Biological Activities of Virgin Coconut and Virgin Olive Oil Mixture against Oral Primary Colonizers: An In Vitro Study.

AIM AND BACKGROUND: This study aimed to explore the potential synergistic interaction of virgin coconut oil (VCO) and virgin olive oil (VOO) mixture against Streptococcus sanguinis, Streptococcus mutans, and Lactobacillus casei in a single and mixture species through the minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), antiadherence, and antibiofilm activities. MATERIALS AND METHODS: The broth microdilution technique was used to individually determine the MIC of both oils and an oil mixture (in the ratio of 1:1) in a 96-well microtiter plate. As for the MBC, the subcultured method was used. The fractional inhibitory concentration index (ΣFIC) was determined to identify the interaction types between both oils. The oil mixture at its MIC was then tested on its antibiofilm and antiadherence effect. RESULTS: The MIC of the oil mixture against the tested microbiota was 50-100%. The oil mixture was bactericidal at 100% concentration for all the mentioned microbes except S. mutans. The ΣFIC value was 2 to 4, indicating that the VCO and VOO acted additively against the microbiota. Meanwhile, the oil mixture at MIC (50% for S. sanguinis and L. casei; 100% for S. mutans and mixture species) exhibited antiadherence and antibiofilm activity toward the microbiota in mixture species. CONCLUSION: The oil mixture possesses antibacterial, antibiofilm, and antiadherence properties toward the tested microbiota, mainly at 50-100% concentration of oil mixture. There was no synergistic interaction found between VCO and VOO. CLINICAL SIGNIFICANCE: Children and individuals with special care may benefit from using the oil mixture, primarily to regulate the biofilm formation and colonization of the bacteria. Furthermore, the oil mixture is natural and nontoxic compared to chemical-based oral healthcare products. How to cite this article: Ng YM, Sockalingam SNMP, Shafiei Z, et al. Biological Activities of Virgin Coconut and Virgin Olive Oil Mixture against Oral Primary Colonizers: An In Vitro Study. J Contemp Dent Pract 2024;25(3):260-266.

Growing a Cystic Fibrosis-Relevant Polymicrobial Biofilm to Probe Community Phenotypes.

Most in vitro models lack the capacity to fully probe bacterial phenotypes emerging from the complex interactions observed in real-life environments. This is particularly true in the context of hard-to-treat, chronic, and polymicrobial biofilm-based infections detected in the airways of individuals living with cystic fibrosis (CF), a multiorgan genetic disease. While multiple microbiome studies have defined the microbial compositions detected in the airway of people with CF (pwCF), no in vitro models thus far have fully integrated critical CF-relevant lung features. Therefore, a significant knowledge gap exists in the capacity to investigate the mechanisms driving the pathogenesis of mixed species CF lung infections. Here, we describe a recently developed four-species microbial community model, including Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus sanguinis, and Prevotella melaninogenica grown in CF-like conditions. Through the utilization of this system, clinically relevant phenotypes such as antimicrobial recalcitrance of several pathogens were observed and explored at the molecular level. The usefulness of this in vitro model resides in its standardized workflow that can facilitate the study of interspecies interactions in the context of chronic CF lung infections.

Functional Analysis of the Major Pilin Proteins of Type IV Pili in Streptococcus sanguinis CGMH010.

The pil gene cluster for Type IV pilus (Tfp) biosynthesis is commonly present and highly conserved in Streptococcus sanguinis. Nevertheless, Tfp-mediated twitching motility is less common among strains, and the factors determining twitching activity are not fully understood. Here, we analyzed the functions of three major pilin proteins (PilA1, PilA2, and PilA3) in the assembly and activity of Tfp in motile S. sanguinis CGMH010. Using various recombinant pilA deletion strains, we found that Tfp composed of different PilA proteins varied morphologically and functionally. Among the three PilA proteins, PilA1 was most critical in the assembly of twitching-active Tfp, and recombinant strains expressing motility generated more structured biofilms under constant shearing forces compared to the non-motile recombinant strains. Although PilA1 and PilA3 shared 94% identity, PilA3 could not compensate for the loss of PilA1, suggesting that the nature of PilA proteins plays an essential role in twitching activity. The single deletion of individual pilA genes had little effect on the invasion of host endothelia by S. sanguinis CGMH010. In contrast, the deletion of all three pilA genes or pilT, encoding the retraction ATPase, abolished Tfp-mediated invasion. Tfp- and PilT-dependent invasion were also detected in the non-motile S. sanguinis SK36, and thus, the retraction of Tfp, but not active twitching, was found to be essential for invasion.

