Antimicrobial effect and microbial adherence are among the important properties that require careful consideration during material selection for maxillofacial prosthesis. The surface roughness of dental materials provides an environment for microbial growth. As the skin is directly in contact with the maxillofacial prosthesis, they are susceptible to microbial infection. Polymethyl methacrylate and silicone elastomers are the most often utilized materials for the construction of maxillofacial prostheses due to their low cost, ease of manipulation, good physical and mechanical qualities, and durability. The maxillofacial prosthesis is worn intraorally or extraorally or both which makes it susceptible to microbial colonisation at any given time leading to infection[9, 18]. Attempts have been made to reduce microbial adherence and further colonization on the surface of the maxillofacial prosthesis through the incorporation of antimicrobial agents, mechanical cleansing agents namely brushing, and chemical cleansing agents and by modifying the material itself[19–21].
Agar diffusion and dilution procedures are frequently employed to assess the in vitro antibacterial activity. Additional techniques exist as well, including the cross-streak method, agar plug diffusion, and antimicrobial gradient[8, 22, 23]. The agar-disc diffusion method is chosen above the other methods because it is the least expensive, can test a huge variety of bacteria and microbial agents, and is simple to interpret the data. As the agar-disc diffusion method is frequently cited as a standardized method for fastidious organisms and used for antibacterial testing against both gram-positive and Gram-negative bacteria, it was used in this study to compare the antimicrobial effect of two PMMA (m-PMMA and c-PMMA) and two silicone elastomers (silicone A-2000 and silicone A-2186) against S. aureus, S. mutans, and C. albicans.
After 24 hours of incubation, the maxillofacial prosthetic materials investigated in this study—PMMA and silicone elastomers—showed no antibacterial properties against any of the tested pathogens. The evaluated samples included c-PMMA, silicone A-2000, and silicone A-2186 that were purchased commercially, whereas m-PMMA was made on-site. The producers of the commercially purchased items provided no information regarding the existence of an antibacterial agent. In contrast, m-PMMA had hydroxyapatite crystal filler (Ca (PO4)6(OH)2), which contains Ca(OH)2 and may produce antibacterial characteristics[24]. The content of Ca(OH)2 in the m-PMMA test samples may be connected to its antimicrobial properties and the expansion of the inhibition zone. Unreacted Ca(OH)2 particles partially dissolve into Ca2+ and OH- ions, which has an antibacterial effect. The cytoplasmatic membrane of the microbe, which is essential to its survival, is damaged once the OH- ion dissipates within the desired medium. Another element of m-PMMA that might exhibit antibacterial properties is BPO[25]. This OH- ion will also build changes in transporting the nutrition and solutes into the microorganisms, resulting in the imbalance of microorganism nutrition[26]. Antimicrobial testing results, however, revealed no inhibitory zone in the m-PMMA samples. A previous study reported that the modifications of HA coating on titanium surface led to reduced bacterial adhesion as a result of interfaces modification[27]. This observation might be the result of the HA concentration in m-PMMA being insufficient or too low to show antibacterial activities. PMMA and silicone elastomer surfaces that are rough encourage bacterial colonization[5, 28]. This study tested the surface roughness of silicone and PMMA elastomers in an effort to link it to microbial adhesion. In this work, the surface roughness value of the examined materials was determined using a stylus profilometer (Surfcom Flex, Seimitsu Co., Ltd., Tokyo, Japan)[29]. In comparison to silicone elastomers, both PMMA showed a much-reduced mean value for surface roughness. Due to their distinct compositions, silicone elastomers and PMMA should have different surface finishes, which could have a significant impact on microbial adherence. PMMA and silicone elastomers were manufactured using the same stone mould as for standardization. BPO is another component in m-PMMA that has the potential of displaying antimicrobial property[30]. The antibacterial property of BPO is attributed to the formation of highly reactive oxygen species that oxidises proteins in bacterial cell membranes. However, the result of the antimicrobial testing showed no inhibition zone in m-PMMA samples. This finding could be due to the concentration of HA in m-PMMA which might not be adequate or too low to demonstrate antimicrobial properties. This is in agreement with a study which reported that ad-equate concentration of antimicrobial agent is crucial in the treatment and management of orthopaedic infections[31].
