Hydrogen peroxide-dependent DNA release and transfer of antibiotic resistance genes in Streptococcus gordonii. J Bacteriol. 2011;193:6912-6922. [PMID: 21984796 DOI: 10.1128/jb.05791-11]

Recent progress in understanding the role of bacterial extracellular DNA: focus on dental biofilm

Dental biofilm is a highly complicated and dynamic structure comprising not only microbial communities but also the surrounding matrix of extracellular polymeric substances (EPS), including polysaccharides, proteins, extracellular DNA (eDNA) and other biopolymers. In recent years, the important role of bacterial eDNA in dental biofilms has gradually attracted attention. In this review, we present recent studies on the presence, dynamic conformation and release of oral bacterial eDNA. Moreover, updated information on functions associated with oral bacterial eDNA in biofilm formation, antibiotic resistance, activation of the immune system and immune evasion is highlighted. Finally, we summarize the role of oral bacterial eDNA as a promising target for the treatment of oral diseases. Increasing insight into the versatile roles of bacterial eDNA in dental biofilms will facilitate the prevention and treatment of biofilm-induced oral infections.

Microbial interactions impact stress tolerance in a model oral community

Understanding the molecular mechanisms governing microbial interactions is crucial for unraveling the complexities of microbial communities and their ecological impacts. Here, we employed a two-species model system comprising the oral bacteria Aggregatibacter actinomycetemcomitans and Streptococcus gordonii to investigate how synergistic and antagonistic interactions between microbes impact their resilience to environmental change and invasion by other microbes. We used an in vitro colony biofilm model and focused on two S. gordonii-produced extracellular molecules, L-lactate and H2O2, which are known to impact fitness of this dual-species community. While the ability of A. actinomycetemcomitans to cross-feed on S. gordonii-produced L-lactate enhanced its fitness during co-culture, this function showed little impact on the ability of co-cultures to resist environmental change. In fact, the ability of A. actinomycetemcomitans to catabolize L-lactate may be detrimental in the presence of tetracycline, highlighting the complexity of interactions under antimicrobial stress. Furthermore, H2O2, known for its antimicrobial properties, had negative impacts on both species in our model system. However, H2O2 production by S. gordonii enhanced A. actinomycetemcomitans tolerance to tetracycline, suggesting a protective role under antibiotic pressure. Finally, S. gordonii significantly inhibited the bacterium Serratia marcescens from invading in vitro biofilms, but this inhibition was lost during co-culture with A. actinomycetemcomitans and in a murine abscess model, where S. gordonii actually promoted S. marcescens invasion. These data indicate that microbial interactions can impact fitness of a bacterial community upon exposure to stresses, but these impacts are highly environment dependent.

IMPORTANCE

Microbial interactions are critical modulators of the emergence of microbial communities and their functions. However, how these interactions impact the fitness of microbes in established communities upon exposure to environmental stresses is poorly understood. Here, we utilized a two-species community consisting of Aggregatibacter actinomycetemcomitans and Streptococcus gordonii to examine the impact of synergistic and antagonistic interactions on microbial resilience to environmental fluctuations and susceptibility to microbial invasion. We focused on the S. gordonii-produced extracellular molecules, L-lactate and H2O2, which have been shown to mediate interactions between these two microbes. We discovered that seemingly beneficial functions, such as A. actinomycetemcomitans cross-feeding on S. gordonii-produced L-Lactate, can paradoxically exacerbate vulnerabilities, such as susceptibility to antibiotics. Moreover, our data highlight the context-dependent nature of microbial interactions, emphasizing that a seemingly potent antimicrobial, such as H2O2, can have both synergistic and antagonistic effects on a microbial community dependent on the environment.

Competitive dynamics and balance between Streptococcus mutans and commensal streptococci in oral microecology

Dental caries, as a biofilm-related disease, is closely linked to dysbiosis in microbial ecology within dental biofilms. Beyond its impact on oral health, bacteria within the oral cavity pose systemic health risks by potentially entering the bloodstream, thereby increasing susceptibility to bacterial endocarditis, among other related diseases. Streptococcus mutans, a principal cariogenic bacterium, possesses virulence factors crucial to the pathogenesis of dental caries. Its ability to adhere to tooth surfaces, produce glucans for biofilm formation, and metabolize sugars into lactic acid contributes to enamel demineralization and the initiation of carious lesions. Its aciduricity and ability to produce bacteriocins enable a competitive advantage, allowing it to thrive in acidic environments and dominate in changing oral microenvironments. In contrast, commensal streptococci, such as Streptococcus sanguinis, Streptococcus gordonii, and Streptococcus salivarius, act as primary colonizers and compete with S. mutans for adherence sites and nutrients during biofilm formation. This competition involves the production of alkali, peroxides, and antibacterial substances, thereby inhibiting S. mutans growth and maintaining microbial balance. This dynamic interaction influences the balance of oral microbiota, with disruptions leading to shifts in microbial composition that are marked by rapid increases in S. mutans abundance, contributing to the onset of dental caries. Thus, understanding the dynamic interactions between commensal and pathogenic bacteria in oral microecology is important for developing effective strategies to promote oral health and prevent dental caries. This review highlights the roles and competitive interactions of commensal bacteria and S. mutans in oral microecology, emphasizing the importance of maintaining oral microbial balance for health, and discusses the pathological implications of perturbations in this balance.

Isolation and Characterization of Escherichia coli from Brazilian Broilers

Avian pathogenic Escherichia coli (APEC) causes colibacillosis, one of the main diseases leading to economic losses in industrial poultry farming due to high morbidity and mortality and its role in the condemnation of chicken carcasses. This study aimed to isolate and characterize APEC obtained from necropsied chickens on Brazilian poultry farms. Samples from birds already necropsied by routine inspection were collected from 100 batches of broiler chickens from six Brazilian states between August and November 2021. Three femurs were collected per batch, and characteristic E. coli colonies were isolated on MacConkey agar and characterized by qualitative PCR for minimal predictive APEC genes, antimicrobial susceptibility testing, and whole genome sequencing to identify species, serogroups, virulence genes, and resistance genes. Phenotypic resistance indices revealed significant resistance to several antibiotics from different antimicrobial classes. The isolates harbored virulence genes linked to APEC pathogenicity, including adhesion, iron acquisition, serum resistance, and toxins. Aminoglycoside resistance genes were detected in 79.36% of isolates, 74.6% had sulfonamide resistance genes, 63.49% showed β-lactam resistance genes, and 49.2% possessed at least one tetracycline resistance gene. This study found a 58% prevalence of avian pathogenic E. coli in Brazilian poultry, with strains showing notable antimicrobial resistance to commonly used antibiotics.

Long-term exposure to food-grade disinfectants causes cross-resistance to antibiotics in Salmonella enterica serovar Typhimurium strains with different antibiograms and sequence types

BACKGROUND

Disinfectants are important in the food industry to prevent the transmission of pathogens. Excessive use of disinfectants may increase the probability of bacteria experiencing long-term exposure and consequently resistance and cross-resistance to antibiotics. This study aims to investigate the cross-resistance of multidrug-resistant, drug-resistant, and drug-susceptible isolates of Salmonella enterica serovar Typhimurium (S. Typhimurium) with different sequence types (STs) to a group of antibiotics after exposure to different food-grade disinfectants.