Mechanical and microbiological properties of orthodontic resin modified with nanostructured silver vanadate decorated with silver nanoparticles (ßAgVO3).

OBJECTIVE: To investigate the impact of incorporating the antimicrobial nanomaterial ß-AgVO3 into orthodontic resin, focusing on degree of conversion, surface characteristics, microhardness, adhesion properties, and antimicrobial activity. METHODS: The 3 M Transbond XT resin underwent modification, resulting in three groups (Control, 2.5% addition, 5% addition) with 20 specimens each. Fourier transform infrared spectroscopy assessed monomer conversion. Laser confocal microscopy examined surface roughness, and microhardness was evaluated using Knoop protocols. Shear strength was measured before and after artificial aging on 36 premolar teeth. Microbiological analysis against S. mutans and S. sanguinis was conducted using the agar diffusion method. RESULTS: Degree of conversion remained unaffected by time (P = 0.797), concentration (P = 0.438), or their interaction (P = 0.187). The 5% group exhibited the lowest surface roughness, differing significantly from the control group (P = 0.045). Microhardness showed no significant differences between concentrations (P = 0.740). Shear strength was highest in the control group (P < 0.001). No significant differences were observed in the samples with or without thermocycling (P = 0.759). Microbial analysis revealed concentration-dependent variations, with the 5% group exhibiting the largest inhibition halo (P < 0.001). CONCLUSIONS: Incorporating ß-AgVO3 at 2.5% and 5% concentrations led to significant differences in surface roughness, adhesion, and antimicrobial activity. Overall, resin modification positively impacted degree of conversion, surface characteristics, microhardness, and antimicrobial activity. Further research is warranted to determine clinically optimal concentrations that maximize antimicrobial benefits while minimizing adverse effects on adhesion properties. CLINICAL SIGNIFICANCE: Incorporating ß-AgVO3 into orthodontic resin could improve patient quality of life by prolonging intervention durability and reducing the impact of cariogenic microorganisms. The study's findings also hold promise for the industry, paving the way for the development of new materials with antimicrobial properties for potential applications in the health sector.

Systematic functional analysis of the Com pilus in Streptococcus sanguinis: a minimalistic type 4 filament dedicated to DNA uptake in monoderm bacteria.

IMPORTANCE: Type 4 filaments (T4F) are nanomachines ubiquitous in prokaryotes, centered on filamentous polymers of type 4 pilins. T4F are exceptionally versatile and widespread virulence factors in bacterial pathogens. The mechanisms of filament assembly and the many functions they facilitate remain poorly understood because of the complexity of T4F machineries. This hinders the development of anti-T4F drugs. The significance of our research lies in characterizing the simplest known T4F-the Com pilus that mediates DNA uptake in competent monoderm bacteria-and showing that four protein components universally conserved in T4F are sufficient for filament assembly. The Com pilus becomes a model for elucidating the mechanisms of T4F assembly.

Infective Endocarditis Caused by Streptococcus sanguinis Resulting in Stroke, Ruptured Infected Pseudoaneurysm of Superior Mesenteric Artery, and Rapidly Progressive Glomerulonephritis.

A 71-year-old-man was admitted to our hospital with a cerebral embolism and diagnosed with infective endocarditis (IE) caused by Streptococcus sanguinis. Mitral valve replacement was performed. About one month later, he experienced sudden abdominal pain and shock due to a ruptured infected mesenteric artery pseudoaneurysm. Forty-four days after abdominal surgery, he presented with rapidly progressive glomerulonephritis with anti-glomerular basement membrane antibodies. He was treated with plasma exchange and prednisolone, and his renal function gradually improved. Since postoperative complications often occur within a few years after surgery for IE, careful follow-up is important, even after antimicrobial therapy and valve surgery.

Noncanonical Streptococcus sanguinis ComCDE circuitry integrates environmental cues in transformation outcome decision.

