Candida-streptococcal mucosal biofilms display distinct structural and virulence characteristics depending on growth conditions and hyphal morphotypes

Oral host-microbe interactions investigated in 3D organotypic models

The oral cavity is inhabited by abundant microbes which continuously interact with the host and influence the host's health. Such host-microbe interactions (HMI) are dynamic and complex processes involving e.g. oral tissues, microbial communities and saliva. Due to difficulties in mimicking the in vivo complexity, it is still unclear how exactly HMI influence the transition between healthy status and disease conditions in the oral cavity. As an advanced approach, three-dimensional (3D) organotypic oral tissues (epithelium and mucosa/gingiva) are being increasingly used to study underlying mechanisms. These in vitro models were designed with different complexity depending on the research questions to be answered. In this review, we summarised the existing 3D oral HMI models, comparing designs and readouts, discussing applications as well as future perspectives.

Fungi and bacteria occupy distinct spatial niches within carious dentin

The role of bacteria in the etiology of dental caries is long established, while the role of fungi has only recently gained more attention. The microbial invasion of dentin in advanced caries especially merits additional research. We evaluated the fungal and bacterial community composition and spatial distribution within carious dentin. Amplicon 16S rRNA gene sequencing together with quantitative PCR was used to profile bacterial and fungal species in caries-free children (n = 43) and 4 stages of caries progression from children with severe early childhood caries (n = 32). Additionally, healthy (n = 10) and carious (n = 10) primary teeth were decalcified, sectioned, and stained with Grocott's methenamine silver, periodic acid Schiff (PAS) and calcofluor white (CW) for fungi. Immunolocalization was also performed using antibodies against fungal β-D-glucan, gram-positive bacterial lipoteichoic acid, gram-negative endotoxin, Streptococcus mutans, and Candida albicans. We also performed field emission scanning electron microscopy (FESEM) to visualize fungi and bacteria within carious dentinal tubules. Bacterial communities observed included a high abundance of S. mutans and the Veillonella parvula group, as expected. There was a higher ratio of fungi to bacteria in dentin-involved lesions compared to less severe lesions with frequent preponderance of C. albicans, C. dubliniensis, and in one case C. tropicalis. Grocott's silver, PAS, CW and immunohistochemistry (IHC) demonstrated the presence of fungi within carious dentinal tubules. Multiplex IHC revealed that fungi, gram-negative, and gram-positive bacteria primarily occupied separate dentinal tubules, with rare instances of colocalization. Similar findings were observed with multiplex immunofluorescence using anti-S. mutans and anti-C. albicans antibodies. Electron microscopy showed monomorphic bacterial and fungal biofilms within distinct dentin tubules. We demonstrate a previously unrecognized phenomenon in which fungi and bacteria occupy distinct spatial niches within carious dentin and seldom co-colonize. The potential significance of this phenomenon in caries progression warrants further exploration.

Living together: The role of Candida albicans in the formation of polymicrobial biofilms in the oral cavity

The oral cavity of humans is colonized by diversity of microbial community, although dominated by bacteria, it is also constituted by a low number of fungi, often represented by Candida albicans. Although in the vast minority, this usually commensal fungus under certain conditions of the host (e.g., immunosuppression or antibiotic therapy), can transform into an invasive pathogen that adheres to mucous membranes and also to medical or dental devices, causing mucosal infections. This transformation is correlated with changes in cell morphology from yeast-like cells to hyphae and is supported by numerous virulence factors exposed by C. albicans cells at the site of infection, such as multifunctional adhesins, degradative enzymes, or toxin. All of them affect the surrounding host cells or proteins, leading to their destruction. However, at the site of infection, C. albicans can interact with different bacterial species and in its filamentous form may produce biofilms-the elaborated consortia of microorganisms, that present increased ability to host colonization and resistance to antimicrobial agents. In this review, we highlight the modification of the infectious potential of C. albicans in contact with different bacterial species, and also consider the mutual bacterial-fungal relationships, involving cooperation, competition, or antagonism, that lead to an increase in the propagation of oral infection. The mycofilm of C. albicans is an excellent hiding place for bacteria, especially those that prefer low oxygen availability, where microbial cells during mutual co-existence can avoid host recognition or elimination by antimicrobial action. However, these microbial relationships, identified mainly in in vitro studies, are modified depending on the complexity of host conditions and microbial dominance in vivo.

Cross-kingdom interaction between Candida albicans and oral bacteria

Candida albicans is a symbiotic fungus that commonly colonizes on oral mucosal surfaces and mainly affects immuno-compromised individuals. Polymicrobial interactions between C. albicans and oral microbes influence the cellular and biochemical composition of the biofilm, contributing to change clinically relevant outcomes of biofilm-related oral diseases, such as pathogenesis, virulence, and drug-resistance. Notably, the symbiotic relationships between C. albicans and oral bacteria have been well-documented in dental caries, oral mucositis, endodontic and periodontal diseases, implant-related infections, and oral cancer. C. albicans interacts with co-existing oral bacteria through physical attachment, extracellular signals, and metabolic cross-feeding. This review discusses the bacterial-fungal interactions between C. albicans and different oral bacteria, with a particular focus on the underlying mechanism and its relevance to the development and clinical management of oral diseases.

