International Journal of Oral Health Sciences

REVIEW ARTICLE
Year
: 2020  |  Volume : 10  |  Issue : 1  |  Page : 6--12

A Review on silver nanoparticles-The powerful nanoweapon against oral pathogens


Suma Bindu Adapa 
 Department of Public Health Dentistry, Maaruti College of Dental Sciences, Bengaluru, Karnataka, India

Correspondence Address:
Dr. Suma Bindu Adapa
Department of Public Health Dentistry, Maaruti College of Dental Sciences, Bengaluru, Karnataka
India

Abstract

In the human oral cavity, complex microbe-host relationships occurs. Dental disease is prevalent in about 89% of the Indian population, of which 72% are residing in the rural areas whose economic values are low to afford the treatment. Indian greeneries are the chief and economical source of medicinal plants and plant products. In recent years, green synthesis of silver nanoparticles (AgNPs) has gained much interest from researchers. As a wide range of metabolites are present in the plant products/extracts, nanoparticles produced by plants are more stable, and the rate of synthesis is faster in comparison to microorganisms. Silver in the form of various compounds have been used in Ayurveda to treat several bacterial infections since time immemorial. The present review explores the huge plant diversity to be utilized for the synthesis of AgNPs and describes the antimicrobial activities of AgNPs against the oral and nonoral microflora.



How to cite this article:
Adapa SB. A Review on silver nanoparticles-The powerful nanoweapon against oral pathogens.Int J Oral Health Sci 2020;10:6-12


How to cite this URL:
Adapa SB. A Review on silver nanoparticles-The powerful nanoweapon against oral pathogens. Int J Oral Health Sci [serial online] 2020 [cited 2020 Aug 8 ];10:6-12
Available from: http://www.ijohsjournal.org/text.asp?2020/10/1/6/289879


Full Text



 Introduction



Oral health is vital for overall health. Dental diseases are recognized as a disease of modern civilization and major public health problem globally.[1] In the past decades, microbiologists, taxonomists, molecular biologists, biochemists, epidemiologists, and dental scientists – led to the identification of the presumed pathogens of human dental caries and periodontal disease/s. The primary etiologic agents of dental caries are the Streptococcus mutans and Streptococcus sobrinus secondarily implicated are the Lactobacillus species. In chronic periodontitis, the species primarily implicated includes Porphyromonas gingivalis (Pg), the spirochetes (including Treponema and Selenomonas species), and secondarily Campylobacter, Fusobacterium, and Bacteroides species, as reviewed by Liljemark WF, Bloomquist C.[2] Micro–organisms play a significant role in oral diseases and control of their activities prevents oral diseases.

Dental disease is prevalent in about 89% of the Indian population, of which 72% are residing in the rural areas whose economic values are low to afford the treatment.[1] Indian greeneries are the chief and economical source of medicinal plants and plant products. Plants are nature's “chemical factories.” They are cost efficient and require little or no maintenance. A vast repertoire of secondary metabolites is found in all plants which possess redox capacity and can be exploited for the biosynthesis of nanoparticles. As a wide range of metabolites are present in the plant products/extracts, nanoparticles produced by plants are more stable and the rate of synthesis is faster in comparison to microorganisms. Silver in the form of various compounds have been used in Ayurveda to treat several bacterial infections since time immemorial. Antimicrobial resistance represents one of the major global public health threats of the 21st century, especially in fields such as Medicine and Dentistry. As several pathogenic bacteria are developing antibiotic resistance, silver nanoparticles (AgNPs) are the new hope to treat them. In general, particles with a size <100 nm are referred to as nanoparticles. Entirely novel and enhanced characteristics such as size, distribution, and morphology have been revealed by these particles in comparison to the larger particles prepared.[3],[4]

The bactericidal activity of AgNPs against the pathogenic, multi-drug resistance (MDR) as well as multidrug susceptible strains of bacteria was studied by many scientists, and it was proved that the AgNPs are the powerful weapons against them.[5]

Although many studies have tested the antibacterial effect of the AgNPs on various organisms, there is a dearth of literature pertaining to their effect against organisms causing oral diseases. Hence, in this communication, we report a review, compiled describing the bactericidal activities of AgNPs against pathogenic oral microbial flora.