Natural competence is the principal driver of streptococcal evolution. While acquisition of new traits could facilitate rapid fitness improvement for bacteria, entry into the competent state is a highly orchestrated event, involving an interplay between various pathways. We present a new type of competence-predation coordination mechanism in Streptococcus sanguinis. Unlike other streptococci that mediate competence through the ComABCDE regulon, several key components are missing in the S. sanguinis ComCDE circuitry. We assembled two synthetic biology devices linking competence-stimulating peptide (CSP) cleavage and export with a quantifiable readout to unravel the unique features of the S. sanguinis circuitry. Our results revealed the ComC precursor cleavage pattern and the two host ABC transporters implicated in the export of the S. sanguinis CSP. Moreover, we discovered a ComCDE-dependent bacteriocin locus. Overall, this study presents a mechanism for commensal streptococci to maximize transformation outcome in a fluid environment through extensive circuitry rewiring.

The effect of bovine trypsin on dental biofilm dispersion: an in vitro study.

To investigate the degradation effect of bovine trypsin on multispecies biofilm of caries-related bacteria and provide an experimental foundation for the prevention of dental caries. Standard strains of S. mutans, S. sanguis, S. gordonii, and L. acidophilus were co-cultured to form 24 h, 48 h, and 72 h biofilms. The experimental groups were treated with bovine trypsin for 30 s, 1 min, and 3 min. Morphological observation and quantitative analysis of extracellular polymeric substances (EPS), live bacteria, and dead bacteria were conducted using the confocal laser scanning microscope (CLSM). The morphological changes of EPS and bacteria were also observed using a scanning electron microscope (SEM). When biofilm was treated for 1 min, the minimal inhibitory concentration (MIC) of bovine trypsin to reduce EPS was 0.5 mg/mL in 24 h and 48 h biofilms, and the MIC of bovine trypsin was 2.5 mg/mL in 72 h biofilms (P < 0.05). When biofilm was treated for 3 min, the MIC of bovine trypsin to reduce EPS was 0.25 mg/mL in 24 h and 48 h biofilms, the MIC of bovine trypsin was 1 mg/mL in 72 h biofilm (P < 0.05). The ratio of live-to-dead bacteria in the treatment group was significantly lower than blank group in 24 h, 48 h, and 72 h multispecies biofilms (P < 0.05). Bovine trypsin can destroy multispecies biofilm structure, disperse biofilm and bacteria flora, and reduce the EPS and bacterial biomass in vitro, which are positively correlated with the application time and concentration.

Amyloid Fibrils Produced by Streptococcus sanguinis Contribute to Biofilm Formation and Immune Evasion.

Bacterial surface proteins assembled into amyloids contribute to biofilm formation and host immune evasion. Streptococcus sanguinis, a pioneer colonizer of teeth commonly involved in cardiovascular infections, expresses about thirty-three proteins anchored to the cell wall by sortase A. Here, we characterized the production of amyloid in S. sanguinis strains differing in biofilm and immune evasion phenotypes and investigated the role of sortase A in amyloidogenesis. Amyloid was identified in biofilms formed by nine strains, using Congo red (CR) staining and cross-polarized light microscopy. Additionally, EGCG, an amyloid inhibitor, impaired biofilm maturation in a strain-specific fashion. The amounts of amyloid-like components quantified in culture fluids of nine strains using thioflavin T and fluorimetry negatively correlated with bacterial binding to complement-activating proteins (SAP, C1q), C3b deposition and rates of opsonophagocytosis in PMNs, implying amyloid production in immune evasion. The deletion of the sortase A gene (srtA) in strain SK36 compromised amyloid production and sucrose-independent biofilm maturation. The srtA mutant further showed increased susceptibility to C3b deposition and altered interactions with PMNs as well as reduced persistence in human blood. These findings highlight the contribution of amyloids to biofilm formation and host immune evasion in S. sanguinis strains, further indicating the participation of sortase A substrates in amyloidogenesis.

Spatiotemporal monitoring of a periodontal multispecies biofilm model: demonstration of prebiotic treatment responses.