Host's Immunity and Candida Species Associated with Denture Stomatitis: A Narrative Review

Denture-related Candida stomatitis, which has been described clinically in the literature, is either localized or generalized inflammation of the oral mucosa in connection with a removable prosthesis. During this inflammatory process, the mycobacterial biofilm and the host’s immune response play an essential role. Among microorganisms of this mixed biofilm, the Candida species proliferates easily and changes from a commensal to an opportunistic pathogen. In this situation, the relationship between the Candida spp. and the host is influenced by the presence of the denture and conditioned both by the immune response and the oral microbiota. Specifically, this fungus is able to hijack the innate immune system of its host to cause infection. Additionally, older edentulous wearers of dentures may experience an imbalanced and decreased oral microbiome diversity. Under these conditions, the immune deficiency of these aging patients often promotes the spread of commensals and pathogens. The present narrative review aimed to analyze the innate and adaptive immune responses of patients with denture stomatitis and more particularly the involvement of Candida albicans sp. associated with this pathology.

Current State and Challenges of the Global Outcomes of Dental Caries Research in the Meta-Omics Era

Despite significant healthcare advances in the 21st century, the exact etiology of dental caries remains unsolved. The past two decades have witnessed a tremendous growth in our understanding of dental caries amid the advent of revolutionary omics technologies. Accordingly, a consensus has been reached that dental caries is a community-scale metabolic disorder, and its etiology is beyond a single causative organism. This conclusion was based on a variety of microbiome studies following the flow of information along the central dogma of biology from genomic data to the end products of metabolism. These studies were facilitated by the unprecedented growth of the next- generation sequencing tools and omics techniques, such as metagenomics and metatranscriptomics, to estimate the community composition of oral microbiome and its functional potential. Furthermore, the rapidly evolving proteomics and metabolomics platforms, including nuclear magnetic resonance spectroscopy and/or mass spectrometry coupled with chromatography, have enabled precise quantification of the translational outcomes. Although the majority supports 'conserved functional changes' as indicators of dysbiosis, it remains unclear how caries dynamics impact the microbiota functions and vice versa, over the course of disease onset and progression. What compounds the situation is the host-microbiota crosstalk. Genome-wide association studies have been undertaken to elucidate the interaction of host genetic variation with the microbiome. However, these studies are challenged by the complex interaction of host genetics and environmental factors. All these complementary approaches need to be orchestrated to capture the key players in this multifactorial disease. Herein, we critically review the milestones in caries research focusing on the state-of-art singular and integrative omics studies, supplemented with a bibliographic network analysis to address the oral microbiome, the host factors, and their interactions. Additionally, we highlight gaps in the dental literature and shed light on critical future research questions and study designs that could unravel the complexities of dental caries, the most globally widespread disease.

Microbial approaches for the assessment of toothpaste efficacy against oral species: A method comparison

Antibacterial properties of toothpastes enable chemical plaque control in limited‐access tooth regions that are mechanically not sufficiently reached by toothbrushes. Therefore, this study aimed to compare different microbial methods to assess antimicrobial toothpaste properties and evaluate different toothpastes in terms of their antibacterial efficacy against different oral microorganisms in an in vitro setting. Six toothpaste suspensions with varying antibacterial supplements were applied to a multispecies biofilm model (Actinomyces oris, Candida albicans, Fusobacterium nucleatum, Streptococcus oralis, and Streptococcus mutans) as well as to each microorganism. A culture method was used to assess the anti‐biofilm effects and two different agar diffusion assays were performed for testing the antimicrobial effect on each microorganism. The measurements of the culture and diffusion analyses were statistically normalized and compared and toothpastes were ranked according to their antimicrobial efficacy. The results of both agar diffusion assays showed a high correlation across all tested species (Spearman correlation coefficients ρ_s > 0.95). The results of the multispecies biofilm model, however, substantially differed in its assessment of antibacterial properties (ρ_s ranging from 0.22 to 0.87) compared to the results of both diffusion assays. Toothpastes with amine fluoride (with and without stannous fluoride), and toothpastes with triclosan resulted in the highest antimicrobial efficacy. Activated carbon supplements in toothpastes were comparable in their antimicrobial action to the negative control NaCl. The appropriate selection of a broad range of oral microorganisms seems crucial when testing the chemical impact of toothpaste and toothpaste supplements.

Cross-kingdom microbial interactions in dental implant-related infections: is Candida albicans a new villain?

Candida albicans, an oral fungal opportunistic pathogen, has shown the ability to colonize implant surfaces and has been frequently isolated from biofilms associated with dental implant-related infections, possibly due to its synergistic interactions with certain oral bacteria. Moreover, evidence suggests that this cross-kingdom interaction on implant can encourage bacterial growth, leading to increased fungal virulence and mucosal damage. However, the role of Candida in implant-related infections has been overlooked and not widely explored or even considered by most microbiological analyses and therapeutic approaches. Thus, we summarized the scientific evidence regarding the ability of C. albicans to colonize implant surfaces, interact in implant-related polymicrobial biofilms, and its possible role in peri-implant infections as far as biologic plausibility. Next, a systematic review of preclinical and clinical studies was conducted to identify the relevance and the gap in the existing literature regarding the role of C. albicans in the pathogenesis of peri-implant infections.

