Cross-kingdom interaction of Candida albicans and Actinomyces viscosus elevated cariogenic virulence

Interactions among microorganisms open up a new world for anti-infectious therapy

The human microbiome, containing bacteria, fungi, and viruses, is a community that coexists peacefully with humans most of the time, but with the potential to cause disease under certain conditions. When the environment changes or certain stimuli are received, microbes may interact with each other, causing or increasing the severity of disease in a host. With the appropriate methods, we can make these microbiota work for us, creating new applications for human health. This review discusses the wide range of interactions between microorganisms that result in an increase in susceptibility to, severity of, and mortality of diseases, and also briefly introduces how microorganisms interact with each other directly or indirectly. The study of microbial interactions and their mechanisms has revealed a new world of treatments for infectious disease. The regulation of the balance between intestinal flora, the correct application of probiotics, and the development of effective drugs by symbiosis all demonstrate the great contributions of the microbiota to human health and its powerful potential value. Consequently, the study of interactions between microorganisms plays an essential role in identifying the causes of diseases and the development of treatments.

Efficient Removal of Dental Plaque Biofilm from Training Typodont Teeth via Water Flosser

Plaque biofilms play critical roles in the development of dental caries. Mechanical plaque control methods are considered to be most effective for plaque removal, such as brushing teeth or using flosser. Recently, water flosser has been paid much attention. Here, we tested the ability of a water flosser to remove the adhered sucrose and the dental plaque biofilms formed by Streptococcus mutans, Streptococcus sanguinis, and Actinobacillus viscosus. We found that the residual sucrose concentration was 3.54 mg/mL in the control group, 1.75 mg/mL in the syringe group (simulating the ordinary mouthwash), and 0 mg/mL in water flosser group. In addition, the residual bacterial concentration was 3.6 × 108 CFU/mL in the control group, 1.6 × 107 CFU/mL in the syringe group, and only 5.5 × 105 CFU/mL in the water flosser group. In summary, water flosser is effective for cleaning the teeth, which may have significant potential in preventing dental caries and maintaining oral health.

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 Microbial Interactions Within the Oral Cavity and Their Implications for Oral Disease

Purpose of Review

This review serves to highlight the cross-kingdom interactions that can occur within the human oral cavity between fungus Candida albicans and oral bacteria, and their impact on the delicate balance between oral health and disease.

Recent Findings

A growing number of physical, chemical, and metabolic networks have been identified that underpin these cross-kingdom interactions. Moreover, these partnerships are often synergistic and can modulate microbial burden or virulence. This, in turn, can drive the onset or progression of oral diseases such as dental caries, periodontitis, denture-associated stomatitis, and oral cancer.

Summary

The impact of cross-kingdom interactions on the cellular, biochemical, and communal composition of oral microbial biofilms is increasingly clear. With growing insight into these processes at the molecular level, so this knowledge can be used to better inform the development of novel strategies to manipulate the oral microbiota to promote oral health and combat oral disease.

Role of mutans streptococci, acid tolerant bacteria and oral Candida species in predicting the onset of early childhood caries

Aim

Early childhood caries is the most common chronic infectious disease in children in the United States. This study, which is part of a larger, longitudinal study exploring oral microbiological components of caries development in children, reports on the impact of total mutans streptococci (MS), total acid tolerant bacteria and Candida species on the development of dental caries in a subset of these children. Of particular interest was the relationship between caries development and co-colonization of mutans streptococci and Candida species.

Methods

Children between the ages of 12 and 47 months displaying no evidence of dental caries were recruited for a longitudinal study (n = 130). Twelve age- and gender-matched pairs were selected. In each pair, one child developed caries during the study, and one did not. Whole mouth plaque samples were collected by swab at baseline and every 6 months thereafter for a duration of 18 months and spiral plated for microbial counts (CFU/ml). Cut-offs based on percent of total cultivable flora were designated for all microbial measures. A scoring system designated the Plaque Microbial Index (PMI) was developed for use in statistical analyses to assess potential predictive factors for caries risk assessment.