Biofilms are complex polymicrobial communities which are often associated with human infections such as the oral disease periodontitis. Studying these complex communities under controlled conditions requires in vitro biofilm model systems that mimic the natural environment as close as possible. This study established a multispecies periodontal model in the drip flow biofilm reactor in order to mimic the continuous flow of nutrients at the air-liquid interface in the oral cavity. The design is engineered to enable real-time characterization. A community of five bacteria, Streptococcus gordonii-GFPmut3*, Streptococcus oralis-GFPmut3*, Streptococcus sanguinis-pVMCherry, Fusobacterium nucleatum, and Porphyromonas gingivalis-SNAP26 is visualized using two distinct fluorescent proteins and the SNAP-tag. The biofilm in the reactor develops into a heterogeneous, spatially uniform, dense, and metabolically active biofilm with relative cell abundances similar to those in a healthy individual. Metabolic activity, structural features, and bacterial composition of the biofilm remain stable from 3 to 6 days. As a proof of concept for our periodontal model, the 3 days developed biofilm is exposed to a prebiotic treatment with L-arginine. Multifaceted effects of L-arginine on the oral biofilm were validated by this model setup. L-arginine showed to inhibit growth and incorporation of the pathogenic species and to reduce biofilm thickness and volume. Additionally, L-arginine is metabolized by Streptococcus gordonii-GFPmut3* and Streptococcus sanguinis-pVMCherry, producing high levels of ornithine and ammonium in the biofilm. In conclusion, our drip flow reactor setup is promising in studying spatiotemporal behavior of a multispecies periodontal community.ImportancePeriodontitis is a multifactorial chronic inflammatory disease in the oral cavity associated with the accumulation of microorganisms in a biofilm. Not the presence of the biofilm as such, but changes in the microbiota (i.e., dysbiosis) drive the development of periodontitis, resulting in the destruction of tooth-supporting tissues. In this respect, novel treatment approaches focus on maintaining the health-associated homeostasis of the resident oral microbiota. To get insight in dynamic biofilm responses, our research presents the establishment of a periodontal biofilm model including Streptococcus gordonii, Streptococcus oralis, Streptococcus sanguinis, Fusobacterium nucleatum, and Porphyromonas gingivalis. The added value of the model setup is the combination of simulating continuously changing natural mouth conditions with spatiotemporal biofilm profiling using non-destructive characterization tools. These applications are limited for periodontal biofilm research and would contribute in understanding treatment mechanisms, short- or long-term exposure effects, the adaptation potential of the biofilm and thus treatment strategies.

Antibacterial Activity of a Lysin LysP53 against Streptococcus mutans.

Dental caries is a common disease affecting quality of life globally. In the present study, we found that a bacteriophage lysin LysP53 against Acinetobacter baumannii possesses selective activity on Streptococcus mutans, the main etiological agent of dental caries, even in low pH caries microenvironments, whereas only minor LysP53 activity was detected against Streptococcus sanguinis, Streptococcus oralis, and Streptococcus mitis. Testing activity against S. mutans planktonic cells showed that 4 µM LysP53 could kill more than 84% of S. mutans within 1 min in buffer with optimal pHs ranging from 4.0 to 6.5. Daily application of LysP53 on biofilms formed in BHI medium supplemented or not with sucrose could reduce exopolysaccharides, expression of genes related to acid resistance and adhesion, and the number of live bacteria in the biofilms. LysP53 treatment also showed similar effects as 0.12% chlorhexidine in preventing enamel demineralization due to S. mutans biofilms, as well as effective removal of S. mutans colonization of tooth surfaces in mice without observed toxic effects. Because of its selective activity against main cariogenic bacteria and good activity in low pH caries microenvironments, it is advantageous to use LysP53 as an active agent for preventing caries.

PepO and CppA modulate Streptococcus sanguinis susceptibility to complement immunity and virulence.

Streptococcus sanguinis is a ubiquitous commensal species of the oral cavity commonly involved as an opportunistic pathogen in cardiovascular infections. In this study, we investigated the functions of endopeptidase O (PepO) and a C3-degrading protease (CppA) in the systemic virulence of S. sanguinis. Isogenic mutants of pepO and cppA obtained in strain SK36 showed increased susceptibility to C3b deposition and to opsonophagocytosis by human polymorphonuclear neutrophils (PMN). These mutants differ, however, in their profiles of binding to serum amyloid P component (SAP) and C1q, whereas both showed reduced interaction with C4b-binding protein (C4BP) and/or factor H (FH) regulators as compared to SK36. The two mutants showed defects in ex vivo persistence in human blood, serum-mediated invasion of HCAEC endothelial cells, and virulence in a Galleria mellonella infection model. The transcriptional activities of pepO and cppA, assessed by RT-qPCR in nine wild-type strains, further indicated strain-specific profiles of pepO/cppA expression. Moreover, non-conserved amino acid substitutions were detected among the strains, mostly in CppA. Phylogenetic comparisons with homologues of streptococcal species of the oral and oropharyngeal sites suggested that S. sanguinis PepO and CppA have independent ancestralities. Thus, this study showed that PepO and CppA are complement evasion proteins expressed by S. sanguinis in a strain-specific manner, which are required for multiple functions associated with cardiovascular virulence.