Effect of different salivary glucose concentrations on dual-species biofilms of Candida albicans and Streptococcus mutans

Diabetes mellitus is one of the most prevalent diseases worldwide. The glucose levels found in the blood of diabetic patients can be reflected in the saliva, which can favor biofilm growth and predispose denture wearers to oral candidiasis. This study aimed to evaluate the effect of different salivary glucose concentrations on dual-species biofilms of Candida albicans and Streptococcus mutans. A 96-h biofilm was developed on acrylic resin specimens exposed to 'feast' (10% sucrose) and 'famine' periods. Biofilms were constantly exposed to salivary glucose concentrations equivalent to 0, 20, 60 and 100 mM. Higher salivary glucose concentrations resulted in increased counts of C. albicans and a higher quantity of insoluble extracellular polysaccharides. All biofilms presented high phospholipase activity. The biofilms were characterized by the predominance of yeast cells and microcolonies in all the groups analyzed with co-localization of both species. Higher salivary glucose concentrations formed more robust and potentially virulent biofilms.

Interactions between Candida albicans and Enterococcus faecalis in an Organotypic Oral Epithelial Model

Candida albicans as an opportunistic pathogen exploits the host immune system and causes a variety of life-threatening infections. The polymorphic nature of this fungus gives it tremendous advantage to breach mucosal barriers and cause oral and disseminated infections. Similar to C. albicans, Enterococcus faecalis is a major opportunistic pathogen, which is of critical concern in immunocompromised patients. There is increasing evidence that E. faecalis co-exists with C. albicans in the human body in disease samples. While the interactive profiles between these two organisms have been studied on abiotic substrates and mouse models, studies on their interactions on human oral mucosal surfaces are non-existent. Here, for the first time, we comprehensively characterized the interactive profiles between laboratory and clinical isolates of C. albicans (SC5314 and BF1) and E. faecalis (OG1RF and P52S) on an organotypic oral mucosal model. Our results demonstrated that the dual species biofilms resulted in profound surface erosion and significantly increased microbial invasion into mucosal compartments, compared to either species alone. Notably, several genes of C. albicans involved in tissue adhesion, hyphal formation, fungal invasion, and biofilm formation were significantly upregulated in the presence of E. faecalis. By contrast, E. faecalis genes involved in quorum sensing, biofilm formation, virulence, and mammalian cell invasion were downregulated. This study highlights the synergistic cross-kingdom interactions between E. faecalis and C. albicans in mucosal tissue invasion.

Antimicrobial and protective effects of non-thermal plasma treatments on the performance of a resinous liner

OBJECTIVE

Overcoming substantial shortcomings of soft liners as physico-chemical changes and liner-biofilm-related infections remains a challenge in the rehabilitation treatment. In this study, protective non-thermal plasma (NTP) treatments were developed on the soft liner surface to improve its surface and physico-chemical properties and to reduce fungal colonization after biofilm inhibition challenge.

METHODS

Resinous liner specimens (Coe-Soft) were prepared and distributed in 3 groups according to the surface treatments: (1) untreated (control); (2) treated with sulfur hexafluoride-based NTP (SF₆); and (3) treated with hexamethyldisiloxane-based NTP (HMDSO). To test the NTP stability and their protective and antimicrobial effect on the liner surface over time, the morphology, chemical composition, roughness, water contact angle, shore A hardness, sorption and solubility were evaluated before and after the specimens were exposed to dual-species biofilm of Candida albicans and Streptococcus oralis for 14 days. Colony forming units and biofilm structure were assessed. Data were submitted to ANOVA and Tukey tests (α = 0.05).

RESULTS

Both treatments modified the surface morphology, increased hydrophobicity and roughness of the liner, and were effective to reduce C. albicans adhesion without affecting the commensal health-associated S. oralis. HMDSO presented chemical stability and lower hardness in both periods, whereas SF₆ exhibited higher initial hardness than control and the highest sorption; contrarily, similar solubility was noted for all groups.

CONCLUSION

HMDSO-based film showed improved physico-chemical properties and inhibited C. albicans biofilm. Thus, it has potential for use to control candida-related stomatitis and improve liner's stability even after being exposed to biofilm inhibition challenge.

Role of glucosyltransferase R in biofilm interactions between Streptococcus oralis and Candida albicans

Streptococcal glucosyltransferases (Gtf) synthesize α-glucan exopolymers which contribute to biofilm matrix. Streptococcus oralis interacts with the opportunistic pathogen Candida albicans to form hypervirulent biofilms. S. oralis 34 has a single gtf gene (gtfR). However, the role of gtfR in single and mixed species biofilms with C. albicans has never been examined. A gtfR deletion mutant, purified GtfR, and recombinant GtfR glucan-binding domain were tested in single and mixed biofilms on different substrata in vitro. A mouse oral infection model was also used. We found that in single species biofilms growing with sucrose on abiotic surfaces S. oralis gtfR increased biofilm matrix, but not bacterial biomass. In biofilms with C. albicans, S. oralis encoding gtfR showed increased bacterial biomass on all surfaces. C. albicans had a positive effect on α-glucan synthesis, and α-glucans increased C. albicans accretion on abiotic surfaces. In single and mixed infection of mice receiving sucrose S. oralis gtfR enhanced mucosal burdens. However, sucrose had a negative impact on C. albicans burdens and reduced S. oralis burdens in co-infected mice. Our data provide new insights on the GtfR-mediated interactions between the two organisms and the influence of biofilm substratum and the mucosal environment on these interactions.