 Silver-Based Antimicrobials



In ancient India medical system (called Ayurveda), silver has been described as therapeutic agent for many diseases. In 1884, during childbirth, it became a common practice to administer drops of aqueous silver nitrate to newborn's eyes to prevent the transmission of Neisseria gonorrhea from infected mother. Silver became commonly used in medical treatments, such as those of wounded soldiers in World War I, to deter microbial growth.[6] The silver ions are highly reactive, and they bind to proteins followed by structural changes in the bacterial cell wall and nuclear membrane, which leads to cell distortion and death. Silver ions have capacity to inhibit the bacterial replication, by binding and denaturing bacterial DNA.[7] AgNPs are prepared by engineering the metallic silver into ultrafine particles by numerous physical chemical and biological methods. The nano-size of material results in specific physicochemical characteristics different than those of the bulk materials or larger particles. This effect is mainly credited to high surface-area-to-volume ratio, which results in increased reactivity; hence, the nano-scale materials are more advantageous than their bulk materials. The metallic nano-particles such as copper, titanium, magnesium, zinc, gold, and alginate have a strong bactericidal potential owing to their large surface-area-to-volume ratio. Among all, AgNPs have proved to be the most effective antimicrobial agent against bacteria, viruses and other eukaryotic microorganisms.[8]

AgNPs have important biological properties as follows: they are effective bactericidal agents against broad spectrum of bacteria, including antibiotic-resistant strains,[9] fast-acting fungicide against common fungi, including Aspergillus, Candida, and Saccharomyces. AgNPs of 5–20 nm diameter can inhibit HIV-1 virus replication.[10] These can not only alter the expression proteinases, which are important in inflammatory and repair processes but also suppress tumor necrosis factor, interleukin (IL)-12 and IL-1b and induce apoptosis of inflammatory cells.[11] Moreover, AgNPs are also responsible for cytokine modulation in wound healing and the inhibition of the biofilm formation.[12] AgNPs are also termed as new-generation of antimicrobials.[13] Feng et al. reported the bactericidal potential of silver ions against Staphylococcus aureus and Escherichia coli.[14] De Souza studied the antimicrobial activity of 19 antibiotics in combination with the silver–water dispersion solution (15-nm diameter AgNP clusters containing silver ions produced by an electro-colloidal silver process). They found that the MDR E. coli, S. aureus, Salmonella typhi, Shigella flexneri and Bacillus subtilis are susceptible to amoxicillin and clindamycin. Interestingly, the combination of silver–water dispersion and amoxicillin or clindamycin showed an additive effect on S. aureus, S. typhi, S. flexneri and B. subtilis.[15]

 Silver-Based Antimicrobials Against Oral Micro-Biota



Espinosa-Cristóbal et al. (2012) conducted a study to determine the inhibitory effect and anti-adherence activity of AgNPs on the adhesion of S. mutans on surfaces of brackets and wires for orthodontic therapies, two sizes of AgNPs were prepared and characterized. The evaluation of S. mutans adhesion was performed with microbiological assays on surfaces of brackets and orthodontic modules in triplicate. Topographic characteristics of orthodontic brackets and wires were made by scanning electron and atomic force microscopies. All AgNP samples inhibited S. mutans adhesion; however, the smaller AgNPs had better inhibition than the larger ones.[16]

In a study conducted by Charanya et al. to study the Comparative evaluation of antimicrobial efficacy of AgNPs and 2% chlorhexidine gluconate when used alone and in combination assessed using agar diffusion method. The results of the study shown that CHX AgNP combined solution exhibited the highest efficacy in comparison to these solutions used alone. The results shown that the highest efficacy against Candida albicans compared to Enterococcus faecalis and Klebsiella pneumonia organisms (frequently found in the root canal space).[17]