Crystal structure of the pilus-specific sortase from early colonizing oral Streptococcus sanguinis captures an active open-lid conformation.

Dental plaque is a complex microbial biofilm community of many species and a major cause of oral infections and infectious endocarditis. Plaque development begins when primary colonizers attach to oral tissues and undergo coaggregation. Primary colonizers facilitate cellular attachment and inter-bacterial interactions through sortase-dependent pili (or fimbriae) extending out from their cell surface. Consequently, the sortase enzyme is viewed as a potential drug target for controlling biofilm formation and avoiding infection. Streptococcus sanguinis is a primary colonizing bacterium whose pili consist of three different pilin subunits that are assembled together by the pilus-specific (C-type) SsaSrtC sortase. Here, we report on the crystal structure determination of the recombinant wild-type and active-site mutant forms of SsaSrtC. Interestingly, the SsaSrtC structure exhibits an open-lid conformation, although a conserved DPX motif is lacking in the lid. Based on molecular docking and structural analysis, we identified the substrate-binding residues essential for pilin recognition and pilus assembly. We also demonstrated that while recombinant SsaSrtC is enzymatically active toward the five-residue LPNTG sorting motif peptide of the pilins, this activity is significantly reduced by the presence of zinc. We further showed that rutin and α-crocin are potential candidate inhibitors of the SsaSrtC sortase via structure-based virtual screening and inhibition assays. The structural knowledge gained from our study will provide the means to develop new approaches that target pilus-mediated attachment, thereby preventing oral biofilm growth and infection.

Heavy Ion Radiation Directly Induced the Shift of Oral Microbiota and Increased the Cariogenicity of Streptococcus mutans.

Radiation caries is one of the most common complications of head and neck radiotherapy. A shift in the oral microbiota is the main factor of radiation caries. A new form of biosafe radiation, heavy ion radiation, is increasingly being applied in clinical treatment due to its superior depth-dose distribution and biological effects. However, how heavy ion radiation directly impacts the oral microbiota and the progress of radiation caries are unknown. Here, unstimulated saliva samples from both healthy and caries volunteers and caries-related bacteria were directly exposed to therapeutic doses of heavy ion radiation to determine the effects of radiation on oral microbiota composition and bacterial cariogenicity. Heavy ion radiation significantly decreased the richness and diversity of oral microbiota from both healthy and caries volunteers, and a higher percentage of Streptococcus was detected in radiation groups. In addition, heavy ion radiation significantly enhanced the cariogenicity of saliva-derived biofilms, including the ratios of the genus Streptococcus and biofilm formation. In the Streptococcus mutans-Streptococcus sanguinis dual-species biofilms, heavy ion radiation increased the ratio of S. mutans. Next, S. mutans was directly exposed to heavy ions, and the radiation significantly upregulated the gtfC and gtfD cariogenic virulence genes to enhance the biofilm formation and exopolysaccharides synthesis of S. mutans. Our study demonstrated, for the first time, that direct exposure to heavy ion radiation can disrupt the oral microbial diversity and balance of dual-species biofilms by increasing the virulence of S. mutans, increasing its cariogenicity, indicating a potential correlation between heavy ions and radiation caries. IMPORTANCE The oral microbiome is crucial to understanding the pathogenesis of radiation caries. Although heavy ion radiation has been used to treat head and neck cancers in some proton therapy centers, its correlation with dental caries, especially its direct effects on the oral microbiome and cariogenic pathogens, has not been reported previously. Here, we showed that the heavy ion radiation directly shifted the oral microbiota from a balanced state to a caries-associated state by increasing the cariogenic virulence of S. mutans. Our study highlighted the direct effect of heavy ion radiation on oral microbiota and the cariogenicity of oral microbes for the first time.

Environmental influences on Streptococcus sanguinis membrane vesicle biogenesis.

