a patient's personal dentures in the oral cavity and experimental MW disinfection in the laboratory for CFU evaluation. It is pertinent to mention that denture-related stomatitis is commonly associated with *Candida albicans* on the denture surface and oral cavity [5]; therefore, further randomized controlled trials assessing the efficacy of microwave disinfection on the *Candida* species are warranted. In light of the findings within the study limitation, MW disinfection in combination with denture cleaning agents are recommended for PMMA denture disinfection in denture wearers.
