Oral diseases are considered the most common noncommunicable diseases and are related to serious local and systemic disorders. Oral pathogens can grow and spread in the oral mucosae and frequently in biomaterials (e.g., dentures or prostheses) under polymicrobial biofilms, leading to several disorders such as dental caries and periodontal disease. Biofilms harbor a complex array of interacting microbes, increasingly unapproachable to antimicrobials and with dynamic processes key to disease pathogenicity, which partially explain the gradual loss of response towards conventional therapeutic regimens. New drugs (synthesized and natural) and other therapies that have revealed promising results for the treatment or control of these mixed biofilms are presented and discussed here.
- oral biofilm
- infection control
- Streptococcus mutans
- Candida spp.
- natural compounds
- antimicrobial resistance
1. Introduction
According to the World Health Organization (WHO), oral diseases are the most common noncommunicable diseases, causing discomfort, pain, disfigurement, and death [1,2]. Dental caries, the most prevalent condition, derives from microbial biofilms (plaque) formed on the tooth surface [1,2]. Presently, it is estimated that 2.4 billion people have caries of permanent teeth and 486 million children have caries of primary teeth [3]. Similarly, periodontal disease (which affects tissues that both surround and support the teeth), and dental caries are both related to bacterial/fungal infections and are significant causes of deciduous tooth decay in over 560 million children, involving hundreds of billions of dollars of expenses per year [1,4]. In low-income populations, the majority of dental caries are left untreated, and affected teeth are most often extracted due to pain and discomfort [1]. The pain and inflammation of severe dental caries can impair eating and sleeping, as well as the overall quality of life. Abscesses can occur and may result in pain and chronic systemic infection and diseases [1]. If not treated, these disorders can also lead to chronic diseases and serious systemic infections (e.g., Alzheimer’s disease, cardiovascular disorders, oral cancer) [3,5].
Oral pathogens easily grow and propagate in the oral cavity, leading to the formation of dental plaque on both soft and hard tissue [6]. Dental plaque is formed by salivary molecules, proteins, bacterial/fungal debris, and sialic acid, which is then colonized by primary colonizers, including Streptococcus sanguis and Actinomyces viscosus. Their colonization is impacted by various food and environmental factors, including pH, carbon sources, and osmolarity [7,8,9]. Then, other bacterial species such as Streptococcus mutans adhere to the primary colonizers, and the tooth surface develops into a bacterial biofilm, known as dental plaque. Beyond dental plaque is dental calculus, which is a complex combination of dental plaque, salvia, and gingival crevicular fluid. The growth and invasiveness of oral pathogens are both regulated by an equilibrium between dental plaque bacteria and the innate immune system. The build-up of calcified dental plaque that extends into the subgingival layer can trigger inflammation due to perturbations of the immune system [7,8]. The calcified dental plaque can trap many biomolecules, viruses, and other bacteria and fungi, and alterations in the oral microbiome can lead to a wide range of diseases over a lifetime [8]. It has been suggested that the human oral microbiome is made up of over 2000 taxa of bacteria and fungi, including a wide range of opportunistic pathogens involved in cardiac, periodontal, respiratory, and other diseases, including cardiovascular and respiratory diseases, diabetes, and osteoporosis [8]. This review focuses on reports published in the last five years, related to one of the most important oral co-infections: Streptococcus mutans–Candida spp. biofilms.