Glucose effect on Candida albicans biofilm during tissue invasion

OBJECTIVE

To evaluate, in vitro, the effect of two glucose concentrations (0.1 mM and 1.0 mM, simulating glucose concentration in saliva of healthy and diabetic individuals) on Candida albicans biofilm grown on epithelial monolayer.

MATERIAL AND METHODS

C. albicans was inoculated on epithelial monolayers supplemented with 0.1 mM, 1.0 mM or no glucose. Control groups without C. albicans were also evaluated. Tissue response was assessed through the production of Interleukin-1α, Interleukin-8, Interleukin-6, Interleukin-10 and tumor necrosis factor-α. The complex of monolayer and biofilms were evaluated by quantitative reverse transcription polymerase chain reaction for expression of E-cadherin (CDH1), Caspase-3 (CASP3), β-defensin-1 (DEFB-1) and β-defensin-3 (DEFB-3). The biofilm architecture was visualized by confocal laser scanning microscopy.

RESULTS

The production of Interleukin-1α and Interleukin-8 were increased in the presence of C. albicans (p < 0.05). Glucose did not interfere in the release of any cytokine evaluated. C. albicans downregulated transcripts for CDH1 (p < 0.05). Glucose did not induce a significant change in CDH1, CASP3, DEFB-1 and DEFB-3 messenger RNA expression. The biofilms were more structured in the presence of glucose, but no difference in the diffusion of hyphae through the epithelial cells were observed.

CONCLUSIONS

The data suggest that glucose concentration does not affect the behavior of C. albicans during tissue invasion and other mechanisms must be related to the greater susceptibility of diabetic individuals to candidiasis.

Biofilm Matrixome: Extracellular Components in Structured Microbial Communities

Biofilms consist of microbial communities embedded in a 3D extracellular matrix. The matrix is composed of a complex array of extracellular polymeric substances (EPS) that contribute to the unique attributes of biofilm lifestyle and virulence. This ensemble of chemically and functionally diverse biomolecules is termed the 'matrixome'. The composition and mechanisms of EPS matrix formation, and its role in biofilm biology, function, and microenvironment are being revealed. This perspective article highlights recent advances about the multifaceted role of the 'matrixome' in the development, physical-chemical properties, and virulence of biofilms. We emphasize that targeting biofilm-specific conditions such as the matrixome could lead to precise and effective antibiofilm approaches. We also discuss the limited knowledge in the context of polymicrobial biofilms, and the need for more in-depth analyses of the EPS matrix in mixed communities that are associated with many human infectious diseases.

Biofilm Interactions of Candida albicans and Mitis Group Streptococci in a Titanium-Mucosal Interface Model

Streptococci from the mitis group (represented mainly by Streptococcus mitis, Streptococcus oralis, Streptococcus sanguinis, and Streptococcus gordonii) form robust biofilms with Candida albicans in different experimental models. These microorganisms have been found in polymicrobial biofilms forming on titanium biomaterial surfaces in humans with peri-implant disease. The purpose of this work was to study mutualistic interactions in biofilms forming on titanium and their effect on the adjacent mucosa, using a relevant infection model. Single and mixed biofilms of C. albicans and each Streptococcus species were grown on titanium disks. Bacterial and fungal biovolume and biomass were quantified in these biofilms. Organotypic mucosal constructs were exposed to preformed titanium surface biofilms to test their effect on secretion of proinflammatory cytokines and cell damage. C. albicans promoted bacterial biofilms of all mitis Streptococcus species on titanium surfaces. This relationship was mutualistic since all bacterial species upregulated the efg1 hypha-associated gene in C. albicans Mixed biofilms caused increased tissue damage but did not increase proinflammatory cytokine responses compared to biofilms comprising Candida alone. Interestingly, spent culture medium from tissues exposed to titanium biofilms suppressed Candida growth on titanium surfaces.IMPORTANCE Our findings provide new insights into the cross-kingdom interaction between C. albicans and Streptococcus species representative of the mitis group. These microorganisms colonize titanium-based dental implant materials, but little is known about their ability to cause inflammation and damage of the adjacent mucosal tissues. Using an in vitro biomaterial-mucosal interface infection model, we showed that mixed biofilms of each species with C. albicans enhance tissue damage. One possible mechanism for this effect is the increased fungal hypha-associated virulence gene expression we observed in mixed biofilms with these species. Interestingly, we also found that the interaction of multispecies biofilms with organotypic mucosal surfaces led to the release of growth-suppressing mediators of Candida, which may represent a homeostatic defense mechanism of the oral mucosa against fungal overgrowth. Thus, our findings provide novel insights into biofilms on biomaterials that may play an important role in the pathogenesis of mucosal infections around titanium implants.

