## 5 Antimicrobial Effect  
With regard to antimicrobial properties, the use of silver nanoparticles can be considered a useful approach to promoting an antimicrobial effect. [PERSON] et al. [10] report that lower concentrations of silver nanoparticles could affect biofilm formation on PMMA surfaces. Moreover, they mention that the agglomerates of silver nanoparticles were more effective against _C. glabrata_ than nanoparticles distributed individually. In the case of _Candida albicans_, the major etiological agent for stomatitis in dental prostheses users, [PERSON] et al. [1] show less adherence to the surface of PMMA-AgNP-based discs, suggesting an antifungal effect and the possibility that it acts as an anti-adherence agent for microorganisms that colonize surfaces. [PERSON] et al. [11] show that the use of silver-coated polymers is a reliable approach to forming PMMA-based materials to treat bacterial colonization. They mention the release of silver ions as an important mechanism that promotes an antibacterial effect against _Escherichia coli_ and _S. epidermidis_, even though the immobilization of silver nanoparticles on the surface achieved effective microbial growth inhibition. [PERSON] et al. [12] indicate that AgNP agglomerates show a slight decrease of the antimicrobial effect, which is observed more at higher concentrations (2 wt %) compared with lower concentrations (1.5 wt %), and well-dispersed nanoparticles. The authors conducted the antimicrobial tests against _P. aeruginosa_ and _S. aureus_ and they studied the antifungal effect against _Cryptococcus neoformans_. In all cases, concentrations lower than 0.75 wt % had no well-defined inhibition zone when observed in agar plates; however, when using concentrations between 1 and 1.5 wt %, they observed positive results against Gram-negative and Gram-positive bacteria and an antifungal effect. [PERSON] et al. [13] formed light-activated PMMA nanofibers doped with photosensitizer meso-tetraphenylporphyrin (TPP), which has been reported as an effective reactive oxygen species (ROS) producer. TPP inactivates bacterial membranes upon light irradiation. The authors used an Ag/TPP/PMMA solution to prepare films by electrospinning. After this, the films were irradiated with a light-emitting diode light source (110 mW power output, 405 nm center wavelength and a light spot of 10 nm) and a 405 nm emitting laser system with 50 mW output. They evaluated the antibacterial effect using _E. faecalis_ and _S. epidermidis_. They reported that the light-activated PMMA nanofibers had a significantly increased inhibition zone compared with non-irradiated nanofibers. Light irradiation leads to the migration and release of AgNPs to the surface from nanofibers or to the formation of AgNP aggregates. The authors suggest that their obtained material is a promising option for the photodynamic inactivation of bacteria. This strategy is known as photodynamic therapy and has been used as a disinfection method in oral cavities [14]. The antimicrobial effect of silver nanoparticles can be induced by the formation of ROS or by releasing silver ions from the material tested, depending on the synthesis method, silver-nanoparticle dispersion and silver concentration in the PMMA matrix. The studies reviewed here show the antibacterial effect of silver nanoparticles against Gram-positive and Gram-negative microorganisms and antifungal effects against different _Candida_ species. According to the results of this review, the crucial factor for the antimicrobial function in PMMA is the additive. Some authors report the use of antibiotics to form bone cements that are used in orthopedic surgery to perform hip and joint implants. By itself, PMMA does not have antimicrobial properties, but by adding additives like silver nanoparticles, researchers are looking to design an appropriate material for dental prostheses, bone cement and other applications [2; 7; 8; 9].