In a study conducted by Lu et al.[18] stable AgNps with different sizes (~5, 15, and 55 nm mean values) were synthesized by using a simple reduction method or hydrothermal method. The antibacterial activities were evaluated by colony counting assay and growth inhibition curve method, and corresponding minimum inhibitory concentration (MIC) against five anaerobic oral pathogenic bacteria and aerobic bacteria E. coli were determined. The results showed that AgNps had apparent antibacterial effects against the anaerobic oral pathogenic bacteria and aerobic bacteria. The MIC values of 5-nm Ag against anaerobic oral pathogenic bacteria Aggregatibacter actinomycetemcomitans (Aa), Fusobacterium nucleatum (Fn), Streptococcus mitis, S. mutans and Streptococcus sanguis were 25, 25, 25, 50, and 50 μg/mL, respectively. The aerobic oral bacteria were more susceptible to AgNPs than the anaerobic bacteria. The results showed that a potential role of AgNps in the inhibition of oral microbial infections.

Espinosa-Cristobal et al.[19] in the year 2009, determined the antibacterial effect of AgNPs on S. mutans. Three sizes of AgNps were used to find MICs against S. mutans. This study concludes that AgNps has antibacterial activity against S. mutans and this property is better when the particle size is diminished.

Sirisha et al.[20] evaluated the antimicrobial effect of AgNPs synthesized with ocimum sanctum leaf extract against four periodontal pathogens, i.e., Fn, Pg, Aa and Prevotella intermedia (Pi), the results suggested that all the test solutions reveal their sensitivity to test microorganism in a dose-dependent manner. On comparison, AgNP found to have an enhanced antimicrobial activity and Aa found to be more susceptible for the same.

In a study conducted by Kale et al.[21] to evaluate the antimicrobial efficacy of AgNPs by calculating the minimum inhibitory and bactericidal concentration against Pg, Aa, Pi. The results shown that Pg, Aa were sensitive to AgNPs. However, Pi was found to be resistant.

dos Santos et al. (2017)[22] conducted a study to determine whether the size and morphology of AgNPs in colloidal solution altered their antimicrobial activity, four-colored colloids were characterized by ultraviolet (UV)-visible spectroscopy and transmission electron microscopy, and their antimicrobial activity (MIC and minimum bactericidal concentration) against S. mutans compared with chlorhexidine and silver diamine fluoride as controls. All the AgNP colloids showed antimicrobial activity. The colloids showed antimicrobial activity higher than silver diamine fluoride and equivalent to chlorhexidine. Thus, the antimicrobial activity of the colloids-containing AgNPs against S. mutans was considered equivalent to that of chlorhexidine, the gold standard antibiotic in dentistry.

 Indian Herbs With Silver Nanoparticles



The use of environmentally benign materials such as bacteria, fungi, plant extracts, and enzymes for the syntheses of AgNPs offers numerous benefits of eco-friendly and compatibility for pharmaceutical and other biomedical applications.

AgNPs synthesized using plant extracts from different sources [Table 1] have been used for analyzing their antimicrobial activities against different microbes is shown in [Table 2].{Table 1}{Table 2}

 Silver Nanoparticles in Dentistry



Dental diseases are the most prevalent chronic diseases worldwide, and a costly burden to health-care services. Dental diseases include dental caries, developmental defects in oral cavity, dental erosion, periodontal disease, premalignant lesions, oral cancer, etc., Dental caries is still the most common and widespread oral disease. Currently, the most widely dental material used to treat caries lesions is composite resin, especially because of its esthetics and load-bearing properties.[38],[39],[40] It has been shown that there is microleakage on composite restoration margins, and these gaps can be colonized by oral bacteria, resulting in secondary caries.[41] In order to prevent or to diminish biofilm accumulation antimicrobial restorative materials have been developed, especially through the incorporation of AgNPs to composite resins and adhesive systems.[42],[43],[44]