METHODS

A panel of 27 S. Typhimurium strains with different antibiograms and STs were exposed to increasing concentrations of five food-grade disinfectants, including hydrogen peroxide (H2O2), benzalkonium chloride (BAC), chlorine dioxide (ClO2), sodium hypochlorite (NaClO), and ethanol. Recovered evolved strains were analyzed using genomic tools and phenotypic tests. Genetic mutations were screened using breseq pipeline and changes in resistance to antibiotics and to the same disinfectant were determined. The relative fitness of evolved strains was also determined.

RESULTS

Following exposure to disinfectants, 22 out of 135 evolved strains increased their resistance to antibiotics from a group of 14 clinically important antibiotics. The results also showed that 9 out of 135 evolved strains had decreased resistance to some antibiotics. Genetic mutations were found in evolved strains. A total of 77.78% of ST34, 58.33% of ST19, and 66.67% of the other STs strains exhibited changes in antibiotic resistance. BAC was the disinfectant that induced the highest number of strains to cross-resistance to antibiotics. Besides, H2O2 induced the highest number of strains with decreased resistance to antibiotics.

CONCLUSIONS

These findings provide a basis for understanding the effect of disinfectants on the antibiotic resistance of S. Typhimurium. This work highlights the link between long-term exposure to disinfectants and the evolution of resistance to antibiotics and provides evidence to promote the regulated use of disinfectants.

Oral polymicrobial communities: Assembly, function, and impact on diseases

Oral microbial communities assemble into complex spatial structures. The sophisticated physical and chemical signaling systems underlying the community enable their collective functional regulation as well as the ability to adapt by integrating environmental information. The combined output of community action, as shaped by both intra-community interactions and host and environmental variables, dictates homeostatic balance or dysbiotic disease such as periodontitis and dental caries. Oral polymicrobial dysbiosis also exerts systemic effects that adversely affect comorbidities, in part due to ectopic colonization of oral pathobionts in extra-oral tissues. Here, we review new and emerging concepts that explain the collective functional properties of oral polymicrobial communities and how these impact health and disease both locally and systemically.

Biofilm ecology associated with dental caries: Understanding of microbial interactions in oral communities leads to development of therapeutic strategies targeting cariogenic biofilms

A biofilm is a sessile community characterized by cells attached to the surface and organized into a complex structural arrangement. Dental caries is a biofilm-dependent oral disease caused by infection with cariogenic pathogens, such as Streptococcus mutans, and associated with frequent exposure to a sugar-rich diet and poor oral hygiene. The virulence of cariogenic biofilms is often associated with the spatial organization of S. mutans enmeshed with exopolysaccharides on tooth surfaces. However, in the oral cavity, S. mutans does not act alone, and several other microbes contribute to cariogenic biofilm formation. Microbial communities in cariogenic biofilms are spatially organized into complex structural arrangements of various microbes and extracellular matrices. The balance of microbiota diversity with reduced diversity and a high proportion of acidogenic-aciduric microbiota within the biofilm is closely related to the disease state. Understanding the characteristics of polymicrobial biofilms and the association of microbial interactions within the biofilm (e.g., symbiosis, cooperation, and competition) in terms of their potential role in the pathogenesis of oral disease would help develop new strategies for interventions in virulent biofilm formation.

Streptococcus mutans membrane vesicles inhibit the biofilm formation of Streptococcus gordonii and Streptococcus sanguinis

Streptococcus mutans, whose main virulence factor is glucosyltransferase (Gtf), has a substantial impact on the development of dental caries. S. mutans membrane vesicles (MVs), which are rich in Gtfs, have been shown to affect biofilm formation of other microorganisms. Streptococcus gordonii and Streptococcus sanguinis are initial colonizers of tooth surfaces, which provide attachment sites for subsequent microorganisms and are crucial in the development of oral biofilms. S. mutans and S. gordonii, as well as S. mutans and S. sanguinis, have a complex competitive and cooperative relationship, but it is unclear whether S. mutans MVs play a role in these interspecific interactions. Therefore, we co-cultured S. mutans MVs, having or lacking Gtfs, with S. gordonii and S. sanguinis. Our results showed that S. mutans MVs inhibited biofilm formation of S. gordonii and S. sanguinis but did not affect their planktonic growth; contrastingly, S. mutans ΔgtfBC mutant MVs had little effect on both their growth and biofilm formation. Additionally, there were fewer and more dispersed bacteria in the biofilms of the S. mutans MV-treated group than that in the control group. Furthermore, the expression levels of the biofilm-related virulence factors GtfG, GtfP, and SpxB in S. gordonii and S. sanguinis were significantly downregulated in response to S. mutans MVs. In conclusion, the results of our study showed that S. mutans MVs inhibited biofilm formation of S. gordonii and S. sanguinis, revealing an important role for MVs in interspecific interactions.

Oral Commensal Streptococci: Gatekeepers of the Oral Cavity

Oral commensal streptococci are primary colonizers of the oral cavity. These streptococci produce many adhesins, metabolites, and antimicrobials that modulate microbial succession and diversity within the oral cavity. Often, oral commensal streptococci antagonize cariogenic and periodontal pathogens such as Streptococcus mutans and Porphyromonas gingivalis, respectively. Mechanisms of antagonism are varied and range from the generation of hydrogen peroxide, competitive metabolite scavenging, the generation of reactive nitrogen intermediates, and bacteriocin production. Furthermore, several oral commensal streptococci have been shown to alter the host immune response at steady state and in response to oral pathogens. Collectively, these features highlight the remarkable ability of oral commensal streptococci to regulate the structure and function of the oral microbiome. In this review, we discuss mechanisms used by oral commensal streptococci to interact with diverse oral pathogens, both physically and through the production of antimicrobials. Finally, we conclude by exploring the critical roles of oral commensal streptococci in modulating the host immune response and maintaining health and homeostasis.

Quantification of Extracellular DNA Network Abundance and Architecture within Streptococcus gordonii Biofilms Reveals Modulatory Factors

Extracellular DNA (eDNA) is an important component of biofilm matrix that serves to maintain biofilm structural integrity, promotes genetic exchange within the biofilm, and provides protection against antimicrobial compounds. Advances in microscopy techniques have provided evidence of the cobweb- or lattice-like structures of eDNA within biofilms from a range of environmental niches. However, methods to reliably assess the abundance and architecture of eDNA remain lacking. This study aimed to address this gap by development of a novel, high-throughput image acquisition and analysis platform for assessment of eDNA networks in situ within biofilms. Utilizing Streptococcus gordonii as the model, the capacity for this imaging system to reliably detect eDNA networks and monitor changes in abundance and architecture (e.g., strand length and branch number) was verified. Evidence was provided of a synergy between glucans and eDNA matrices, while it was revealed that surface-bound nuclease SsnA could modify these eDNA structures under conditions permissive for enzymatic activity. Moreover, cross talk between the competence and hexaheptapeptide permease systems was shown to regulate eDNA release by S. gordonii. This novel imaging system can be applied across the wider field of biofilm research, with potential to significantly advance interrogation of the mechanisms by which the eDNA network architecture develops, how it can influence biofilm properties, and how it may be targeted for therapeutic benefit. IMPORTANCE Extracellular DNA (eDNA) is critical for maintaining the structural integrity of many microbial biofilms, making it an attractive target for the management of biofilms. However, our knowledge and targeting of eDNA are currently hindered by a lack of tools for the quantitative assessment of eDNA networks within biofilms. Here, we demonstrate use of a novel image acquisition and analysis platform with the capacity to reliably monitor the abundance and architecture of eDNA networks. Application of this tool to Streptococcus gordonii biofilms has provided new insights into how eDNA networks are stabilized within the biofilm and the pathways that can regulate eDNA release. This highlights how exploitation of this novel imaging system across the wider field of biofilm research has potential to significantly advance interrogation of the mechanisms by which the eDNA network architecture develops, how it can influence biofilm properties, and how it may be targeted for therapeutic benefit.