The 280 grid sandpapers were used in this investigation to polish both groups of PMMA samples. Given that the fitting surface of the prosthesis was not polished, the polished exterior of the tested PMMA might not accurately represent the fitting surface in a clinical setting. A few earlier research compared the surface roughness of PMMA and silicone elastomers. Their studies focused on the relationship between the microorganisms' adherence and the materials' surface roughness[15, 32–35]. In contrast to smooth surfaces, which prevented bacterial adherence and biofilm deposition, McAllister et al. discovered that the unevenness of polymeric surfaces encouraged adherence of bacteria and biofilm production. In this investigation, there were no statistically significant differences in the mean value of surface roughness between silicone A-2000 and A-2186 or between m-PMMA and c-PMMA. As a result, the surface attributes of m-PMMA, particularly the surface roughness, were unaffected by the addition of fillers.
PMMA, silicone elastomers, and maxillofacial prosthetic materials all have surfaces that are home to microorganisms and provide a favorable environment for their growth. Microbial adherence and colonization on the maxillofacial prosthesis are influenced by a variety of circumstances. Previous research work demonstrated that the extent of microbial adhesion was influenced by surface materials and different types of bacteria[34, 36]. The microbial adhesion was shown to vary in this investigation depending on the topography of the biomaterial. However, the finding of this study might closely represent the polished surface of the prosthesis in situ in which a polished surface could probably less likely to attract microbial adherence. The result obtained was in agreement with a study that demonstrated a higher extent of microbial adherence on silicone elastomers than the commercially obtained PMMA[37].
Through physicochemical interactions, bacteria cling to the surface of the maxillofacial prosthetic material during the adhesion process. The physiochemical interactions include hydrophobicity, surface roughness, surface free energy, wettability, chemical composition and the topography of the material surface itself. In general, rough surfaces attract more microbial adherence than smooth surfaces[5]. The higher the surface charge of the material, the more microorganisms will adhere to it[38, 39]. In relation to wettability, the higher the wettability, the less the adherence of microorganisms[39]. The mechanisms of microbial adherence occur in a two-stage process. The initial adherence of microorganisms was described in terms of the surface-free energies of the surfaces and the microorganisms. Additionally, the hydrophobicity of microorganisms has also been theorized as a reason for an increase in microbial adherence. The second phase of the microbial adherence process involves specific adherence-receptor interactions that bind stereo-chemically to receptors on the surfaces which are necessary for the tight binding of the microorganisms to the surface for colonization[40].
Comparing silicone, A-2000 and silicone A-2186 to m-PMMA and c-PMMA, S. mutans adhesion was likewise noticeably lower. Similar results were discovered in a study by Al-Aaskari et al. (2014), which showed that silicone surfaces exhibited greater microbial adherence than PMMA surfaces[17]. This could be attributed to the polishing effects on the PMMA that produced smoother surface than that of unpolished silicone elastomers. Another study also reported that PMMA tested samples which were subjected to polishing demonstrated less surface roughness and fewer microorganisms[15].
The study's findings showed that there was no discernible difference in C. albicans adhesion to any of the four studied materials. The cause could be attributed to the fact that C. albicans's adhesion to the surfaces of the examined materials was unaffected by their surface roughness.
Surface chemistry, wettability, surface energy and hydrophobicity are among the factors that can influence bacterial adherence. Microbial adhesion is connected to the polymer's composition and may be brought on by changes in surface chemistry. Bacterial adhesion may be impacted by the polymer's release into the nearby environment[34]. In this study, HA, PLA, and BPO fillers were added to m-PMMA to improve its mechanical qualities. hence causing changes in the surface chemistry of the tested material. HA and PLA were added to increase the flexural strength and to improve the impact strength of PMMA, respectively, whilst BPO initiates the polymerization reaction of PMMA[41]. This finding supports our study regarding the addition of fillers. Not only the addition of fillers to PMMA would change the mechanical properties, but it might have also displayed some degree of antibacterial effect due to the presence of HA, hence reducing the number of bacteria adhered to it as it has some antimicrobial activity[42]. BPO is also a medication that is known to possess antimicrobial effects[43], which possibly further helped to reduce bacterial adherence on the surfaces of m-PMMA. The present study also demonstrated S. aureus and S. mutans have more affinity towards silicone surfaces than that acrylic resin surfaces.