Results

Children who developed caries were significantly more likely to harbor higher percentages of acid tolerant bacteria (p = 0.003), MS (p < 0.001) and have Candida species present (p < 0.001) at ≥1 visit leading up to caries onset. Mean PMI scores derived from the aforementioned microbial measures, were higher for caries active children than caries free children (p = 0.000147). Co-colonization of MS and Candida species was significantly associated with caries development (p < 0.001) and detection of both at the same visit had a 100% positive predictive value and 60% negative predictive value for caries development.

Conclusion

In children who developed caries, there was a statistically significant association with the percent of total flora that was acid tolerant, the percent of MS, the presence of Candida and co-colonization of MS and Candida species. Combining these microbial measures into PMI scores further delineated children who developed caries from those who remained caries-free. These microbiological measures show potential as predictive factors and risk assessment tools for caries development.

Diarylureas: New Promising Small Molecules against Streptococcus mutans for the Treatment of Dental Caries

Dental caries is a biofilm-mediated disease that represents a worldwide oral health issue. Streptococcus mutans has been ascertained as the main cariogenic pathogen responsible for human dental caries, with a high ability to form biofilms, regulated by the quorum sensing. Diarylureas represent a class of organic compounds that show numerous biological activities, including the antimicrobial one. Two small molecules belonging to this class, specifically to diphenylureas, BPU (1,3-bis[3,5-bis(trifluoromethyl)phenyl]urea) and DMTU (1,3-di-m-tolyl-urea), showed interesting results in studies regarding the antimicrobial activity against the cariogenic bacterium S. mutans. Since there are not many antimicrobials used for the prevention and treatment of caries, further studies on these two interesting compounds and other diarylureas against S. mutans may be useful to design new effective agents for the treatment of caries with generally low cytotoxicity.

The advances in the regulation of immune microenvironment by Candida albicans and macrophage cross-talk

Candida albicans (C. albicans) is the most common causative agent of invasive fungal infections in hospitals. The body defends against and eliminates C. albicans infection by various mechanisms of immune response, and the latter mechanism of immune evasion is a major challenge in the clinical management of C. albicans infection. The role of macrophages in combating C. albicans infection has only recently been recognized, but the mechanisms remain to be elucidated. This review focuses on the interaction between C. albicans and macrophages (macrophages), which causes the body to generate an immune response or C. albicans immune escape, and then regulates the body's immune microenvironment, to explore the effect of C. albicans virulence resistance vs. macrophage killing and clarify the role and mechanism of C. albicans pathogenesis. In general, a thorough understanding of the molecular principles driving antifungal drug resistance is essential for the development of innovative treatments that can counteract both existing and emerging fungal threats.

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.

Candida albicans CHK1 gene regulates its cross-kingdom interactions with Streptococcus mutans to promote caries

The cross-kingdom interactions between Candida albicans and Streptococcus mutans have played important roles in early childhood caries (ECC). However, the key pathways of C. albicans promoting the cariogenicity of S. mutans are still unclear. Here, we found that C. albicans CHK1 gene was highly upregulated in their dual-species biofilms. C. albicans chk1Δ/Δ significantly reduced the synergistical growth promotion, biofilm formation, and exopolysaccharides (EPS) production of S. mutans, the key cariogenic agent, compared to C. albicans wild type (WT) and CHK1 complementary strains. C. albicans WT upregulated the expressions of S. mutans EPS biosynthesis genes gtfB, gtfC, and gtfD, and their regulatory genes vicR and vicK, but chk1Δ/Δ had no effects. Both C. albicans WT and chk1Δ/Δ failed to promote the biofilm formation and EPS production of S. mutans ΔvicK and antisense-vicR strains, indicating that C. albicans CHK1 upregulated S. mutans vicR and vicK to increase the EPS biosynthesis gene expression, then enhanced the EPS production and biofilm formation to promote the cariogenicity. In rat caries model, the coinfection with chk1Δ/Δ and S. mutans decreased the colonization of S. mutans and developed less caries especially the severe caries compared to that from the combinations of S. mutans with C. albicans WT, indicating the essential role of C. albicans CHK1 gene in the development of dental caries. Our study for the first time demonstrated the key roles of C. albicans CHK1 gene in dental caries and suggested that it may be a practical target to reduce or treat ECC. KEY POINTS: • C. albicans CHK1 gene is important for its interaction with S. mutans. • CHK1 regulates S. mutans two-component system to promote its cariogenicity. • CHK1 gene regulates the cariogenicity of S. mutans in rat dental caries.