Understanding the Matrix: The Role of Extracellular DNA in Oral Biofilms

Dental plaque is the key etiological agent in caries formation and the development of the prevalent chronic oral inflammatory disease, periodontitis. The dental plaque biofilm comprises a diverse range of microbial species encased within a rich extracellular matrix, of which extracellular DNA (eDNA) has been identified as an important component. The molecular mechanisms of eDNA release and the structure of eDNA have yet to be fully characterized. Nonetheless, key functions that have been proposed for eDNA include maintaining biofilm structural integrity, initiating adhesion to dental surfaces, acting as a nutrient source, and facilitating horizontal gene transfer. Thus, eDNA is a potential therapeutic target for the management of oral disease–associated biofilm. This review aims to summarize advances in the understanding of the mechanisms of eDNA release from oral microorganisms and in the methods of eDNA detection and quantification within oral biofilms.

eDNA Inactivation and Biofilm Inhibition by the PolymericBiocide Polyhexamethylene Guanidine Hydrochloride (PHMG-Cl)

The choice of effective biocides used for routine hospital practice should consider the role of disinfectants in the maintenance and development of local resistome and how they might affect antibiotic resistance gene transfer within the hospital microbial population. Currently, there is little understanding of how different biocides contribute to eDNA release that may contribute to gene transfer and subsequent environmental retention. Here, we investigated how different biocides affect the release of eDNA from mature biofilms of two opportunistic model strains Pseudomonas aeruginosa ATCC 27853 (PA) and Staphylococcus aureus ATCC 25923 (SA) and contribute to the hospital resistome in the form of surface and water contaminants and dust particles. The effect of four groups of biocides, alcohols, hydrogen peroxide, quaternary ammonium compounds, and the polymeric biocide polyhexamethylene guanidine hydrochloride (PHMG-Cl), was evaluated using PA and SA biofilms. Most biocides, except for PHMG-Cl and 70% ethanol, caused substantial eDNA release, and PHMG-Cl was found to block biofilm development when used at concentrations of 0.5% and 0.1%. This might be associated with the formation of DNA–PHMG-Cl complexes as PHMG-Cl is predicted to bind to AT base pairs by molecular docking assays. PHMG-Cl was found to bind high-molecular DNA and plasmid DNA and continued to inactivate DNA on surfaces even after 4 weeks. PHMG-Cl also effectively inactivated biofilm-associated antibiotic resistance gene eDNA released by a pan-drug-resistant Klebsiella strain, which demonstrates the potential of a polymeric biocide as a new surface-active agent to combat the spread of antibiotic resistance in hospital settings.

Features of microbiocenosis and production of hydrogen peroxide by gum symbionts in adolescents with catarrhal gingivitis and chronic gastroduodenitis

Introduction. The key links in the etiology and pathogenesis of periodontal tissue diseases are the quantitative and qualitative changes in the composition of the microflora of the oral cavity, with the simultaneous deterioration of oral hygiene, and reduction of local and general immunity, which occurs more often in the presence of somatic diseases. Aims. The aim of the paper was to study the features of the microbiocenosis of periodontal tissues and the production of hydrogen peroxide by gum symbionts in adolescents with catarrhal gingivitis and chronic gastroduodenitis. Methods. The condition of the microbiocenosis of the gums of 83 adolescents from 12 to 18 years, which was divided into groups depending on the diagnosed catarrhal gingivitis and chronic gastroduodenitis, was studied. Bacteriological examination was performed to isolate pure cultures of microorganisms and to identify them according to generally accepted microbiological methods. The ability of the selected cultures to produce hydrogen peroxide was studied on an indicator medium with potassium-iodine-starch system, by the iodometric method. Results. The results of microbiological studies showed significant changes in qualitative and quantitative indicators of the microbiocenosis of the gingival mucosa in the affected area of patients with gingivitis, compared with dentally and somatically healthy individuals in the control group. In the group of clinically healthy adolescents, hydrogen peroxide producers were found on the mucous membrane of the gums only in 5.0 ± 1.15% of the examinees. In catarrhal gingivitis, hydrogen peroxide producers were found in 52.4 ± 2.4 % of the examined main group subjects (p < 0.01) and in the 50.0 ± 2.5 % of the examined comparison group subjects (p < 0.01). Conclusion. Among adolescents with catarrhal gingivitis, which occurs on the background of chronic gastroduodenitis, there were more pronounced quantitative and qualitative changes in the microbiocenosis of the gums. The hydrogen peroxide produced by them can act as an additional damaging factor in the pathogenesis of the inflammatory process of the gingival area.

Single DNase or Proteinase Treatment Induces Change in Composition and Structural Integrity of Multispecies Oral Biofilms

Biofilm virulence is mainly based on its bacterial cell surrounding biofilm matrix, which contains a scaffold of exopolysaccharides, carbohydrates, proteins, lipids, and nucleic acids. Targeting these nucleid acids or proteins could enable an efficient biofilm control. Therefore, the study aimed to test the effect of deoxyribonuclease I (DNase I) and proteinase K on oral biofilms. Six-species biofilms (Streptococcus mutans, Streptococcus oralis, Actinomyces oris, Fusobacterium nucleatum, Veillonella dispar, and Candida albicans) were exposed to DNase I (0.001 mg/mL, 0.002 mg/mL) or proteinase K (0.05 mg/mL, 0.1 mg/mL) for 1 h during biofilm formation. After 64 h, biofilms were harvested, quantified by culture analysis and visualized by image analysis using CLSM (confocal laser scanning microscopy). Statistical analysis was performed by ANOVA, followed by the Tukey test at a 5% significance level. The biofilm treatment with proteinase K induced a significant increase of Logs₁₀ counts in S. mutans and a decrease in C. albicans, while biofilm thickness was reduced from 28.5 μm (control) to 9.07 μm (0.05 mg/mL) and 7.4 μm (0.1 mg/mL). Treatment with DNase I had no effect on the total bacterial growth within the biofilm. Targeting proteins of biofilms by proteinase K are promising adjunctive tool for biofilm control.

Reactive Oxygen Species in Pathogen Clearance: The Killing Mechanisms, the Adaption Response, and the Side Effects

Reactive oxygen species (ROS) are attractive weapons in both antibiotic-mediated killing and host-mediated killing. However, the involvement of ROS in antibiotic-mediated killing and complexities in host environments challenge the paradigm. In the case of bacterial pathogens, the examples of some certain pathogens thriving under ROS conditions prompt us to focus on the adaption mechanism that pathogens evolve to cope with ROS. Based on these, we here summarized the mechanisms of ROS-mediated killing of either antibiotics or the host, the examples of bacterial adaption that successful pathogens evolved to defend or thrive under ROS conditions, and the potential side effects of ROS in pathogen clearance. A brief section for new antibacterial strategies centered around ROS was also addressed.

