Dental caries is the most prevalent non-communicable disease and has become a serious public health problem throughout the world [1], [2]. The acid-producing bacteria regularly grow and infect the tooth surface, eroding the tooth structure and causing demineralization and tooth decay [3], [4]. Dental caries affects an estimated 2.3 billion people worldwide, including more than 530 million children, making it one of the most prevalent chronic diseases [5]. Dental treatment is costly and accounts for an average of 5% of total medical costs and 20% of private healthcare expenses in most high-income countries, which is beyond the financial capacity of healthcare systems in most developing countries [6]. S. mutans is a facultative anaerobic, Gram-positive species which is commonly found in the human oral cavity and is a major contributor to dental caries. In the oral cavity, it uses carbohydrates to produce acid through fermentation and to synthesize intra- and extracellular polysaccharides, which act as a binding agent on the tooth surface and promote the formation of biofilm. The many layers of biofilm on the tooth surface form a complex structure called plaque, which has considered the main cause of dental caries [7]. In order to prevent dental caries, a strategy had to be developed to inhibit the growth of S. mutans and the formation of a biofilm on the tooth surface [7], [8].

Antibiotics and fluorides have been used for several decades to prevent oral infections and plaque formation. However, this type of treatment can have serious consequences. The prolonged use of antibiotics has led to promote the antibiotic-resistant strains and the excessive use of fluoride in toothpaste could damage the tooth enamel and cause dental fluorosis [9], [10]. Fluoride could also lead to a reduction in the probiotic community of microflora in the oral cavity [11], [12]. Due to their negligible side effects for humans, the growing technology of metallic nanoparticles is now widely used. In recent years, this technology has generated new ideas and strategies for disease control in humans and animals [13], [14], [15]. Green synthesis of nanoparticles is a bottom-up approach where the nanoparticles are synthesized through the oxidation/reduction process of metallic ions by the organic moieties from the biological resources. The green synthesized nanoparticles-based approaches have a variety of applications in the treatment of various diseases. Therefore, these nanoparticles have been considered by many researchers around the world. Metallic nanoparticles are widely used in the biomedical industry. They have antimicrobial potential and are therefore used in the pharmaceutical industry for antibacterial coatings of implantable devices or wound dressings [16], [17], [18]. Metallic nanoparticles have various applications in the fields of diagnostics, therapy, health and nutrition. Recently, numerous reports have been published on the biomedical applications of metallic nanoparticles [19], [20], [21], [22], [23], [24].

Due to the extraordinary involvement of selenium in the regulation of the immune system, SeNPs have an advantage over other nanoparticles. SeNPs also have a very high nutritional value. They are taken as dietary supplements and play an important role in the composition of at least 25 human selenoproteins and selenocysteine-containing enzymes. As a result, the physiological functions of our body are carried out properly [16], [25], [26], [27], [28]. The microorganisms can convert inorganic Se to organic Se, producing effective and useful products that are important for the efficient use of Se resources. There are three major metabolic pathways through which bacteria interact with Se: biosynthesis, energy transduction, and detoxification. Proteins or extracellular polymeric substances produced by microorganisms interact with elemental Se and act as a corona of elemental Se nanomaterials. The presence of this corona increases the production of biogenic (organic) form of Se [29]. Biogenic selenium nanoparticles have the lowest toxicity, significant biological activities and high utilization value compared to organic and inorganic selenium. Therefore, biogenic nanoparticles are considered relatively safe for both human and animal use [30], [31]. Biogenic SeNPs have significant antioxidant, anticancer, antiviral, antibacterial and antifungal activities [32], [33]. Non-biogenic/physical methods are costly and require high temperatures, low pH, and undesirable chemicals that could render the nanoparticles toxic, making them risky for human consumption. Therefore, many researchers in this decade are focusing on biogenic methods to produce SeNPs using probiotic bacteria as they are cost-effective, environmentally friendly and safe for human use [34], [35].

The objectives of this study were: 1). to develop a biogenic method for the production of SeNPs by L. plantarum KNF-5, 2). to determine the characterization of SeNPs, 3). to determine the inhibitory effect of SeNPs on S. mutans ATCC 25175, 4). to determine the anti-biofilm potential of SeNPS against S. mutans ATCC 25175 and 5). to investigate the expression of biofilm-producing genes after treatment with SeNPs. The outcomes of this study will provide a new option for the prevention and treatment of oral diseases.