Natural products from traditional medicine as promising agents targeting at different stages of oral biofilm development

Oral cavity is an ideal habitat for more than 1,000 species of microorganisms. The diverse oral microbes form biofilms over the hard and soft tissues in the oral cavity, affecting the oral ecological balance and the development of oral diseases, such as caries, apical periodontitis, and periodontitis. Currently, antibiotics are the primary agents against infectious diseases; however, the emergence of drug resistance and the disruption of oral microecology have challenged their applications. The discovery of new antibiotic-independent agents is a promising strategy against biofilm-induced infections. Natural products from traditional medicine have shown potential antibiofilm activities in the oral cavity with high safety, cost-effectiveness, and minimal adverse drug reactions. Aiming to highlight the importance and functions of natural products from traditional medicine against oral biofilms, here we summarized and discussed the antibiofilm effects of natural products targeting at different stages of the biofilm formation process, including adhesion, proliferation, maturation, and dispersion, and their effects on multi-species biofilms. The perspective of antibiofilm agents for oral infectious diseases to restore the balance of oral microecology is also discussed.

The Arginine Biosynthesis Pathway of Candida albicans Regulates Its Cross-Kingdom Interaction with Actinomyces viscosus to Promote Root Caries

The cross-kingdom interactions between Candida albicans and Actinomyces viscosus play critical roles in root caries. However, the key pathway by which C. albicans regulates its interactions with A. viscosus is unclear. Here, we first employed 39 volunteers with root caries and 37 caries-free volunteers, and found that the abundances of C. albicans and A. viscosus were significantly increased in the individuals with root caries and showed a strong positive correlation. Their dual-species combination synergistically promoted biofilm formation and root caries in rats. The arginine biosynthesis pathway of C. albicans was significantly upregulated in dual-species biofilms and dental plaques from another 10 root caries volunteers compared with the 10 caries-free volunteers. The exogenous addition of arginine increased the cariogenicity of the dual-species biofilm. The C. albicansARG4, a key gene from the arginine biosynthesis pathway, null mutant failed to promote dual-species biofilm formation and root caries in rats; however, the addition of arginine restored its synergistic actions with A. viscosus. Our results identified the critical roles of the C. albicans arginine biosynthesis pathway in its cross-kingdom interactions with A. viscosus for the first time and indicated that targeting this pathway was a practical way to treat root caries caused by multiple species.

IMPORTANCE Root caries is a critical problem that threatens the oral health of the elderly population. Our results identified the essential roles of the C. albicans arginine biosynthesis pathway in its cross-kingdom interactions with A. viscosus in root caries for the first time and indicated that targeting this pathway was a practical way to treat root caries caused by multiple species.

The Oral Microbiota: Community Composition, Influencing Factors, Pathogenesis, and Interventions

The human oral cavity provides a habitat for oral microbial communities. The complexity of its anatomical structure, its connectivity to the outside, and its moist environment contribute to the complexity and ecological site specificity of the microbiome colonized therein. Complex endogenous and exogenous factors affect the occurrence and development of the oral microbiota, and maintain it in a dynamic balance. The dysbiotic state, in which the microbial composition is altered and the microecological balance between host and microorganisms is disturbed, can lead to oral and even systemic diseases. In this review, we discuss the current research on the composition of the oral microbiota, the factors influencing it, and its relationships with common oral diseases. We focus on the specificity of the microbiota at different niches in the oral cavity, the communities of the oral microbiome, the mycobiome, and the virome within oral biofilms, and interventions targeting oral pathogens associated with disease. With these data, we aim to extend our understanding of oral microorganisms and provide new ideas for the clinical management of infectious oral diseases.