Magnesium-Dependent Promotion of H2O2 Production Increases Ecological Competitiveness of Oral Commensal Streptococci

The pyruvate oxidase (SpxB)-dependent production of H₂O₂ is widely distributed among oral commensal streptococci. Several studies confirmed the ability of H₂O₂ to antagonize susceptible oral bacterial species, including caries-associated Streptococcus mutans as well as several periodontal pathobionts. Here we report a potential mechanism to bolster oral commensal streptococcal H₂O₂ production by magnesium (Mg²⁺) supplementation. Magnesium is a cofactor for SpxB catalytic activity, and supplementation increases the production of H₂O₂ in vitro. We demonstrate that Mg²⁺ affects spxB transcription and SpxB abundance in Streptococcus sanguinis and Streptococcus gordonii. The competitiveness of low-passage commensal streptococcal clinical isolates is positively influenced in antagonism assays against S. mutans. In growth conditions normally selective for S. mutans, Mg²⁺ supplementation is able to increase the abundance of S. sanguinis in dual-species biofilms. Using an in vivo biophotonic imaging platform, we further demonstrate that dietary Mg²⁺ supplementation significantly improves S. gordonii oral colonization in mice. In summary, our results support a role for Mg²⁺ supplementation as a potential prebiotic to promote establishment of oral health-associated commensal streptococci.

Antibiofilm Activity of Polyamide 11 Modified with Thermally Stable Polymeric Biocide Polyhexamethylene Guanidine 2-Naphtalenesulfonate

The choice of efficient antimicrobial additives for polyamide resins is very difficult because of their high processing temperatures of up to 300 °C. In this study, a new, thermally stable polymeric biocide, polyhexamethylene guanidine 2-naphtalenesulfonate (PHMG-NS), was synthesised. According to thermogravimetric analysis, PHMG-NS has a thermal degradation point of 357 °C, confirming its potential use in joint melt processing with polyamide resins. Polyamide 11 (PA-11) films containing 5, 7 and 10 wt% of PHMG-NS were prepared by compression molding and subsequently characterised by FTIR spectroscopy. The surface properties were evaluated both by contact angle, and contactless induction. The incorporation of 10 wt% of PHMG-NS into PA-11 films was found to increase the positive surface charge density by almost two orders of magnitude. PA-11/PHMG-NS composites were found to have a thermal decomposition point at about 400 °C. Mechanical testing showed no change of the tensile strength of polyamide films containing PHMG-NS up to 7 wt%. Antibiofilm activity against the opportunistic bacteria Staphylococcus aureus and Escherichia coli was demonstrated for films containing 7 or 10 wt% of PHMG-NS, through a local biocide effect possibly based on an influence on the bacterial eDNA. The biocide hardly leached from the PA-11 matrix into water, at a rate of less than 1% from its total content for 21 days.

Genetics of sanguinis-Group Streptococci in Health and Disease

With the application of increasingly advanced "omics" technologies to the study of our resident oral microbiota, the presence of a defined, health-associated microbial community has been recognized. Within this community, sanguinis-group streptococci, comprising the closely related Streptococcus sanguinis and Streptococcus gordonii, together with Streptococcus parasanguinis, often predominate. Their ubiquitous and abundant nature reflects the evolution of these bacteria as highly effective colonizers of the oral cavity. Through interactions with host tissues and other microbes, and the capacity to readily adapt to prevailing environmental conditions, sanguinis-group streptococci are able to shape accretion of the oral plaque biofilm and promote development of a microbial community that exists in harmony with its host. Nonetheless, upon gaining access to the blood stream, those very same colonization capabilities can confer upon sanguinis-group streptococci the ability to promote systemic disease. This article focuses on the role of sanguinis-group streptococci as the commensurate commensals, highlighting those aspects of their biology that enable the coordination of health-associated biofilm development. This includes the molecular mechanisms, both synergistic and antagonistic, that underpin adhesion to substrata, intercellular communication, and polymicrobial community formation. As our knowledge of these processes advances, so will the opportunities to exploit this understanding for future development of novel strategies to control oral and extraoral disease.

Biocidal Agents Used for Disinfection Can Enhance Antibiotic Resistance in Gram-Negative Species

Biocidal agents used for disinfection are usually not suspected to enhance cross-resistance to antibiotics. The aim of this review was therefore to evaluate the effect of 13 biocidal agents at sublethal concentrations on antibiotic resistance in Gram-negative species. A medline search was performed for each biocidal agent on antibiotic tolerance, antibiotic resistance, horizontal gene transfer, and efflux pump. In cells adapted to benzalkonium chloride a new resistance was most frequently found to ampicillin (eight species), cefotaxime (six species), and sulfamethoxazole (three species), some of them with relevance for healthcare-associated infections such as Enterobacter cloacae or Escherichia coli. With chlorhexidine a new resistance was often found to ceftazidime, sulfamethoxazole and imipenem (eight species each) as well as cefotaxime and tetracycline (seven species each). Cross-resistance to antibiotics was also found with triclosan, octenidine, sodium hypochlorite, and didecyldimethylammonium chloride. No cross-resistance to antibiotics has been described after low level exposure to ethanol, propanol, peracetic acid, polyhexanide, povidone iodine, glutaraldehyde, and hydrogen peroxide. Taking into account that some biocidal agents used in disinfectants have no health benefit (e.g., in alcohol-based hand rubs) but may cause antibiotic resistance it is obvious to prefer products without them.

Live and let die: Hydrogen peroxide production by the commensal flora and its role in maintaining a symbiotic microbiome

The majority of commensal oral streptococci are able to generate hydrogen peroxide (H₂ O₂ ) during aerobic growth, which can diffuse through the cell membrane and inhibit competing species in close proximity. Competing H₂ O₂ production is mainly dependent upon the pyruvate oxidase SpxB, and to a lesser extent the lactate oxidase LctO, both of which are important for energy generation in aerobic environments. Several studies point to a broad impact of H₂ O₂ production in the oral environment, including a potential role in biofilm homeostasis, signaling, and interspecies interactions. Here, we summarize the current research regarding oral streptococcal H₂ O₂ generation, resistance mechanisms, and the ecological impact of H₂ O₂ production. We also discuss the potential therapeutic utility of H₂ O₂ for the prevention/treatment of dysbiotic diseases as well as its potential role as a biomarker of oral health.

Susceptibility patterns and the role of extracellular DNA in Staphylococcus epidermidis biofilm resistance to physico-chemical stress exposure

Background

Over 65% of human infections are ascribed to bacterial biofilms that are often highly resistant to antibiotics and host immunity. Staphylococcus epidermidis is the predominant cause of recurrent nosocomial and biofilm-related infections. However, the susceptibility patterns of S. epidermidis biofilms to physico-chemical stress induced by commonly recommended disinfectants [(heat, sodium chloride (NaCl), sodium hypochlorite (NaOCl) and hydrogen peroxide (H₂O₂)] in domestic and human healthcare settings remains largely unknown. Further, the molecular mechanisms of bacterial biofilms resistance to the physico-chemical stresses remain unclear. Growing evidence demonstrates that extracellular DNA (eDNA) protects bacterial biofilms against antibiotics. However, the role of eDNA as a potential mechanism underlying S. epidermidis biofilms resistance to physico-chemical stress exposure is yet to be understood. Therefore, this study aimed to evaluate the susceptibility patterns of and eDNA release by S. epidermidis biofilm and planktonic cells to physico-chemical stress exposure.

