Metals in their pure or alloy forms are commonly used in dentistry despite the introduction of advanced materials like resins and ceramics, which can be largely attributed to the mechanical properties of metallic alloys (Upadhyay et al., 2006). The intra-oral environment has several factors that could predispose such metal alloy-based dental appliances to corrosion. These factors include varying temperature, oxygenation, mechanical forces, acidity, and alkalinity of external agents (foods, drugs), microorganisms, local anaerobic environments (e.g., subgingival). Some of the metals used in dentistry are amalgams of silver-tin, copper, noble metal alloys of gold and silver palladium, base metal alloys of nickel, cobalt, iron, and titanium alloys. Though most of the alloys are passivized and resistant to corrosion, the susceptibility still exists because of the predisposing factors in the oral environment (Bayramoglu et al., 2000; Karov and Hinberg, 2001). The clinical relevance of corrosion of dental appliances in the oral environment is due to some major clinical implications. The first is the potential toxic risk posed by the corrosion by-products. The second is that the corroded dental appliance could lose its functional integrity. The risk of allergy to the unbounded metal elements when released by corrosion should also be considered. A study by zora et al. suggested that corrosion products may pose a risk in immunologically susceptible patients (Venclíková et al., 2007).

The role of microorganisms in corrosion is extensively discussed in the sewage and pipeline industry, although the literature is relatively scanty when it comes to the biological environment, including the oral cavity (Mystkowska et al., 2018). Thus, the present systematic review was formulated to assess the association between microorganisms and corrosion of intra-oral dental appliances.

The present systematic review adhered strictly to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (Moher et al., 2009; Hutton et al., 2015) (Figure 1).

Microbial Corrosion of metal is induced by activities of microorganisms like bacteria, fungi, and algae (Wilson et al., 1997; Daubert et al., 2018). The bacteria more commonly attributed to corrosion are SRB, SOB, iron-oxidizing/ reducing bacteria, manganese-oxidizing bacteria, Pseudomonas, bacteria secreting organic acids, and slime. Among the fungi, Cladosporium, Aspergillus, Penicillium, and Paecilomyces (Iverson, 1987), and Candida albicans (Souza et al., 2010) are associated with metallic alloy corrosion. Bluegreen algae and a species of red algae (Graciollasia sp.) are the algae associated with corrosion (Iverson, 1987). In the present article, the published literature was reviewed to assess the association between oral microbes and corrosion in intra-oral dental materials.

It was observed that various species like Streptococcus, Actinomyces, Veilonella, SRB, SOB, were reported to cause corrosion intraorally. There can be two categories of microbial corrosion based on the involvement of oxygen, anaerobic, and aerobic corrosion. SRB is a classic example of anaerobic corrosion while SOB is a prime example of aerobic corrosion. The basic process of corrosion involves a flow of electricity between certain areas of a metal surface through a solution that can conduct an electric current. Organisms like SOB secrete organic acids as part of their fermentation process which in turn stimulates anodic reactions. Sulphuric acid produced by SOB reduces the pH which in turn favors the growth of iron and manganese-oxidizing bacteria. These microbes oxidize manganese and iron metal alloys and cause their corrosion (Maruthamuthu et al., 2005). Pavlic et al. (2019), Sridhar et al. (2016), Pozhitkov et al. (2015), and Vaidyanathan et al. (1991) reported that a difference in pH could have contributed to the microbial corrosion. Literature suggests that the lowering pH, although may not corrode as the pH does not reach the depassivation point, it is plausible that it may favor the process (Nash and Kelly, 1993; Schiff et al., 2002). Mabilleau et al. (2006) suggested that S.mitis releases lactic acid in the microenvironment and it is likely that this compound is the main candidate to explain Ti corrosion. Some organisms stimulate cathodic reactions by consuming hydrogen. Sulfate-reducing bacteria (SRB) consume hydrogen through hydrogenase enzymes thereby depolarizing the cathode enhancing the process of corrosion (Mystkowska, 2016). SRB also utilize lactate produced by other bacteria in the biofilm as a carbon source and reduce sulfate to sulfide. Sulfide combines with iron in SUS alloys to form ferrous sulfide as the corrosion product. Cwalina et al. (2017) found that both groups of bacteria of sulfur cycle, SRB, and SOB colonize NiTi and Ti alloys, with a lower pH favoring the growth of SRB and causing further corrosion. Both SRB and SOB are capable of corroding NiTi and Ti alloys even though Kameda et al. (2014) found a higher degree of corrosion in SUS and none in Ti. This in turn could be because Ti is more resistant to corrosion by electric current.