Streptococcus mutans and Candida spp.: Relevance in Oral Biofilms
The colonization of teeth by cariogenic bacteria is one of the most important risk factors in the development of dental diseases, with S. mutans being the primary species associated with the early dental caries process [10]. Streptococcus mutans is a Gram-positive, facultative anaerobic bacteria, and the primary etiological agent of dental plaques and dental caries [11,12]. This species is closely related to the streptococci group that inhabits the mouth, pharynx, and intestine and is well adapted to form biofilms due to its ability to form amyloids, which are very prevalent in natural biofilms [12]. Also, it colonizes the dental surface, causing damage to the hard tooth structure in the presence of fermentable carbohydrates, since it is a recognized acid-producing bacterium (e.g., sucrose and fructose). Moreover, S. mutans can adhere to the enamel salivary pellicle and other plaque bacteria, where they produce acidic metabolites, build up glycogen stores, and synthesize extracellular polysaccharides (EPS), glucans, and fructans from dietary sugars, leading to increased dental caries. In fact, there are several acidogenic and aciduric species in dental plaque associated with dental caries development, but S. mutans is the primary producer of EPS, which makes these biofilms difficult to control [11].
Along with the thousands of Streptococcus spp., over 100 fungal phylotypes also colonize the oral cavity, including many Candida spp. [13]. They are often found in the oral cavity of healthy individuals, with Candida albicans being the most predominant species (~60%–70%), followed by Candida tropicalis and Candida glabrata [13,14]. Usually, Candida spp. are commensal; however, in specific situations, these fungi may become parasitic, causing oral candidiasis particularly in immunocompromised patients, such as those with HIV/AIDS [13,15,16]. These fungal species are present in coating surfaces, dentine, as well as in the cementum surface. Interestingly, C. albicans grows in enamel cracks, grooves and flows into the crevices of cavities, as well as deeply penetrating open dentinal tubules [13]. Several Candida spp. have been isolated from caries as well as from root and dentinal caries in both children and adults, with prevalence ranging from 66% to 97% in pediatric populations and 31% to 56% in adult populations [15]. Thus, C. albicans can contribute to dentine and root caries in both children and adults.
Remarkably, Candida spp. cannot effectively form plaque biofilms directly or bind to S. mutans unless sugar is present. In a 2017 study, it was found that an enzyme known as glucosyltransferase B (GftB, which promotes firm cell clustering, and increased cohesion of plaque), secreted by S. mutans, uses sugar from the diet to manufacture glue-like polymers called glucans. Candida spp. increases glucan formation, causing the formation of a sticky biofilm that allows the yeast to adhere to teeth and bind to S. mutans [17]. These researchers further showed that the outer portion of the Candida spp. cell wall, composed of molecules called mannans, was involved in binding GftB, and that mutant Candida strains that lacked the mannan components had impaired binding to S. mutans GftB and a reduced biofilm load [17]. They further tested this hypothesis on biofilm formation in a rodent model of early childhood caries. Rodents infected with both S. mutans and either the wild type or defective mutant Candida strains showed that animals infected with wild-type Candida spp. had abundant biofilm formation, while those infected with the mutant Candida strains had reduced biofilm formation by up to ~5-fold [17].
Glucosyltransferases (Gtfs)-derived EPS have been revealed to be a key mediator of co-species biofilm development and their co-existence with C. albicans induces the expression of virulence genes in S. mutans. In fact, water-soluble glucan synthesis is regulated by the dexA gene and affects biofilm aggregation and cariogenic pathogenicity in S. mutans [18]. The co-cultivation altered S. mutans signal transduction and the transcription of genes (comC and ciaRH) associated with fitness and virulence [19]. Arzmi and co-workers [20] suggested that polymicrobial biofilms differentially modulate oral microorganisms’ phenotypes. Yang and co-workers [21] found that antigen I/II of S. mutans is important for Candida spp. incorporation into the biofilm and is also required for increased acid production. S. mutans not only modulates biofilm formation, but also attenuates C. albicans virulence [22,23]. Additionally, the possibility of a quorum sensing system stimulation of S. mutans by C. albicans was also demonstrated, consequently changing its virulence properties [24].