Results

S. epidermidis biofilms exposed to physico-chemical stress conditions commonly recommended for disinfection [heat (60 °C), 1.72 M NaCl, solution containing 150 μL of waterguard (0.178 M NaOCl) in 1 L of water or 1.77 M H₂O₂] for 30 and 60 min exhibited lower log reductions of CFU/mL than the corresponding planktonic cells (p < 0.0001). The eDNA released by sub-lethal heat (50 °C)-treated S. epidermidis biofilm and planktonic cells was not statistically different (p = 0.8501). However, 50 °C-treated S. epidermidis biofilm cells released significantly increased eDNA than the untreated controls (p = 0.0098). The eDNA released by 0.8 M NaCl-treated S. epidermidis biofilm and planktonic cells was not significantly different (p = 0.9697). Conversely, 5 mM NaOCl-treated S. epidermidis biofilms exhibited significantly increased eDNA release than the corresponding planktonic cells (p = 0.0015). Further, the 50 μM H₂O₂-treated S. epidermidis biofilms released significantly more eDNA than the corresponding planktonic cells (p = 0.021).

Conclusions

S. epidermidis biofilms were less susceptible to physico-chemical stress induced by the four commonly recommended disinfectants than the analogous planktonic cells. Further, S. epidermidis biofilms enhanced eDNA release in response to the sub-lethal heat and oxidative stress exposure than the corresponding planktonic cells suggesting a role of eDNA in biofilms resistance to the physico-chemical stresses.

Determination and identification of antibiotic-resistant oral streptococci isolated from active dental infections in adults

OBJECTIVE

To determine and identify antibiotic-resistant bacteria (ARB) of oral streptococci from active dental infections in adults and its association with age and gender.

MATERIAL AND METHODS

This cross-sectional study included 59 subjects from 18 to 62 years old. Ninety-eighth samples obtained from the subjects were cultivated in agar plates containing antibiotics amoxicillin/clavulanic acid (A-CA), clindamycin, and moxifloxacin (concentrations of 16, 32 or 64 µg/ml). PCR assay was performed to identify bacterial species.

RESULTS

The bacterial species that showed more antibiotic-resistance (AR) was S. mutans (45.9%), followed by S. gordonii (21.6%), S. oralis (17.6%), S. sanguinis (9.5%), S. salivarius (5.4%) and S. sobrinus (0%). Moreover, clindamycin (59.4%) showed the highest frequency of AR. Moxifloxacin and A-CA showed an susceptibility >99.1%, while clindamycin showed the lowest efficacy (93.3%); there was a significant statistically difference (p < .01). The age group between 26 and 50 years old (32.2%) and females (28.8%) showed more multiresistance. Clindamycin showed a statistical difference (p < .05) when comparing groups by gender.

CONCLUSIONS

Clindamycin was the antibiotic with the highest frequency of ARB and lower bactericidal effect. Moxifloxacin and A-CA showed the highest efficacy and the lowest ARB frequency. Streptococcus mutans was the bacterial specie that showed an increased frequency of AR.

Plasticity of the Pyruvate Node Modulates Hydrogen Peroxide Production and Acid Tolerance in Multiple Oral Streptococci

Commensal Streptococcus sanguinis and Streptococcus gordonii are pioneer oral biofilm colonizers. Characteristic for both is the SpxB-dependent production of H₂O₂, which is crucial for inhibiting competing biofilm members, especially the cariogenic species Streptococcus mutans H₂O₂ production is strongly affected by environmental conditions, but few mechanisms are known. Dental plaque pH is one of the key parameters dictating dental plaque ecology and ultimately oral health status. Therefore, the objective of the current study was to characterize the effects of environmental pH on H₂O₂ production by S. sanguinis and S. gordoniiS. sanguinis H₂O₂ production was not found to be affected by moderate changes in environmental pH, whereas S. gordonii H₂O₂ production declined markedly in response to lower pH. Further investigation into the pyruvate node, the central metabolic switch modulating H₂O₂ or lactic acid production, revealed increased lactic acid levels for S. gordonii at pH 6. The bias for lactic acid production at pH 6 resulted in concomitant improvement in the survival of S. gordonii at low pH and seems to constitute part of the acid tolerance response of S. gordonii Differential responses to pH similarly affect other oral streptococcal species, suggesting that the observed results are part of a larger phenomenon linking environmental pH, central metabolism, and the capacity to produce antagonistic amounts of H₂O₂IMPORTANCE Oral biofilms are subject to frequent and dramatic changes in pH. S. sanguinis and S. gordonii can compete with caries- and periodontitis-associated pathogens by generating H₂O₂ Therefore, it is crucial to understand how S. sanguinis and S. gordonii adapt to low pH and maintain their competitiveness under acid stress. The present study provides evidence that certain oral bacteria respond to environmental pH changes by tuning their metabolic output in favor of lactic acid production, to increase their acid survival, while others maintain their H₂O₂ production at a constant level. The differential control of H₂O₂ production provides important insights into the role of environmental conditions for growth competition of the oral flora.

Murein Hydrolase LytF of Streptococcus sanguinis and the Ecological Consequences of Competence Development

The overall health of the oral cavity is dependent on proper homeostasis between health-associated bacterial colonizers and bacteria known to promote dental caries. Streptococcus sanguinis is a health-associated commensal organism, a known early colonizer of the acquired tooth pellicle, and is naturally competent. We have shown that LytF, a competence-controlled murein hydrolase, is capable of inducing the release of extracellular DNA (eDNA) from oral bacteria. Precipitated LytF and purified LytF were used as treatments against planktonic cultures and biofilms. Larger amounts of eDNA were released from cultures treated with protein samples containing LytF. Additionally, LytF could affect biofilm formation and cellular morphology. Biofilm formation was significantly decreased in the lytF-complemented strain, in which increased amounts of LytF are present. The same strain also exhibited cell morphology defects in both planktonic cultures and biofilms. Furthermore, the LytF cell morphology phenotype was reproducible in wild-type cells using purified LytF protein. In sum, our findings demonstrate that LytF can induce the release of eDNA from oral bacteria, and they suggest that, without proper regulation of LytF, cells display morphological abnormalities that contribute to biofilm malformation. In the context of the oral biofilm, LytF may play important roles as part of the competence and biofilm development programs, as well as increasing the availability of eDNA.IMPORTANCEStreptococcus sanguinis, a commensal organism in the oral cavity and one of the pioneer colonizers of the tooth surface, is associated with the overall health of the oral environment. Our laboratory showed previously that, under aerobic conditions, S. sanguinis can produce H₂O₂ to inhibit the growth of bacterial species that promote dental caries. This production of H₂O₂ by S. sanguinis also induces the release of eDNA, which is essential for proper biofilm formation. Under anaerobic conditions, S. sanguinis does not produce H₂O₂ but DNA is still released. Determining how S. sanguinis releases DNA is thus essential to understand biofilm formation in the oral cavity.

Composition and immuno-stimulatory properties of extracellular DNA from mouse gut flora

AIM

To demonstrate that specific bacteria might release bacterial extracellular DNA (eDNA) to exert immunomodulatory functions in the mouse small intestine.

METHODS

Extracellular DNA was extracted using phosphate buffered saline with 0.5 mmol/L dithiothreitol combined with two phenol extractions. TOTO-1 iodide, a cell-impermeant and high-affinity nucleic acid stain, was used to confirm the existence of eDNA in the mucus layers of the small intestine and colon in healthy Male C57BL/6 mice. Composition difference of eDNA and intracellular DNA (iDNA) of the small intestinal mucus was studied by Illumina sequencing and terminal restriction fragment length polymorphism (T-RFLP). Stimulation of cytokine production by eDNA was studied in RAW264.7 cells in vitro.