In it together: Candida-bacterial oral biofilms and therapeutic strategies

Under natural environmental settings or in the human body, the majority of microorganisms exist in complex polymicrobial biofilms adhered to abiotic and biotic surfaces. These microorganisms exhibit symbiotic, mutualistic, synergistic, or antagonistic relationships with other species during biofilm colonization and development. These polymicrobial interactions are heterogeneous, complex and hard to control, thereby often yielding worse outcomes than monospecies infections. Concerning fungi, Candida spp., in particular, Candida albicans is often detected with various bacterial species in oral biofilms. These Candida-bacterial interactions may induce the transition of C. albicans from commensal to pathobiont or dysbiotic organism. Consequently, Candida-bacterial interactions are largely associated with various oral diseases, including dental caries, denture stomatitis, periodontitis, peri-implantitis, and oral cancer. Given the severity of oral diseases caused by cross-kingdom consortia that develop hard-to-remove and highly drug-resistant biofilms, fundamental research is warranted to strategically develop cost-effective and safe therapies to prevent and treat cross-kingdom interactions and subsequent biofilm development. While studies have shed some light, targeting fungal-involved polymicrobial biofilms has been limited. This mini-review outlines the key features of Candida-bacterial interactions and their impact on various oral diseases. In addition, current knowledge on therapeutic strategies to target Candida-bacterial polymicrobial biofilms is discussed.

Candida albicans and Early Childhood Caries

Early childhood caries (ECC) is a highly prevalent and costly chronic oral infectious disease in preschool children. Candida albicans has been frequently detected in children and has demonstrated cariogenic traits. However, since ECC is a multifactorial infectious disease with many predisposing non-microbial factors, it remains to be elucidated whether the presence and accumulation of C. albicans in ECC is merely a consequence of the adaptation of C. albicans to a cariogenic oral environment, or it plays an active role in the initiation and progression of dental caries. This review aims to summarize the current knowledge on C. albicans and the risk of ECC, with a focus on its synergistic relationship with the cariogenic pathogen Streptococcus mutans. We also highlight recent advances in the development of approaches to disrupt C. albicans-S. mutans cross-kingdom biofilms in ECC prevention and treatment. Longitudinal clinical studies, including interventional clinical trials targeting C. albicans, are necessary to ascertain if C. albicans indeed contributes in a significant manner to the initiation and progression of ECC. In addition, further work is needed to understand the influence of other bacteria and fungi of oral microbiota on C. albicans-S. mutans interactions in ECC.

Recent Advances and Opportunities in the Study of Candida albicans Polymicrobial Biofilms

It is well known that the opportunistic pathogenic yeast, Candida albicans, can form polymicrobial biofilms with a variety of bacteria, both in vitro and in vivo, and that these polymicrobial biofilms can impact the course and management of disease. Although specific interactions are often described as either synergistic or antagonistic, this may be an oversimplification. Polymicrobial biofilms are complex two-way interacting communities, regulated by inter-domain (inter-kingdom) signaling and various molecular mechanisms. This review article will highlight advances over the last six years (2016-2021) regarding the unique biology of polymicrobial biofilms formed by C. albicans and bacteria, including regulation of their formation. In addition, some of the consequences of these interactions, such as the influence of co-existence on antimicrobial susceptibility and virulence, will be discussed. Since the aim of this knowledge is to inform possible alternative treatment options, recent studies on the discovery of novel anti-biofilm compounds will also be included. Throughout, an attempt will be made to identify ongoing challenges in this area.