Corrosion cells also occur when two areas are in contact with different concentrations of the same solution, like a difference in concentration of oxygen. The less-aerated zone acts as an anode, which undergoes corrosion. One of the factors causing such oxygenation difference is the heterogeneous layer of a biofilm with bacteria like Streptococcus mutans which use oxygen and create a difference in degrees of oxygen concentration based on their presence or absence in the biofilm (Alasvand Zarasvand and Rai, 2014). This is the main mechanism behind the corrosion of S.mutans. Fukushima et al. (2014) also suggested a similar mechanism in their study. Actinomyces viscosus consumes oxygen and shifts the anodic curve toward more negative potentials causing corrosion of metals (Laurent et al., 2001). In addition to other reasons, Díaz et al. (2018) has suggested that surface roughness promotes the corrosion of the Ti surface by S.mutans by creating retentive areas for the bacteria.

There were few contrary findings regarding microbial corrosion. Lu et al. (2017) stated that S. mutans formed a biofilm on the metal surface which enhances corrosion resistance by creating physical barriers that prevented oxygen interactions with the metal surfaces. Lucchetti et al. (2015) too found no significant effect of bacteria like Eikenella corrodens on corrosion of metal alloys. A study by Maruthamuthu et al. (2005) has shown that passivity and corrosion resistance of some SUS and NiTi was improved by bacteria whereas some SUS was shown to decrease. In a study by Liu et al. (2018) found that rapid electrochemical anodization treatment used on Ti2448 alloys increased their biocorrosion resistance. Regarding the prevention against microbial corrosion, Liu et al. (2018) suggested that a new beta-type Ti alloy with a hybrid oxide layer produced by the electrochemical anodization treatment provided better protection against corrosion by microorganisms by lowering the anodic and cathodic current densities. Jorand et al. (2015) showed that the SRB is resistant to ampicillin therapy which might sound that fighting corrosion against these organisms might be difficult. Microbial corrosion needs higher attention in dentistry as more evidence is gathered regarding their role in inducing intra-oral corrosion of alloys. The present systematic review provides insight into the various microorganisms implicated in causing corrosion of intraoral metallic alloy-based dental appliances. Also, the various mechanisms for a microbe induced metallic alloy corrosion are elaborated. Out of the 25 articles reviewed, 23 articles suggested that microorganisms are capable of causing corrosion while 2 articles (Maruthamuthu et al., 2005; Lu et al., 2017) suggested that they protect against corrosion and one suggested no significant effect (Lucchetti et al., 2015).

Although all the 25 studies had assessed the role of microorganisms in corrosion of dental appliance, most were in-vitro studies simulating the intra-oral conditions. Also, there were several variables including methodology used to assess the corrosion, the research design (in-vitro/in-vivo microenvironment) employed, the microbe and the metal/alloys assessed which led to large-scale heterogeneity in the collected data. In addition, in studies like Pozhitkov et al., the results did not specify the microorganisms responsible for the corrosion (Pozhitkov et al., 2015). Given the significant number of variables in the included studies and the lack of specificity in reporting the the causative microbe, a quantitative analysis was not possible.

The review identified several microorganisms to be closely associated with corrosion of intraoral metallic alloy-based dental appliance. Despite the association, it is vital to acknowledge that most of the included studies were based on in-vitro models. Thus, large-scale multi-center prospective clinical studies with a homogenous research design are required to validate the findings of the present systematic review.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

UG, AF, LM, SP, SV, and AR contributed to the conception of the work, data acquisition, analysis, drafting the work, and final approval of the version to be published. MK, SF, DM, and HB contributed to the interpretation of data, revising it critically for important intellectual content, and final approval of the version to be published. All authors agree to be accountable for all aspects of the work ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.