2. Compounds with Activity against Oral Infections
Oral microbial communities are some of the most complex microbial floras in the human body, consisting of thousands of bacterial and hundreds of fungal species [25]. Culture-independent molecular methods, such as proteomics and 16S rRNA sequencing, have demonstrated that S. mutans was the dominant species, with elevated levels of other streptococci including Streptococcus sanguinis, Streptococcus mitis, and Streptococcus salivarius. Candida spp. combined with Streptococcus spp. usually increases the virulence in invasive candidiasis, early childhood caries, or peri-implantitis [13,26,27]. The interactions between the various species in these mixed biofilms may be synergistic, in that the presence of one microorganism generates a niche for other pathogenic microorganisms, which serves to facilitate organisms’ retention. Thus, Streptococcus spp. and Candida spp. create special synergistic consortia on solid oral surfaces [13,26].
Streptococcus mutans strains are considered the most cariogenic bacteria; however, Candida spp. can increase their cariogenicity. Bacterial and fungal cells are able to produce glucan as an extracellular polysaccharide that aids cariogenic biofilm formation [28]. Bacteria also convert dietary sucrose and free glucose or fructose into a diverse range of soluble and, particularly, insoluble extracellular polysaccharides (EPS) (e.g., water-insoluble glucans) through exoenzymes, such as Gtfs, and EPS, to build blocks of cariogenic biofilms. These structures promote the tooth surface colonization by S. mutans and additional microorganisms into dental plaque (e.g., Porphyromonas gingivalis, streptococci, Fusobacterium spp., Prevotella spp.), while forming the scaffold core or matrix of the biofilm. In addition, the EPS matrix creates a special diffusion-limiting barrier, facilitating acidic microenvironments creation at the biofilm–tooth interface, which is critical for adjacent tooth enamel dissolution [29]. Furthermore, S. mutans displays many other virulence attributes, including its ability to produce acid and to tolerate an acidic environment.
Among Candida spp., C. albicans is the most prevalent on dental tissues, reaching 60%–70%, followed by C. tropicalis and C. glabrata [30]. As indicated earlier, these yeasts are usually commensal, but in some situations they can become parasitic, causing oral candidiasis [30,31]. The virulence attributes of C. albicans are the acidogenicity and aciduric nature, along with the ability to develop profuse biofilms, to ferment and assimilate dietary sugars, and to produce collagenolytic proteinases [15]. Also, C. albicans modulates the pH in dual-species biofilms to values above the critical pH where enamel dissolves. Although the species present low cariogenicity, biofilms formed by mixed populations of C. albicans and S. mutans are more voluminous [32]. The presence of Candida spp. enhances S. mutans growth, fitness, and accumulation within biofilms. It is documented that C. albicans growth stimulates S. mutans development via biofilm-derived metabolites [33]. Also, C. albicans mannans mediate S. mutans exoenzyme GtfB binding to modulate cross-kingdom biofilm development [17]. Candida spp.-derived β-1,3-glucans contribute to the EPS matrix structure, while fungal mannan and β-glucan provide sites for GtfB binding and activity; thus, the coexistence of S. mutans with C. albicans can cause dental caries progression or disease recurrence in the future [34]. It was also suggested that S. mutans co-cultivation with C. albicans influences carbohydrate utilization by bacterial cells [35,36], and the analysis of metabolites confirmed the increase in carbohydrate metabolism, with amounts of formate elevated by co-cultured biofilms [37]. Nonetheless, biofilm biomass and metabolic activity were both strain- and growth medium-dependent [37]. Liu and co-workers showed that nicotine promotes S. mutans attachment. The enhancement of the synergistic relationship may contribute to caries development in smokers [38,39]. On the other hand, there are contradictory reports in this matter. It was proposed that, by definition, C. albicans is not a cariogenic microorganism; it could prevent caries by actively increasing pH and preventing mineral loss [40]. Nonetheless, a study published in 2016 concluded that children with severe early childhood caries were likely infected by their mothers, as the mothers of these children were highly infected with C. albicans [41]. In fact, genetic testing of Candida strains from children and their mothers showed that most strains were genetically related [41].