OUP accepted manuscript

This proof-of-principle study aims to find commensal oral bacteria that can produce extracellular polymeric substances (EPS), which have similar lubrication properties to saliva and could serve as saliva substitutes. Saliva and plaque samples were collected from 21 generally healthy individuals. Primary screening was done by conventional culturing and Gram-staining; all species selected for further analysis were identified by MALDI-TOF and deposited in DSMZ. Lactobacillus gasseri (DSM32453 and DSM32455), Lactobacillus rhamnosus (DSM32452), Lactobacillus paracasei (DSM32454), and Streptococcus sanguinis (DSM32456) produced 413.6, 415.7, 431.1, 426.8, and 877.6 µg/ml of EPS, respectively. At the same time calcium dissolution could not be detected for both L. gasseri strains, minimal dissolution for the other three: S. sanguinis 0.3 mm, and 3.7 mm for L. rhamnosus and L. paracasei. There were no differences found between the EPS samples and the saliva for the effect of shear rate on the viscosity and for the effect of sliding speed on lubrication properties. In conclusion, five commensal bacterial strains have been isolated, all able to produce EPS and lead to no or to low calcium dissolution. EPS produced exhibits rheological and tribological properties comparable to human saliva. A total of four out of five selected strains are probiotic and, therefore, may exhibit additional beneficial influence within the oral cavity.

Effects of Bleaching Associated with Er:YAG and Nd:YAG Laser on Enamel Structure and Bacterial Biofilm Formation

OBJECTIVE

To compare the effects of bleaching associated with Er:YAG and Nd:YAG laser on enamel structure and mixed biofilm formation on teeth surfaces.

MATERIALS AND METHODS

Sixty-eight enamel samples were randomly divided into four groups (n = 17), control, Opalescence Boost only, Opalescence Boost plus Er: YAG laser, and Opalescence Boost plus Nd:YAG laser. The structure was observed using SEM after bleaching. Subsequently, the treated enamel samples were also cultured in suspensions of Streptococcus mutans, Streptococcus sanguis, Actinomyces viscosus, and Fusobacterium nucleatum (Fn) for 24 and 48 h. Biofilm formation was quantified by crystal violet staining, and the structure was visualized by confocal laser scanning microscopy. The data were analyzed using the Kruskal-Wallis method.

RESULTS

The enamel structure significantly changed after bleaching. There was no obvious difference in the biofilm formation after 24 h; however, after 48 hours, the amount of biofilm increased significantly. Remarkably, the amount was significantly higher on enamel bleached only, however, there was no significant difference between samples bleached with Er:YAG or Nd:YAG laser compared to the control.

CONCLUSIONS

Bleaching only appeared to markedly promote biofilm formation after 48 h, and the biofilms on samples bleached with Er:YAG or Nd:YAG laser did not change significantly, showing that bleaching with Er:YAG or Nd:YAG laser can be safely applied in clinical practice.

Cariogenic Biofilm: Pathology-Related Phenotypes and Targeted Therapy

The initiation and development of cariogenic (that is, caries-related) biofilms are the result of the disruption of homeostasis in the oral microenvironment. There is a daily accumulation of dental biofilm on the surface of teeth and its matrix of extracellular polymers supports the host in its defense against invading microbes, thus helping to achieve oral microbial homeostasis. However, the homeostasis can be broken down under certain circumstances such as during long-term exposure to a low pH environment which results in the dominance of acidogenic and acid-tolerating species in the dental biofilm and, thus, triggers the shift of harmless biofilm to an acidic one. This work aims to explore microbial diversity and the quorum sensing of dental biofilm and their important contributions to oral health and disease. The complex and multispecies ecosystems of the cariogenic biofilm pose significant challenges for the modulation of the oral microenvironment. Promising treatment strategies are those that target cariogenic niches with high specificity without disrupting the balance of the surrounding oral microbiota. Here, we summarized the recent advances in modulating cariogenic biofilm and/or controlling its pathogenic traits.