Candida spp./Bacteria Mixed Biofilms

The ability to form biofilms is a common feature of microorganisms, such as bacteria or fungi. These consortiums can colonize a variety of surfaces, such as host tissues, dentures, and catheters, resulting in infections highly resistant to drugs, when compared with their planktonic counterparts. This refractory effect is particularly critical in polymicrobial biofilms involving both fungi and bacteria. This review emphasizes Candida spp.-bacteria biofilms, the epidemiology of this community, the challenges in the eradication of such biofilms, and the most relevant treatments.

Interactions between oral commensal Candida and oral bacterial communities in immunocompromised and healthy children

Candida species are usually found as commensal microorganisms in the oral cavity of healthy people. During chemotherapy, cytostatic drugs lead to depletion of the oral flora with the emergence of a dominant bacterial species. The transition from commensal to pathogenic state, further associated with yeast colonization and oral mucositis implies a replacement of the dominant microorganism by Candida albicans. This process goes plausibly through cooperation between C. albicans and bacteria. This study focused on the first step of cooperation between microorganisms isolated from the same oral flora either of leukemic or healthy children. C. albicans isolated from 8/20 children were cultured to display their noninvasive blastosporic yeast form and mixed with their dominant bacteria to study the capacity of planktonic aggregation and the early state of biofilm formation. None of the dominant bacteria opposed the presence of yeast, on the contrary, an interesting cooperation was observed. This behavior is apparently different from that observed when mixing the type strains. In fact, three mutated C. albicans strains display, by their spontaneous ability to form filament, enhanced risks of virulence for leukemic ill carriers. Despite such risks, neither oral nor systemic pathology were observed in ill patients probably because the study was conducted during the first course of chemotherapy and Candida colonization is related to the number of chemotherapeutic cycles. The presence of C. albicans during the initial cycle represents, by its ability to interact with oral bacteria, an actual threat for further cures.

The oral microbiota: dynamic communities and host interactions

The dynamic and polymicrobial oral microbiome is a direct precursor of diseases such as dental caries and periodontitis, two of the most prevalent microbially induced disorders worldwide. Distinct microenvironments at oral barriers harbour unique microbial communities, which are regulated through sophisticated signalling systems and by host and environmental factors. The collective function of microbial communities is a major driver of homeostasis or dysbiosis and ultimately health or disease. Despite different aetiologies, periodontitis and caries are each driven by a feedforward loop between the microbiota and host factors (inflammation and dietary sugars, respectively) that favours the emergence and persistence of dysbiosis. In this Review, we discuss current knowledge and emerging mechanisms governing oral polymicrobial synergy and dysbiosis that have both enhanced our understanding of pathogenic mechanisms and aided the design of innovative therapeutic approaches for oral diseases.

Candida Interactions with the Oral Bacterial Microbiota

The human oral cavity is normally colonized by a wide range of microorganisms, including bacteria, fungi, Archaea, viruses, and protozoa. Within the different oral microenvironments these organisms are often found as part of highly organized microbial communities termed biofilms, which display consortial behavior. Formation and maintenance of these biofilms are highly dependent on the direct interactions between the different members of the microbiota, as well as on the released factors that influence the surrounding microbial populations. These complex biofilm dynamics influence oral health and disease. In the latest years there has been an increased recognition of the important role that interkingdom interactions, in particular those between fungi and bacteria, play within the oral cavity. Candida spp., and in particular C. albicans, are among the most important fungi colonizing the oral cavity of humans and have been found to participate in these complex microbial oral biofilms. C. albicans has been reported to interact with individual members of the oral bacterial microbiota, leading to either synergistic or antagonistic relationships. In this review we describe some of the better characterized interactions between Candida spp. and oral bacteria.

Influence of Streptococcus mitis and Streptococcus sanguinis on virulence of Candida albicans: in vitro and in vivo studies

The aim was to evaluate in vitro possible interactions, gene expression, and biofilm formation in species of Candida albicans, Streptococcus mitis, and Streptococcus sanguinis and their in vivo pathogenicity. The in vitro analysis evaluated the effects of S. mitis and S. sanguinis on C. albicans's biofilm formation by CFU count, filamentation capacity, and adhesion (ALS1, ALS3, HWP1) and transcriptional regulatory gene (BCR1, CPH1, EFG1) expression. In vivo studies evaluated the pathogenicity of the interaction of the microorganisms on Galleria mellonella, with analyses of the CFU per milliliter count and filamentation. In vitro results indicated that there was an observed decrease in CFU (79.4-71.5%) in multi-species biofilms. The interaction with S. mitis inhibited filamentation, which seems to increase its virulence factor with over-expression of genes ALS1, ALS3, and HWP1 as well the interaction with S. sanguinis as ALS3 and HWP1. S. mitis upregulated BRC1, CPH1, and EFG1. The histological images of in vivo study indicate an increase in the filamentation of C. albicans when in interaction with the other species. It was concluded that S. mitis interaction suggests increased virulence factors of C. albicans, with periods of lower virulence and proto-cooperation in the interaction with S. sanguinis.