Progress in understanding the etiology, epidemiology, and microbiology of periodontal pocket flora has called for new antimicrobial therapeutic schemes for oral diseases [42]. Distinctive means and compounds may be used to prevent oral infections. Biomolecules produced by S. mutans, lactoferrin, and probiotics, applying chemicals and photodynamic therapy, can support the management of oral candidiasis [43].
2.1. Chemical Compounds with Activity against Streptococcus mutans and Candida spp. Biofilms
| Main Effect(s) | Compound | Targeted Species | Reference(s) |
|---|---|---|---|
| Pathogen toxicity | aPDT: chloroaluminium phthalocyanine (cationic nanoemulsion); hypericin-glucamine Rose Bengal in α-cyclodextrin; curcumin; methylene blue; toluidine blue O erythrosine with green light or Photodithazine®; rose Bengal, methylene blue and curcumin with white light | Mainly C. albicans, C. glabrata, S. mutans, and S. sanguis, but also other species | [44,45,46,47,48,49,50,51,52,53,54,55] |
| Antibiofilm | Chlorhexidine with low concentrations but with the addition of cis-2-decenoic acid | S. mutans and C. albicans, and bacterial–fungal dual-species consortia | [56] |
| Chlorhexidine gluconate with tyrosol | Single and mixed-species oral biofilms | [57,58] | |
| Fluoride-releasing copolymer: methyl methacrylate, 2-hydroxyethyl methacrylate with polymethyl methacrylate (incorporating sodium fluoride) | Acidogenic mixed-species biofilms | [59] | |
| Antimicrobial | 2.5% sodium hypochlorite, 2% chlorhexidine, and ozonated water | Mono- and dual-species biofilms of S. mutans and C. albicans | [25] |
| Chlorhexidine carrier nanosystem based on iron oxide magnetic nanoparticles and chitosan | [60] | ||
| Chitosan, silver nanoparticles and ozonated olive oil | Several endodontic pathogens, including S. mutans and C. albicans mixed biofilms | [61] | |
| Chitosan and carboxymethyl chitosan | Biofilms of Streptococcus spp.and Candida spp. | [62,63,64,65] | |
| Materials containing silver nanoparticles alone or with polyphosphates | C. albicans and S. mutans | [66,67] | |
| Dimethylaminododecyl methacrylate modified denture base resin | Several microorganisms associated on the dental base | [68] | |
| Poly-(2-tert-butilaminoethyl) methacrylate | S. mutans | [69] | |
| Cetylpyridinium chloride and cetyltrimethylammonium bromide with plant terpinen-4-ol (Synthetic surfactants+) | S. mutans and C. albicans | [70] | |
| Nanoparticles of amphiphilic silanes with Chlorin e6 | Streptococcus genus | [71] | |
| Antimicrobial and antibiofilm | Hexagonal boron nitride nanoparticles | S. mutans, Staphylococcus pasteuri, S. mutans-Candida spp. | [72] |
| Silver nanoparticles combined to calcium glycerophosphate or nanostructured silver vanadate (dental acrylic resins) | Several microorganisms associated with dental prostheses | [73,74] | |
| Modified pH-responsive cationic poly (ethylene glycol)-block-poly (2-(((2-aminoethyl) carbamoyl) oxy) ethyl methacrylate | Acidogenic mixed-species biofilms | [75] | |
| Extracellular polysaccharides (EPS) inhibitors | Combination fluconazole with povidone iodine | C. albicans or mixed biofilm formation with S. mutans | [76] |
| Thiazolidinediones, such as thiazolidinedione-8 | Mixed biofilm formed by Candida spp. and bacterial strains (S. mutans) | [77] | |
| Lactams, such as γ-Alkylidene-γ-lactams solubilized in 3.5% dimethyl sulfoxide | S. mutans and C. glabrata | [78] |
2.2. Natural Compounds with Bioactivity Against Streptococcus mutans and Candida spp. Biofilms
Due to the incidence of oral disease, increased resistance by bacteria and fungi to antimicrobials, and the adverse effects of some drugs currently used in dentistry (and general medicine), there is a great need for alternative prophylaxis and treatment options that are not only safer but also cost-effective. While several antimicrobial agents are commercially available, these chemicals can alter the oral microbiota and have undesirable side effects (e.g., nausea, diarrhea, vomiting, tooth staining). Henceforth, traditional medicine and the search for alternative products are still important. In fact, natural products offer an assortment of chemical structures and possess an extensive variety of biological properties; thus, they are a source for new pharmaceuticals. Bioactive secondary metabolites have been revealed to be useful as new antimicrobial and antibiofilm drugs, such as numerous furanones, alkaloids, and flavonoids, from many plants and marine organisms [87]. Table 2 presents natural compounds and plant extracts with antimicrobial potential, particularly focused on anti-Streptococcus spp. anti-Candida spp. activities.