Anticaries activity of GERM CLEAN in Streptococcus mutans and Candida albicans dual-species biofilm

OBJECTIVE

To evaluate the antimicrobial effects of a peptide containing novel oral spray GERM CLEAN on dual-species biofilm formed by Streptococcus mutans and Candida albicans and to investigate whether GERM CLEAN inhibits the demineralization procedure of bovine enamel in vitro.

METHODS

The antimicrobial effects of GERM CLEAN on dual-species biofilm were analyzed by initial adherence rate calculation, water-insoluble exopolysaccharides quantification, total biomass quantification, and colony-forming units (CFUs) counting. Scanning electron microscopy and confocal laser scanning microscopy were applied to evaluate the impacts of GERM CLEAN on the biofilm structure. Further, the effects of GERM CLEAN on acidogenicity of dual-species were appraised via glycolytic pH drop analysis and hydroxyapatite dissolution measurement. The percentage of Surface Microhardness Reduction (%SMHR) evaluation, Atomic Force Micrograph (AFM) examination, and Transverse Microradiography (TMR) analysis after pH cycling were used to determine whether GERM CLEAN inhibited the demineralization of bovine enamel.

RESULTS

GERM CLEAN decreased the adherence rate, water-insoluble EPS production, biofilm formation, and acidogenicity of the dual-species. Moreover, GERM CLEAN significantly inhibited the demineralization status of bovine enamels.

CONCLUSION

This peptide containing novel oral spray GERM CLEAN has antimicrobial potential toward the dual-species. GERM CLEAN can also impede the demineralization procedure of enamel.

Candida biome of severe early childhood caries (S-ECC) and its cariogenic virulence traits

The protected niche of deep-caries lesions is a distinctive ecosystem. We assessed the Candida biome and its cariogenic traits from dentin samples of 50 children with severe-early childhood caries (S-ECC). Asymptomatic, primary molars belonging to International Caries Detection and Assessment-ICDAS caries-code 5 and 6 were analyzed, and C. albicans (10-isolates), C. tropicalis (10), C. krusei (10), and C. glabrata (5) isolated from the lesions were then evaluated for their biofilm formation, acidogenicity, and the production of secreted hydrolases: hemolysins, phospholipase, proteinase and DNase. Candida were isolated from 14/43 ICDAS-5 lesions (32.5%) and 44/57 ICDAS-6 lesions (77.2%). Compared to, ICDAS-5, a significantly higher frequency of multi-species infestation was observed in ICDAS-6 lesions (p=0.001). All four candidal species (above) showed prolific biofilm growth, and an equal potency for tooth demineralization. A significant interspecies difference in the mean phospholipase, as well as proteinase activity was noted (p < 0.05), with C. albicans being the predominant hydrolase producer. Further, a positive correlation between phospholipase and proteinase activity of Candida-isolates was noted (r = 0.818, p < 0.001). Our data suggest that candidal mycobiota with their potent cariogenic traits may significantly contribute to the development and progression of S-ECC.

Pathogenetic Impact of Bacterial-Fungal Interactions

Polymicrobial infections are of paramount importance because of the potential severity of clinical manifestations, often associated with increased resistance to antimicrobial treatment. The intricate interplay with the host and the immune system, and the impact on microbiome imbalance, are of importance in this context. The equilibrium of microbiota in the human host is critical for preventing potential dysbiosis and the ensuing development of disease. Bacteria and fungi can communicate via signaling molecules, and produce metabolites and toxins capable of modulating the immune response or altering the efficacy of treatment. Most of the bacterial–fungal interactions described to date focus on the human fungal pathogen Candida albicans and different bacteria. In this review, we discuss more than twenty different bacterial–fungal interactions involving several clinically important human pathogens. The interactions, which can be synergistic or antagonistic, both in vitro and in vivo, are addressed with a focus on the quorum-sensing molecules produced, the response of the immune system, and the impact on clinical outcome.