RESULTS

TOTO-1 iodide staining confirmed existence of eDNA in loose mucus layer of the mouse colon and thin surface mucus layer of the small intestine. Illumina sequencing analysis and T-RFLP revealed that the composition of the eDNA in the small intestinal mucus was significantly different from that of the iDNA of the small intestinal mucus bacteria. Illumina Miseq sequencing showed that the eDNA sequences came mainly from Gram-negative bacteria of Bacteroidales S24-7. By contrast, predominant bacteria of the small intestinal flora comprised Gram-positive bacteria. Both eDNA and iDNA were added to native or lipopolysaccharide-stimulated Raw267.4 macrophages, respectively. The eDNA induced significantly lower tumor necrosis factor-α/interleukin-10 (IL-10) and IL-6/IL-10 ratios than iDNA, suggesting the predominance for maintaining immune homeostasis of the gut.

CONCLUSION

Our results indicated that degraded bacterial genomic DNA was mainly released by Gram-negative bacteria, especially Bacteroidales-S24-7 and Stenotrophomonas genus in gut mucus of mice. They decreased pro-inflammatory activity compared to total gut flora genomic DNA.

[The origin of hydrogen peroxide in oral cavity and its role in oral microecology balance]

Hydrogen peroxide, an important antimicrobial agent in oral cavity, plays a significant role in the balance of oral microecology. At the early stage of biofilm formation, about 80% of the detected initial colonizers belong to the genus Streptococcus. These oral streptococci use different oxidase to produce hydrogen peroxide. Recent studies showed that the produced hydrogen peroxide plays a critical role in modulating oral microecology. Hydrogen peroxide modulates biofilm development attributed to its growth inhibitory nature. Hydrogen peroxide production is closely associated with extracellular DNA(eDNA) release from microbe and the development of its competent cell which are critical for biofilm development and also serves as source for horizontal gene transfer. Microbe also can reduce the damage to themselves through several detoxification mechanisms. Moreover, hydrogen peroxide is also involved in the regulation of interactions between oral microorganisms and host. Taken together, hydrogen peroxide is an imperative ecological factor that contributes to the microbial equilibrium in the oral cavity. Here we will give a brief review of both the origin and the function in the oral microecology balance of hydrogen peroxide.

Extracellular DNA Contributes to Dental Biofilm Stability

Extracellular DNA (eDNA) is a major matrix component of many bacterial biofilms. While the presence of eDNA and its role in biofilm stability have been demonstrated for several laboratory biofilms of oral bacteria, there is no data available on the presence and function of eDNA in in vivo grown dental biofilms. This study aimed to determine whether eDNA was part of the matrix in biofilms grown in situ in the absence of sucrose and whether treatment with DNase dispersed biofilms grown for 2.5, 5, 7.5, 16.5, or 24 h. Three hundred biofilms from 10 study participants were collected and treated with either DNase or heat-inactivated DNase for 1 h. The bacterial biovolume was determined with digital image analysis. Staining with TOTO®-1 allowed visualization of eDNA both on bacterial cell surfaces and, with a cloud-like appearance, in the intercellular space. DNase treatment strongly reduced the amount of biofilm in very early stages of growth (up to 7.5 h), but the treatment effect decreased with increasing biofilm age. This study proves the involvement of eDNA in dental biofilm formation and its importance for biofilm stability in the earliest stages. Further research is required to uncover the interplay of eDNA and other matrix components and to explore the therapeutic potential of DNase treatment for biofilm control.

A Critical Role for Extracellular DNA in Dental Plaque Formation

Extracellular DNA (eDNA) has been identified in the matrix of many different monospecies biofilms in vitro, including some of those produced by oral bacteria. In many cases, eDNA stabilizes the structure of monospecies biofilms. Here, the authors aimed to determine whether eDNA is an important component of natural, mixed-species oral biofilms, such as plaque on natural teeth or dental implants. To visualize eDNA in oral biofilms, approaches for fluorescently stained eDNA with either anti-DNA antibodies or an ultrasensitive cell-impermeant dye, YOYO-1, were first developed using Enterococcus faecalis, an organism that has previously been shown to produce extensive eDNA structures within biofilms. Oral biofilms were modelled as in vitro "microcosms" on glass coverslips inoculated with the natural microbial population of human saliva and cultured statically in artificial saliva medium. Using antibodies and YOYO-1, eDNA was found to be distributed throughout microcosm biofilms, and was particularly abundant in the immediate vicinity of cells. Similar arrangements of eDNA were detected in biofilms on crowns and overdenture abutments of dental implants that had been recovered from patients during the restorative phase of treatment, and in subgingival dental plaque of periodontitis patients, indicating that eDNA is a common component of natural oral biofilms. In model oral biofilms, treatment with a DNA-degrading enzyme, NucB from Bacillus licheniformis, strongly inhibited the accumulation of biofilms. The bacterial species diversity was significantly reduced by treatment with NucB and particularly strong reductions were observed in the abundance of anaerobic, proteolytic bacteria such as Peptostreptococcus, Porphyromonas and Prevotella. Preformed biofilms were not significantly reduced by NucB treatment, indicating that eDNA is more important or more exposed during the early stages of biofilm formation. Overall, these data demonstrate that dental plaque eDNA is potentially an important target for oral biofilm control.

SpxB-mediated H2 O2 induces programmed cell death in Streptococcus sanguinis

Streptococcus sanguinis (S. sanguinis) is a commensal oral streptococci that produces hydrogen peroxide (H2 O2 ), and this production is dependent on pyruvate oxidase (SpxB) activity. In addition to its well-known role in intraspecies or interspecies competitions, recent studies have shown that H2 O2 produced by S. sanguinis under aerobic conditions not only upregulates biofilm formation and eDNA release but also regulates cell death without obvious cell lysis. Here, we report that S. sanguinis exhibits characteristic hallmarks of eukaryotic apoptosis when it encounters endogenous and exogenous H2 O2 . As the most common mode of programmed cell death (PCD), apoptosis is accompanied by a series of biochemical and morphological events, including DNA fragmentation, chromosome condensation, membrane potential depolarization, phosphatidylserine (PS) exposure, and caspase substrate binding protein activity changes. In addition, we also provide genetic evidence that there is decreased expression of the related DNA repair genes comEA, recA, dnaC, dinG, and pcrA in the wild-type compared to the isogenic spxB mutant in S. sanguinis. Our data suggest that endogenous H2 O2 is the most important agent in this development process in S. sanguinis.

Extracellular DNA in oral microbial biofilms

The extracellular matrix of microbial biofilms is critical for surface adhesion and nutrient homeostasis. Evidence is accumulating that extracellular DNA plays a number of important roles in biofilm integrity and formation on hard and soft tissues in the oral cavity. Here, we summarise recent developments in the field and consider the potential of targeting DNA for oral biofilm control.

[Effects of growth stages and pH value on the expression of autolytic enzyme atIS gene of Streptococcus gordonii]

OBJECTIVE

This study aimed to detect the difference in the expression levels of autolysin atIS gene of Streptococcus gordonii (S. gordonii) at different growth stages and pH values, as well as to analyze the factors regulating atlS gene expression in S. gordonii.