Different bacterial species and strains attach to different surface materials in different ways. Surface charge is a crucial physical factor that may have an impact on bacterial adhesion[35]. The addition of HA, BPO, and PLA fillers to modified PMMA may have an effect on the surface chemistry, changing the surface energy of the material and, as a result, reducing bacterial adhesion. The silicone elastomers silicone A-2000 and silicone A-2186, on the other hand, may have had similar surface energies towards S. aureus and S. mutans.
The wettability of the surface material is correlated with microbial adherence. PMMA is an example of a material with high wettability which is hydrophilic and silicone is an example of a material with low wettability which is hydrophobic in nature. A previous study also reported that, the wettability of silicone elastomer was significantly less than that of acrylic resin[44]. Therefore, the wettability character coupled with surface irregularities and porosities of silicone elastomers further promote bacterial adherence.
The m-PMMA as modified acrylic causes fewer adherences of S. aureus and S. mutans towards its surface. According to Park et al. (2003), increasing the amount of methacrylic acid in PMMA modification made the material more hydrophilic, which lowered the adherence of microbes[35]. This is in good agreement with the results of this investigation.
S. aureus is an enclosed coagulase-negative staphylococci (CNS) that exhibits surface hydrophobicity, whereas silicone elastomers are hydrophobic by nature[45]. The amount of bacterial adherence to acrylic resin, which is less hydrophobic than silicone elastomer, was much less. This investigation also found no discernible difference between silicone A-2000 and silicone A-2186 in terms of S. aureus and S. mutans adhesion. Although silicone elastomers come in a variety of forms, their surfaces may have a comparable hydrophobic propensity for bacteria. S. aureus and S. mutans are both gramme positive, encapsulated bacteria, however they react to surfaces differently depending on their affinities. These bacteria' hydrophobic and hydrophilic affinities are influenced by various hydrophobic and hydrophilic substrata in the materials[46].
The instruments available for surface roughness measurement evaluation are the stylus profilometer, laser profilometer, atomic force microscopy (AFM), confocal microscopy and optical interferometry. Among those mentioned, the stylus profilometer and AFM are mostly used to evaluate surface roughness. The stylus profilometers are relatively inexpensive, portable and practical for manufacturing environments and no sample preparation is needed[47]. The AFM is an instrument that traces the surface topography of the sample with a sharp probe while monitoring the interaction forces working between the probe and sample surface. The advantage of the AFM is that it can visualise non-conductive materials in a non-vacuous (i.e., air or liquid) environment[48].
Figures 3 show SEM pictures of bacterial adhesion to silicone elastomers (silicone A-2000 and silicone A-2186) and PMMA (m-PMMA and c-PMMA). S. aureus demonstrated grape-like clusters of variable distributions, whilst S. mutans distributed as short chains of diplococci in shape. It was shown that S. aureus and S. mutans were less tightly packed and more widely dispersed on the surfaces of PMMA than they were on the silicone elastomers. In addition, both silicone elastomers appear to have a rougher and irregular surface than that of PMMA. C. albicans are seen as budding yeast to hyphal formation which can be described as the feathery or spidery outgrowths from the main colony[49, 50]. Larger microbial cells, like yeasts, are easier to remove from smooth surfaces than bacteria from a therapeutic standpoint[51]. The samples go through a few washing steps during SEM preparation, which may remove more C. albicans from the smooth surfaces than other microorganisms.
Firstly, this was an in vitro study which was based mainly on different types of material in relation to specific microorganisms. Apart from that, in the modification of the PMMA, the samples used were restricted by the amount of HA (2%) and BPO (0.5%) which were not enough to exert antimicrobial effect. The tested maxillofacial prosthetic materials (m-PMMA, c-PMMA, silicone A-2000 and silicone A-2186) did not exhibit any antimicrobial activity against S. aureus, S. mutans, and C. albicans. All of the investigated materials (m-PMMA, c-PMMA, silicone A-2000, and silicone A-2186) had surface imperfections to some extent, according to the SEM examination, which encouraged S. aureus, S. mutans, and C. albicans to cling to their surfaces.
There are also possibilities to add different antimicrobial agents to m-PMMA without altering the percentage of HA and BPO fillers. Future studies on the current topic can be suggested to be carried out on other types of microorganisms.