Tipping the Balance: C. albicans Adaptation in Polymicrobial Environments

Candida albicans is a pleiomorphic fungus which co-exists with commensal bacteria in mucosal and skin sites of mammalian hosts. It is also a major co-isolated organism from polymicrobial systemic infections, with high potential for morbidity or mortality in immunocompromised patients. Traditionally, resident mucosal bacteria have been thought to antagonize C. albicans in its ability to colonize or cause infection. However, recent investigations have revealed synergistic relationships with certain bacterial species that colonize the same mucosal sites with C. albicans. Such relationships broaden the research landscape in pathogenesis but also contribute to clinical challenges in the prevention or treatment of mucosal candidiasis. This review sheds light on interactions of C. albicans and mucosal bacteria, with special emphasis on the effects of the resident bacterial microbiota on C. albicans physiology as they relate to its adaptation in mucosal sites as a commensal colonizer or as a pathogenic organism.

Chemotherapy-induced oral mucositis and associated infections in a novel organotypic model

Oral mucositis is a common side effect of cancer chemotherapy, with significant adverse impact on the delivery of anti-neoplastic treatment. There is a lack of consensus regarding the role of oral commensal microorganisms in the initiation or progression of mucositis because relevant experimental models are non-existent. The goal of this study was to develop an in vitro mucosal injury model that mimics chemotherapy-induced mucositis, where the effect of oral commensals can be studied. A novel organotypic model of chemotherapy-induced mucositis was developed based on a human oral epithelial cell line and a fibroblast-embedded collagen matrix. Treatment of organotypic constructs with 5-fluorouracil (5-FU) reproduced major histopathologic characteristics of oral mucositis, such as DNA synthesis inhibition, apoptosis and cytoplasmic vacuolation, without compromising the three-dimensional structure of the multilayer organotypic mucosa. Although structural integrity of the model was preserved, 5-FU treatment resulted in a widening of epithelial intercellular spaces, characterized by E-cadherin dissolution from adherens junctions. In a neutrophil transmigration assay we discovered that this treatment facilitated transport of neutrophils through epithelial layers. Moreover, 5-FU treatment stimulated key proinflammatory cytokines that are associated with the pathogenesis of oral mucositis. 5-FU treatment of mucosal constructs did not significantly affect fungal or bacterial biofilm growth under the conditions tested in this study; however, it exacerbated the inflammatory response to certain bacterial and fungal commensals. These findings suggest that commensals may play a role in the pathogenesis of oral mucositis by amplifying the proinflammatory signals to mucosa that is injured by cytotoxic chemotherapy.

Community Development between Porphyromonas gingivalis and Candida albicans Mediated by InlJ and Als3

The pleiomorphic yeast Candida albicans is a significant pathogen in immunocompromised individuals. In the oral cavity, C. albicans is an inhabitant of polymicrobial communities, and interspecies interactions promote hyphal formation and biofilm formation. C. albicans colonizes the subgingival area, and the frequency of colonization increases in periodontal disease. In this study, we investigated the interactions between C. albicans and the periodontal pathogen Porphyromonas gingivalisC. albicans and P. gingivalis were found to coadhere in both the planktonic and sessile phases. Loss of the internalin-family protein InlJ abrogated adhesion of P. gingivalis to C. albicans, and recombinant InlJ protein competitively inhibited interspecies binding. A mutant of C. albicans deficient in expression of major hyphal protein Als3 showed diminished binding to P. gingivalis, and InlJ interacted with Als3 heterologously expressed in Saccharomyces cerevisiae Transcriptional profiling by RNA sequencing (RNA-Seq) established that 57 genes were uniquely upregulated in an InlJ-dependent manner in P. gingivalis-C. albicans communities, with overrepresentation of those corresponding to 31 gene ontology terms, including those associated with growth and division. Of potential relevance to the disease process, C. albicans induced upregulation of components of the type IX secretion apparatus. Collectively, these findings indicate that InlJ-Als3-dependent binding facilitates interdomain community development between C. albicans and P. gingivalis and that P. gingivalis has the potential for increased virulence within such communities.IMPORTANCE Many diseases involve the concerted actions of microorganisms assembled in polymicrobial communities. Inflammatory periodontal diseases are among the most common infections of humans and result in destruction of gum tissue and, ultimately, in loss of teeth. In periodontal disease, pathogenic communities can include the fungus Candida albicans; however, the contribution of C. albicans to the synergistic virulence of the community is poorly understood. Here we characterize the interactions between C. albicans and the keystone bacterial pathogen Porphyromonas gingivalis and show that coadhesion mediated by specific proteins results in major changes in gene expression by P. gingivalis, which could serve to increase pathogenic potential. The work provides significant insights into interdomain interactions that can enhance our understanding of diseases involving a multiplicity of microbial pathogens.

Effect of a Lactobacillus Salivarius Probiotic on a Double-Species Streptococcus Mutans and Candida Albicans Caries Biofilm