Dentures, mostly constituted by poly (methyl methacrylate) (PMMA) acrylic resin, have their inner surface considerably rough, and this roughness, allied to other factors (e.g., poor hygiene, HIV infection, etc.), contributes to the emergence of denture stomatitis.[45],[46]Candida species colonize denture surfaces forming a biofilm, which acts as a keyfactor to denture stomatitis development.[47] Acosta-Torres et al.[48] developed a PMMA-containing 1 μg/mL of AgNPs and they compared this new compound to unmodified PMMA. It has been observed that PMMA AgNPs specimens showed significantly less C. albicans adherence compared to PMMA (P < 0.05). Accordingly, Nam[49] has incorporated AgNPs into a commercial tissue conditioner, in the following concentrations: 0.1%, 0.5%, 1.0%, 2.0%, and 3.0%. Their inhibitory effect was evaluated against S. aureus, S. mutans, and C. albicans after 24 h and 72 h. The authors have reported that the modified tissue conditioner presented antimicrobial properties even at lower concentrations, that is, 0.1% (for S. mutans and S. aureus) and 0.5% (for C. albicans).

Several studies have demonstrated that bacteria are the main etiologic agent of pulpal infection and periradicular lesion formation.[50] Various materials have been used as root canal fillings, among which gutta-percha is one of the most used.[51] Since elimination of bacteria in root canals is the key to treatment success endodontic materials should ideally provide some antimicrobial activity. Iranian researchers (2008)[52] have introduced nanosilver gutta-percha, as an attempt to improve the antibacterial effect of gutta-percha. The new material, which is standard gutta-percha coated with AgNPs, has demonstrated significant effect against E. faecalis, S. aureus, C. albicans, and E. coli.

Aiming to improve antimicrobial potential of mineral trioxide aggregate (MTA), Samiei et al.[53] modified MTA by adding AgNPs, at 1% weight. Its effect against oral bacteria and fungi species was assessed. Results have showed that AgNPs-containing MTA possesses higher antimicrobial effect against E. faecalis, C. albicans, and Pseudomonas aeruginosa, compared to unmodified MTA.

In study performed by Zhao et al.[54] AgNPs were incorporated into titania nanotubes (TiO2-NTs) on Ti implants, in a process involving silver nitrate immersion and UV radiation. The antibacterial effect against S. aureus was assessed, and results have shown the inhibition of planktonic bacteria during the first several days. Moreover, AgNPs-coating Ti implants have presented ability to prevent bacteria adhesion for up to 30 days, which are considered sufficient time to prevent postinfection in early stages.

AgNPs-containing dental materials present good antimicrobial properties [Table 3]. However, much is still to be discovered.{Table 3} [59]

 Conclusion



This review discussed the broad spectrum of bioactivity of AgNPs, clinical application of AgNPs in dentistry and the bactericidal potential of AgNPs against the oral aerobic and anaerobic bacteria making them promising agents to fight infections. The AgNPs showed good antibacterial activity against several oral and nonoral bacteria strains. An increasing awareness toward green dentistry and use of green route for synthesis of metal nano-particles making them a nano-weapons to fight against oral diseases. Benefit of synthesis of AgNPs using plant extracts is that it is an economical, energy efficient, cost effective; provide healthier work places and communities, protecting human health and environment leading to lesser waste and safer products. This multiactional nanoweapon can be used for the prevention and treatment of oral diseases.

Strengths and limitations

In the dental sciences, the importance of review articles is rising, especially when clinicians wants to update their knowledge and generate guidelines about a topic, reviews acts as a starting point. Good review method is critical because they provide an unbiased point of view for the reader regarding the current literature. The limitations of a literature review are the complete reliance on previously published research.

Future perspectives

Many reports have been published about the syntheses of AgNPs using plant extracts and their antimicrobial properties. Future investigations should be focused on biosynthesis of AgNPs from different greeneries in India to overcome the challenges of economic burden and raising trends of oral diseases. Researcher should design effective drug delivery agent and treating oral diseases besides ensuring higher safety and efficacy.

Acknowledgment

The author would like to express special thanks to husband Mr. Vikram Kishore for reading and giving valuable advice on editing the article.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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