| Year | In vitro Assays | Natural Compound/Extract | Effect | Reference(s) |
|---|---|---|---|---|
| 2018 | Antibacterial and antifungal bioactivity | Acacia arabica (extract) | Antibacterial source of anticariogenic agents | [88] |
| 2018 | Antibacterial and antifungal bioactivity | Myracrodruon urundeuva and Qualea grandiflora (hydroalcoholic extracts) | Activity against S. mutans biofilm | [89] |
| 2018 | Antibacterial and antifungal bioactivity | Cissampelos torulosa, Spirostachys africana, Clematis brachiata, Englerophytum magalismonatanum (extracts) | Activity against both Streptococcus spp. and Candida spp. | [90] |
| 2017 | Antibacterial and antifungal bioactivity Cytotoxicity and genotoxicity: murine macrophages (RAW 264.7), human gingival fibroblasts (FMM-1), human breast carcinoma cells (MCF-7), and cervical carcinoma cells (HeLa) |
Thymus vulgaris and Rosmarinus officinalis (extracts) | Antimicrobial and anti-inflammatory effects against oral pathogens | [91,92] |
| 2017 | Antibacterial and antifungal bioactivity Antibacterial, antifungal, and antiadhesion in a tissue conditioner |
Azadirachta indica (leaf extract) | Potential antimicrobial agent against both S. mutans and C. albicans | [93] |
| 2017 | Antibacterial and antifungal, antibiofilm and antioxidant bioactivity | Camellia japonica and Thuja orientalis | Significantly inhibited the microbial grow of oral pathogens | [94] |
| 2016 | Antibacterial and antifungal, antibiofilm bioactivity Cytotoxicity and anti-inflammatory effects: human oral epithelial cells |
Houttuynia cordata (herbal tea) | Antibiofilm effects against S. mutans and C. albicans | [95] |
| 2015 | Antibacterial and antifungal bioactivity | Ricinus communis and sodium hypochlorite (cleanser solutions) | Effective in controlling denture biofilms | [96] |
| 2014 | Anti-adherent properties (antibiofilm) | Schinus terebinthifolius and Croton urucurana (methanol and acetate methanol extract fractions in hydroalcoholic and dimethylsulfoxide) | Antibiofilm activity against S. mutans and C. albicans | [97] |
Plant-derived compounds are a good source of therapeutic agents and inhibitors of dental caries, periodontal diseases, and candidiasis. For example, α-mangostin or lawsone methyl ether showed antimicrobial, antibiofilm, and anti-inflammatory activities. Indeed, an oral spray containing these natural chemicals was effective against common oral pathogens [98]. Lupinifolin from Albizia myriophylla wood exhibits good anti-S. mutans activity by damaging bacterial membranes, resulting in cell leakage [99]. Essential oils and bioactive fractions from Aloysia gratissima, Baccharis dracunculifolia, Coriandrum sativum, Cyperus articulatus, and Lippia sidoides were also evaluated as antimicrobials against S. mutans and Candida spp. A significant reduction in extracellular polysaccharides and bacteria was observed for A. gratissima and L. sidoides, indicating that these fractions disrupted biofilm integrity. Plus, C. sativum oils drastically affected C. albicans viability [100], and might be considered as alternative anticaries agents. In turn, quercetin and kaempferol or farnesol with myricetin showed favorable properties in terms of controlling some virulence factors of S. mutans and C. albicans biofilms [101,102,103,104]. It was suggested that tyrosol decreased the metabolic activity and number of viable cells in single and mixed-species biofilms [105]. Correspondingly, β-caryophyllene—a bicyclic sesquiterpene of numerous essential oils—may inhibit cariogenic biofilms and may be a candidate agent for the prevention of dental caries [28]. These results highlight the promising antimicrobial activity of plants for the treatment of dental caries and oral candidiasis.