METHODS

S. gordonii wild strains (ATCC 35105) were collected at different growth stages (early exponential phase, mid-exponential phase, late exponential stage, and platform stage) and pH values (pH 7 and pH 5.5), and total RNA was extracted by using a conventional method. Fluorescence quantitative polymerase chain reaction (FQ-PCR) was used to measure the relative mRNA expression of atlS gene, with bacterial 16S rRNA as internal reference, for a comparison of the mRNA levels of atlS gene expression in S.gordonii at different growth stages and pH values.

RESULTS

FQ-PCR results showed that atlS gene expression increased with gradually increasing growth stage under neutral conditions and was higher than that under acidic conditions (P < 0.05).

CONCLUSION

The atlS gene expression in S. gordonii is influenced by growth stage and pH value factors.

Comparison of genes required for H2O2 resistance in Streptococcus gordonii and Streptococcus sanguinis

Hydrogen peroxide (H2O2) is produced by several members of the genus Streptococcus mainly through the pyruvate oxidase SpxB under aerobic growth conditions. The acute toxic nature of H2O2 raises the interesting question of how streptococci cope with intrinsically produced H2O2, which subsequently accumulates in the microenvironment and threatens the closely surrounding population. Here, we investigate the H2O2 susceptibility of oral Streptococcus gordonii and Streptococcus sanguinis and elucidate potential mechanisms of how they protect themselves from the deleterious effect of H2O2. Both organisms are considered primary colonizers and occupy the same intraoral niche making them potential targets for H2O2 produced by other species. We demonstrate that S. gordonii produces relatively more H2O2 and has a greater ability for resistance to H2O2 stress. Functional studies show that, unlike in Streptococcus pneumoniae, H2O2 resistance is not dependent on a functional SpxB and confirms the important role of the ferritin-like DNA-binding protein Dps. However, the observed increased H2O2 resistance of S. gordonii over S. sanguinis is likely to be caused by an oxidative stress protection machinery present even under anaerobic conditions, while S. sanguinis requires a longer period of time for adaptation. The ability to produce more H2O2 and be more resistant to H2O2 might aid S. gordonii in the competitive oral biofilm environment, since it is lower in abundance yet manages to survive quite efficiently in the oral biofilm.

The impact of horizontal gene transfer on the adaptive ability of the human oral microbiome

The oral microbiome is composed of a multitude of different species of bacteria, each capable of occupying one or more of the many different niches found within the human oral cavity. This community exhibits many types of complex interactions which enable it to colonize and rapidly respond to changes in the environment in which they live. One of these interactions is the transfer, or acquisition, of DNA within this environment, either from co-resident bacterial species or from exogenous sources. Horizontal gene transfer in the oral cavity gives some of the resident bacteria the opportunity to sample a truly enormous metagenome affording them considerable adaptive potential which may be key to survival in such a varying environment. In this review the underlying mechanisms of HGT are discussed in relation to the oral microbiome with numerous examples described where the direct acquisition of exogenous DNA has contributed to the fitness of the bacterial host within the human oral cavity.

Streptococcus mutans extracellular DNA is upregulated during growth in biofilms, actively released via membrane vesicles, and influenced by components of the protein secretion machinery

Streptococcus mutans, a major etiological agent of human dental caries, lives primarily on the tooth surface in biofilms. Limited information is available concerning the extracellular DNA (eDNA) as a scaffolding matrix in S. mutans biofilms. This study demonstrates that S. mutans produces eDNA by multiple avenues, including lysis-independent membrane vesicles. Unlike eDNAs from cell lysis that were abundant and mainly concentrated around broken cells or cell debris with floating open ends, eDNAs produced via the lysis-independent pathway appeared scattered but in a structured network under scanning electron microscopy. Compared to eDNA production of planktonic cultures, eDNA production in 5- and 24-h biofilms was increased by >3- and >1.6-fold, respectively. The addition of DNase I to growth medium significantly reduced biofilm formation. In an in vitro adherence assay, added chromosomal DNA alone had a limited effect on S. mutans adherence to saliva-coated hydroxylapatite beads, but in conjunction with glucans synthesized using purified glucosyltransferase B, the adherence was significantly enhanced. Deletion of sortase A, the transpeptidase that covalently couples multiple surface-associated proteins to the cell wall peptidoglycan, significantly reduced eDNA in both planktonic and biofilm cultures. Sortase A deficiency did not have a significant effect on membrane vesicle production; however, the protein profile of the mutant membrane vesicles was significantly altered, including reduction of adhesin P1 and glucan-binding proteins B and C. Relative to the wild type, deficiency of protein secretion and membrane protein insertion machinery components, including Ffh, YidC1, and YidC2, also caused significant reductions in eDNA.

Mechanisms of Candida biofilm drug resistance

Candida commonly adheres to implanted medical devices, growing as a resilient biofilm capable of withstanding extraordinarily high antifungal concentrations. As currently available antifungals have minimal activity against biofilms, new drugs to treat these recalcitrant infections are urgently needed. Recent investigations have begun to shed light on the mechanisms behind the profound resistance associated with the biofilm mode of growth. This resistance appears to be multifactorial, involving both mechanisms similar to conventional, planktonic antifungal resistance, such as increased efflux pump activity, as well as mechanisms specific to the biofilm lifestyle. A unique biofilm property is the production of an extracellular matrix. Two components of this material, β-glucan and extracellular DNA, promote biofilm resistance to multiple antifungals. Biofilm formation also engages several stress response pathways that impair the activity of azole drugs. Resistance within a biofilm is often heterogeneous, with the development of a subpopulation of resistant persister cells. In this article we review the molecular mechanisms underlying Candida biofilm antifungal resistance and their relative contributions during various growth phases.

Probing oral microbial functionality--expression of spxB in plaque samples

The Human Oral Microbiome Database (HOMD) provides an extensive collection of genome sequences from oral bacteria. The sequence information is a static snapshot of the microbial potential of the so far sequenced species. A major challenge is to connect the microbial potential encoded in the metagenome to an actual function in the in vivo oral biofilm. In the present study we took a reductionist approach and identified a considerably conserved metabolic gene, spxB to be encoded by a majority of oral streptococci using the HOMD metagenome information. spxB encodes the pyruvate oxidase responsible for the production of growth inhibiting amounts of hydrogen peroxide (H₂O₂) and has previously been shown as important in the interspecies competition in the oral biofilm. Here we demonstrate a strong correlation of H₂O₂ production and the presence of the spxB gene in dental plaque. Using Real-Time RT PCR we show that spxB is expressed in freshly isolated human plaque samples from several donors and that the expression is relative constant when followed over time in one individual. This is the first demonstration of an oral community encoded gene expressed in vivo suggesting a functional role of spxB in oral biofilm physiology. This also demonstrates a possible strategy to connect the microbial potential of the metagenome to its functionality in future studies by identifying similar highly conserved genes in the oral microbial community.

Is biofilm formation related to the hypermutator phenotype in clinical Enterobacteriaceae isolates?