Membrane vesicles are produced by Gram-negative and Gram-positive bacteria. While membrane vesicles are potent elicitors of eukaryotic cells and involved in cell-cell communication, information is scarce about their general biology in the context of community members and the environment. Streptococcus sanguinis, a Gram-positive oral commensal, is prevalent in the oral cavity and well-characterized for its ability to antagonize oral pathobionts. We have found that production and dissemination of membrane vesicles by S. sanguinis is dependent on environmental and community factors. Co-culture with interacting commensal Corynebacterium durum, as well as with the periodontal pathobiont Filifactor alocis had no effect on S. sanguinis vesicle number and size, whereas the periodontal pathobiont Porphyromonas gingivalis abolished S. sanguinis vesicle production. Using both correlation and differential expression analyses to examine the transcriptomic changes underlying vesicle production, we found that differential expression of genes encoding proteins related to the cytoplasmic membrane and peptidoglycan correlate with the abundance of membrane vesicles. Proteomic characterizations of the vesicle cargo identified a variety of proteins, including those predicted to influence host interactions or host immune responses. Cell culture studies of gingival epithelial cells demonstrated that both crude and highly purified membrane vesicles could induce the expression of IL-8, TNF-α, IL-1ß, and Gro-α within 6 hours of inoculation at levels comparable to whole cells. Our findings suggest that production of membrane vesicles by S. sanguinis is heavily influenced by community and environmental factors and plays an important role in communication with host cells.

Glycerol metabolism supports oral commensal interactions.

During oral biofilm development, interspecies interactions drive species distribution and biofilm architecture. To understand what molecular mechanisms determine these interactions, we used information gained from recent biogeographical investigations demonstrating an association of corynebacteria with streptococci. We previously reported that Streptococcus sanguinis and Corynebacterium durum have a close relationship through the production of membrane vesicle and fatty acids leading to S. sanguinis chain elongation and overall increased fitness supporting their commensal state. Here we present the molecular mechanisms of this interspecies interaction. Coculture experiments for transcriptomic analysis identified several differentially expressed genes in S. sanguinis. Due to its connection to fatty acid synthesis, we focused on the glycerol-operon. We further explored the differentially expressed type IV pili genes due to their connection to motility and biofilm adhesion. Gene inactivation of the glycerol kinase glpK had a profound impact on the ability of S. sanguinis to metabolize C. durum secreted glycerol and impaired chain elongation important for their interaction. Investigations on the effect of type IV pili revealed a reduction of S. sanguinis twitching motility in the presence of C. durum, which was caused by a decrease in type IV pili abundance on the surface of S. sanguinis as determined by SEM. In conclusion, we identified that the ability to metabolize C. durum produced glycerol is crucial for the interaction of C. durum and S. sanguinis. Reduced twitching motility could lead to a closer interaction of both species, supporting niche development in the oral cavity and potentially shaping symbiotic health-associated biofilm communities.

BacterAI maps microbial metabolism without prior knowledge.

Training artificial intelligence (AI) systems to perform autonomous experiments would vastly increase the throughput of microbiology; however, few microbes have large enough datasets for training such a system. In the present study, we introduce BacterAI, an automated science platform that maps microbial metabolism but requires no prior knowledge. BacterAI learns by converting scientific questions into simple games that it plays with laboratory robots. The agent then distils its findings into logical rules that can be interpreted by human scientists. We use BacterAI to learn the amino acid requirements for two oral streptococci: Streptococcus gordonii and Streptococcus sanguinis. We then show how transfer learning can accelerate BacterAI when investigating new environments or larger media with up to 39 ingredients. Scientific gameplay and BacterAI enable the unbiased, autonomous study of organisms for which no training data exist.

In Situ Raman Analysis of Biofilm Exopolysaccharides Formed in Streptococcus mutans and Streptococcus sanguinis Commensal Cultures.

This study probed in vitro the mechanisms of competition/coexistence between Streptococcus sanguinis (known for being correlated with health in the oral cavity) and Streptococcus mutans (responsible for aciduric oral environment and formation of caries) by means of quantitative Raman spectroscopy and imaging. In situ Raman assessments of live bacterial culture/coculture focusing on biofilm exopolysaccharides supported the hypothesis that both species engaged in antagonistic interactions. Experiments of simultaneous colonization always resulted in coexistence, but they also revealed fundamental alterations of the biofilm with respect to their water-insoluble glucan structure. Raman spectra (collected at fixed time but different bacterial ratios) showed clear changes in chemical bonds in glucans, which pointed to an action by Streptococcus sanguinis to discontinue the impermeability of the biofilm constructed by Streptococcus mutans. The concurrent effects of glycosidic bond cleavage in water-insoluble α - 1,3-glucan and oxidation at various sites in glucans' molecular chains supported the hypothesis that secretion of oxygen radicals was the main "chemical weapon" used by Streptococcus sanguinis in coculture.