The aim of the study was to evaluate the anti-cariogenic effects of Lactobacillus salivarius by reducing pathogenic species and biofilm mass in a double-species biofilm model. Coexistence of S. mutans with C. albicans can cause dental caries progression or recurrence of the disease in the future. Fifty-nine children with diagnosed early childhood caries (ECC) were recruited onto the study. The condition of the children's dentition was defined according to the World Health Organization guidelines. The participants were divided into children with initial enamel demineralization and children showing dentin damage. The study was performed on the S. mutans and C. albicans clinical strains, isolated from dental plaque of patients with ECC. The effect of a probiotic containing Lactobacillus salivarius on the ability of S. mutans and C. albicans to produce a double-species biofilm was investigated in an in vitro model. The biomass of the formed/non-degraded biofilm was analyzed on the basis of its crystal violet staining. The number of colonies of S. mutans and C. albicans (CFU/mL, colony forming units/mL) forming the biofilm was determined. Microorganism morphology in the biofilm was evaluated using a scanning electron microscope (SEM). In vitro analysis demonstrated that the presence of S. mutans increased the number of C. albicans colonies (CFU/mL); the double-species biofilm mass and hyphal forms produced in it by the yeast. L. salivarius inhibited the cariogenic biofilm formation of C. albicans and S. mutans. Under the influence of the probiotic; the biofilm mass and the number of S. mutans; C. albicans and S. mutans with C. albicans colonies in the biofilm was decreased. Moreover; it can be noted that after the addition of the probiotic; fungi did not form hyphae or germ tubes of pathogenic potential. These results suggest that L. salivarius can secrete intermediates capable of inhibiting the formation of cariogenic S. mutans and C. albicans biofilm; and may inhibit fungal morphological transformation and thereby reduce the pathogenicity of C. albicans; weakening its pathogenic potential. Further research is required to prove or disprove the long-term effects of the preparation and to achieve preventive methods.

Candida albicans and Pseudomonas aeruginosa Interact To Enhance Virulence of Mucosal Infection in Transparent Zebrafish

Polymicrobial infections often include both fungi and bacteria and can complicate patient treatment and resolution of infection. Cross-kingdom interactions among bacteria, fungi, and/or the immune system during infection can enhance or block virulence mechanisms and influence disease progression. The fungus Candida albicans and the bacterium Pseudomonas aeruginosa are coisolated in the context of polymicrobial infection at a variety of sites throughout the body, including mucosal tissues such as the lung. In vitro, C. albicans and P. aeruginosa have a bidirectional and largely antagonistic relationship. Their interactions in vivo remain poorly understood, specifically regarding host responses in mediating infection. In this study, we examine trikingdom interactions using a transparent juvenile zebrafish to model mucosal lung infection and show that C. albicans and P. aeruginosa are synergistically virulent. We find that high C. albicans burden, fungal epithelial invasion, swimbladder edema, and epithelial extrusion events serve as predictive factors for mortality in our infection model. Longitudinal analyses of fungal, bacterial, and immune dynamics during coinfection suggest that enhanced morbidity is associated with exacerbated C. albicans pathogenesis and elevated inflammation. The P. aeruginosa quorum-sensing-deficient ΔlasR mutant also enhances C. albicans pathogenicity in coinfection and induces extrusion of the swimbladder. Together, these observations suggest that C. albicans-P. aeruginosa cross talk in vivo can benefit both organisms to the detriment of the host.

A Sustained-Release Membrane of Thiazolidinedione-8: Effect on Formation of a Candida/Bacteria Mixed Biofilm on Hydroxyapatite in a Continuous Flow Model

Thiazolidinediones (TZDs) have been found to act as effective quorum sensing quenchers, capable of preventing biofilm formation. Our previous studies demonstrated a profound antibiofilm effect of the TZD derivative thiazolidinedione-8 (S-8), either in solution or incorporated into a sustained-release membrane (SRM-S-8) under batch conditions. In the present study, we used a constant depth film fermenter model in order to investigate the impact of SRM-S-8 on mixed C. albicans-S. mutans biofilm development, under flow conditions. We found that essential parameters of cospecies biofilm maintenance and maturation, such as metabolic activity, biofilm thickness, roughness, extracellular polysaccharides production, and morphology of both pathogens, were altered by SRM-S-8 in the flow system. We propose that prolonged and sustained release of S-8 in a flow-through system allows better penetration of the active agent to deeper layers of the mixed biofilm, thereby increasing its activity against both pathogens. In conclusion, the use of a locally applied sustained-release drug delivery system of S-8 can affect the dental polymicrobial biofilm, resulting in clinical improvements and a better patient compliance.

Candida albicans mannans mediate Streptococcus mutans exoenzyme GtfB binding to modulate cross-kingdom biofilm development in vivo

Candida albicans is frequently detected with heavy infection by Streptococcus mutans in plaque-biofilms from children with early-childhood caries (ECC). This cross-kingdom biofilm contains an extensive matrix of extracellular α-glucans that is produced by an exoenzyme (GtfB) secreted by S. mutans. Here, we report that mannans located on the outer surface of C. albicans cell-wall mediates GtfB binding, enhancing glucan-matrix production and modulating bacterial-fungal association within biofilms formed in vivo. Using single-molecule atomic force microscopy, we determined that GtfB binds with remarkable affinity to mannans and to the C. albicans surface, forming a highly stable and strong bond (1-2 nN). However, GtfB binding properties to C. albicans was compromised in strains defective in O-mannan (pmt4ΔΔ) or N-mannan outer chain (och1ΔΔ). In particular, the binding strength of GtfB on och1ΔΔ strain was severely disrupted (>3-fold reduction vs. parental strain). In turn, the GtfB amount on the fungal surface was significantly reduced, and the ability of C. albicans mutant strains to develop mixed-species biofilms with S. mutans was impaired. This phenotype was independent of hyphae or established fungal-biofilm regulators (EFG1, BCR1). Notably, the mechanical stability of the defective biofilms was weakened, resulting in near complete biomass removal by shear forces. In addition, these in vitro findings were confirmed in vivo using a rodent biofilm model. Specifically, we observed that C. albicans och1ΔΔ was unable to form cross-kingdom biofilms on the tooth surface of rats co-infected with S. mutans. Likewise, co-infection with S. mutans defective in GtfB was also incapable of forming mixed-species biofilms. Taken together, the data support a mechanism whereby S. mutans-secreted GtfB binds to the mannan layer of C. albicans to promote extracellular matrix formation and their co-existence within biofilms. Enhanced understanding of GtfB-Candida interactions may provide new perspectives for devising effective therapies to disrupt this cross-kingdom relationship associated with an important childhood oral disease.