Propolis, rich in flavonoids, has a long history of use as a natural treatment for a host of health problems. It is also used as an ingredient in certain medicinal products applied directly to the skin, or in mouthwash and toothpaste. Propolis is under preliminary research for the potential development of new drugs associated with the control of C. albicans and immunomodulatory effects. Propolis and miswak (Salvadora persica tree), used in toothpaste, dental varnishes, and mouthwash, led to a significant reduction in the colony-forming units of oral biofilms [106,107].
Antimicrobial peptides have also been widely tested for controlling bacterial biofilms. Shang and colleagues [108] tested the efficacy of peptides from Rana chensinensis skin secretions in preventing biofilm formation by cariogenic and periodontic pathogens. Peptide L-K6, a temporin-1CEb analog, exhibited high antimicrobial activity against tested oral pathogens and was able to inhibit S. mutans biofilm formation. This peptide significantly reduced cell viability within oral biofilms. Its anti-inflammatory activity was correlated with L-K6 binding to LPS and dissociating LPS aggregates. Likewise, cyclic dipeptides (CDPs) are common metabolites widely biosynthesized by cyclodipeptide synthases or nonribosomal peptide synthetases by both prokaryotic and eukaryotic cells. Examples of CDPs that inhibit biofilm formation by Streptococcus epidermidis include cyclo-(l-Leucyl-l-Tyrosyl) isolated from mold Penicillium sp. and cyclo-(l-Leucyl-l-Prolyl) isolated from Bacillus amyoliquefaciens. The antibiofilm activity of 75 synthetic CDPs was assessed against oral pathogens, allowing for the identification of five novel CDPs that inhibit biofilm formation and adherence properties. Among them, five new active compounds were identified as preventing biofilm formation by S. mutans and C. albicans on the hydroxylapatite surface [109].
Similarly, the application of probiotics and antagonistic microorganisms for oral pathogens can bring about tangible benefits, namely boosting host immunity and disturbing the pathogen’s environment via competition for space and nutrients. Krzyściak et al. [35] evaluated the anticariogenic effects of Lactobacillus salivarius by reducing pathogenic species in biofilm models. This microorganism has demonstrated the ability to secrete intermediates capable of inhibiting the formation of cariogenic S. mutans and C. albicans biofilms. In other studies, single and mixed biofilms were inhibited by probiotic lactobacilli [110,111]. Therefore, they seem to be useful as an adjunctive therapeutic mode against oral Candida spp. infections. Another study reported that the Candida sorbosivorans SSE-24 strain was used to stimulate erythritol production at a level of 60 g/L. It was detected a significant inhibitory effect of erythritol on the growth and biofilm formation of S. mutans [112]. Interestingly, a novel phage (ɸAPCM01) with activity against S. mutans biofilms was recently isolated from human saliva [113], and the inhibition of S. mutans biofilms was also found by the use of liamocins from Aureobasidium pullulans [114]. Chemically, liamocins are mannitol oils specific to Streptococcus spp., having the potential to act as new inhibitors of oral streptococcal biofilms that should not affect normal oral microflora.
This entry is adapted from the peer-reviewed paper 10.3390/jcm9020517