Streptococcus oralis and Candida albicans Synergistically Activate μ-Calpain to Degrade E-cadherin From Oral Epithelial Junctions

Streptococcus oralis forms robust mucosal biofilms with Candida albicans that have increased pathogenic potential. In this study, using oral epithelial cultures, organotypic oral mucosal constructs, and a mouse model of oral infection, we demonstrated that S. oralis augmented C. albicans invasion through epithelial junctions. C. albicans and S. oralis decreased epithelial E-cadherin levels by synergistically increasing µ-calpain, a proteolytic enzyme that targets E-cadherin. In the mouse coinfection model this was accompanied by increased fungal kidney dissemination. Coinfection with a secreted aspartyl protease (sap) mutant sap2456 and S. oralis increased μ-calpain and triggered mucosal invasion and systemic dissemination, suggesting that fungal protease activity is not required for invasion during coinfection. We conclude that C. albicans and S. oralis synergize to activate host enzymes that cleave epithelial junction proteins and increase fungal invasion.

Fungal morphogenetic changes inside the mammalian host

One of the main features of the majority of pathogenic fungi is the ability to switch between different types of morphological forms. These changes include the transition between cells of different shapes (such as the formation of pseudohyphae and hyphae), or the massive growth of the blastoconidia and formation of titan cells. Morphological changes occur during infection, and there is extensive evidence that they play a key role in processes required for disease, such as adhesion, invasion and dissemination, immune recognition evasion, and phagocytosis avoidance. In the present review, we will provide an overview of how morphological transitions contribute to the development of fungal disease, with special emphasis in two cases: Candida albicans as an example of yeast that switches between blastoconidia and filaments, and Cryptococcus neoformans as an example of a fungus that changes the size without modifying the shape of the cell.

Role of Candida albicans secreted aspartyl protease Sap9 in interkingdom biofilm formation

The fungus Candida albicans colonizes oral cavity surfaces and is carried by up to 60% of human populations. Biofilm development by C. albicans may be modulated by oral streptococci, such as Streptococcus gordonii, S. oralis or S. mutans, so as to augment pathogenicity. In this study we sought to determine if the cell wall-associated secreted aspartyl proteinase Sap9 was necessary for hyphal adhesin functions associated with biofilm community development. A sap9Δ mutant of C. albicans SC5314 formed biofilms that were flatter, and contained fewer blastospores and more hyphal filaments than the parent strain. This phenotypic difference was accentuated under flow (shear) conditions and in the presence of S. gordonii. Dual-species biofilms of C. albicans sap9Δ with S. oralis, S. sanguinis, S. parasanguinis, S. mutans and Enterococcus faecalis all contained more matted hyphae and more bacteria bound to substratum compared to C. albicans wild type. sap9Δ mutant hyphae showed significantly increased cell surface hydrophobicity, ∼25% increased levels of binding C. albicans cell wall protein Als3, and reduced interaction with Eap1, implicating Sap9 in fungal cell-cell recognition. These observations suggest that Sap9 is associated with protein-receptor interactions between fungal cells, and with interkingdom communication in the formation of polymicrobial biofilm communities.

Transcriptome analysis of Streptococcus gordonii Challis DL1 indicates a role for the biofilm-associated fruRBA operon in response to Candida albicans

Multiple levels of interkingdom signaling have been implicated in maintaining the ecological balance between Candida albicans and commensal streptococci to assure a state of oral health. To better understand the molecular mechanisms involved in the initial streptococcal response to the presence of C. albicans that can initiate oral surface colonization and biofilm formation, hypha-forming cells were incubated with Streptococcus gordonii cells for 30 min to assess the streptococcal transcriptome response. A genome-wide microarray analysis and quantitative polymerase chain reaction validation of S. gordonii transcripts identified a number of genes, the majority of which were involved in metabolic functions that were differentially expressed in the presence of hyphae. The fruR, fruB, and fruA genes encoding the transcriptional regulator, fructose-1-phosphate kinase, and fructose-specific permease, respectively, of the phosphoenolpyruvate-dependent fructose phosphotransferase system, were consistently upregulated. An S. gordonii mutant in which these genes were deleted by allelic replacement formed an architecturally distinct, less robust biofilm with C. albicans than did parental strain cells. Complementing the mutant with plasmid borne fruR, fruB, and fruA genes caused phenotype reversion, indicating that the genes in this operon played a role in dual-species biofilm formation. This genome-wide analysis of the S. gordonii transcriptional response to C. albicans has identified several genes that have potential roles in interkingdom signaling and responses.