In bacteria, complex adaptive processes are involved during transition from the planktonic to the biofilm mode of growth, and mutator strains are more prone to producing biofilms. Enterobacteriaceae species were isolated from urinary tract infections (UTIs; 222 strains) and from bloodstream infections (BSIs; 213 strains). Relationship between the hypermutable phenotype and biofilm forming capacity was investigated in these clinical strains. Mutation frequencies were estimated by monitoring the capacity of each strain to generate mutations that conferred rifampicin resistance on supplemented medium. Initiation of biofilm formation was assayed by determining the ability of the cells to adhere to a 96-well polystyrene microtitre plate. UTI Enterobacteriaceae strains showed significantly higher biofilm-forming capacity: 63.1% (54.0% for E. coli strains) vs. 42.3% for BSI strains (47.7% for E. coli). Strains isolated from UTIs did not present higher mutation frequencies than those from BSIs: contrary to what has been widely described for P. aeruginosa strains, isolated from pulmonary samples in patients suffering from cystic fibrosis, no relationship was found between the hypermutator phenotype in Enterobacteriaceae and the ability to initiate a biofilm.

Functional analysis of paralogous thiol-disulfide oxidoreductases in Streptococcus gordonii

Disulfide bonds are important for the stability of many extracellular proteins, including bacterial virulence factors. Formation of these bonds is catalyzed by thiol-disulfide oxidoreductases (TDORs). Little is known about their formation in Gram-positive bacteria, particularly among facultative anaerobic Firmicutes, such as streptococci. To investigate disulfide bond formation in Streptococcus gordonii, we identified five putative TDORs from the sequenced genome. Each of the putative TDOR genes was insertionally inactivated with an erythromycin resistance cassette, and the mutants were analyzed for autolysis, extracellular DNA release, biofilm formation, bacteriocin production, and genetic competence. This analysis revealed a single TDOR, SdbA, which exhibited a pleiotropic mutant phenotype. Using an in silico analysis approach, we identified the major autolysin AtlS as a natural substrate of SdbA and showed that SdbA is critical to the formation of a disulfide bond that is required for autolytic activity. Analysis by BLAST search revealed homologs to SdbA in other Gram-positive species. This study provides the first in vivo evidence of an oxidoreductase, SdbA, that affects multiple phenotypes in a Gram-positive bacterium. SdbA shows low sequence homology to previously identified oxidoreductases, suggesting that it may belong to a different class of enzymes. Our results demonstrate that SdbA is required for disulfide bond formation in S. gordonii and indicate that this enzyme may represent a novel type of oxidoreductase in Gram-positive bacteria.

Role of LytF and AtlS in eDNA release by Streptococcus gordonii

Extracellular DNA (eDNA) is an important component of the biofilm matrix produced by many bacteria. In general, the release of eDNA is associated with the activity of muralytic enzymes leading to obvious cell lysis. In the Gram-positive oral commensal Streptococcus gordonii, eDNA release is dependent on pyruvate oxidase generated hydrogen peroxide (H₂O₂). Addition of H₂O₂ to cells grown under conditions non-permissive for H₂O₂ production causes eDNA release. Furthermore, eDNA release is maximal under aerobic growth conditions known to induce pyruvate oxidase gene expression and H₂O₂ production. Obvious cell lysis, however, does not occur. Two enzymes have been recently associated with eDNA release in S. gordonii. The autolysin AtlS and the competence regulated murein hydrolase LytF. In the present report, we investigated the role of both proteins in the H₂O₂ dependent eDNA release process. Single and double mutants in the respective genes for LytF and AtlS released less eDNA under normal growth conditions, but the AtlS mutant was still inducible for eDNA release by external H₂O₂. Moreover, we showed that the AtlS mutation interfered with the ability of S. gordonii to produce eDNA release inducing amounts of H₂O₂. Our data support a role of LytF in the H₂O₂ eDNA dependent release of S. gordonii as part of the competence stress pathway responding to oxidative stress.

A systems-level approach for investigating Pseudomonas aeruginosa biofilm formation

Prevention of the initiation of biofilm formation is the most important step for combating biofilm-associated pathogens, as the ability of pathogens to resist antibiotics is enhanced 10 to 1000 times once biofilms are formed. Genes essential to bacterial growth in the planktonic state are potential targets to treat biofilm-associated pathogens. However, the biofilm formation capability of strains with mutations in these essential genes must be evaluated, since the pathogen might form a biofilm before it is eliminated. In order to address this issue, this work proposes a systems-level approach to quantifying the biofilm formation capability of mutants to determine target genes that are essential for bacterial metabolism in the planktonic state but do not induce biofilm formation in their mutants. The changes of fluxes through the reactions associated with the genes positively related to biofilm formation are used as soft sensors in the flux balance analysis to quantify the trend of biofilm formation upon the mutation of an essential gene. The essential genes whose mutants are predicted not to induce biofilm formation are regarded as gene targets. The proposed approach was applied to identify target genes to treat Pseudomonas aeruginosa infections. It is interesting to find that most essential gene mutants exhibit high potential to induce the biofilm formation while most non-essential gene mutants do not. Critically, we identified four essential genes, lysC, cysH, adk, and galU, that constitute gene targets to treat P. aeruginosa. They have been suggested by existing experimental data as potential drug targets for their crucial role in the survival or virulence of P. aeruginosa. It is also interesting to find that P. aeruginosa tends to survive the essential-gene mutation treatment by mainly enhancing fluxes through 8 metabolic reactions that regulate acetate metabolism, arginine metabolism, and glutamate metabolism.

Antibiotic resistance: from Darwin to Lederberg to Keynes

The emergence and spread of antibiotic-resistant bacteria reflects both, a gradual, completely Darwinian evolution, which mostly yields slight decreases in antibiotic susceptibility, along with phenotypes that are not precisely characterized as "resistance"; and sudden changes, from full susceptibility to full resistance, which are driven by a vast array of horizontal gene transfer mechanisms. Antibiotics select for more than just antibiotic resistance (i.e., increased virulence and enhanced gene exchange abilities); and many non-antibiotic agents or conditions select for or maintain antibiotic resistance traits as a result of a complex network of underlying and often overlapping mechanisms. Thus, the development of new antibiotics and thoughtful, integrated anti-infective strategies is needed to address the immediate and long-term threat of antibiotic resistance. Since the biology of resistance is complex, these new drugs and strategies will not come from free-market forces, or from "incentives" for pharmaceutical companies.

Homologous recombination is facilitated in starving populations of Pseudomonas putida by phenol stress and affected by chromosomal location of the recombination target

Homologous recombination (HR) has a major impact in bacterial evolution. Most of the knowledge about the mechanisms and control of HR in bacteria has been obtained in fast growing bacteria. However, in their natural environment bacteria frequently meet adverse conditions which restrict the growth of cells. We have constructed a test system to investigate HR between a plasmid and a chromosome in carbon-starved populations of the soil bacterium Pseudomonas putida restoring the expression of phenol monooxygenase gene pheA. Our results show that prolonged starvation of P. putida in the presence of phenol stimulates HR. The emergence of recombinants on selective plates containing phenol as an only carbon source for the growth of recombinants is facilitated by reactive oxygen species and suppressed by DNA mismatch repair enzymes. Importantly, the chromosomal location of the HR target influences the frequency and dynamics of HR events. In silico analysis of binding sites of nucleoid-associated proteins (NAPs) revealed that chromosomal DNA regions which flank the test system in bacteria exhibiting a lower HR frequency are enriched in binding sites for a subset of NAPs compared to those which express a higher frequency of HR. We hypothesize that the binding of these proteins imposes differences in local structural organization of the genome that could affect the accessibility of the chromosomal DNA to HR processes and thereby the frequency of HR.

Identification of the Streptococcus mutans LytST two-component regulon reveals its contribution to oxidative stress tolerance

Background

The S. mutans LrgA/B holin-like proteins have been shown to affect biofilm formation and oxidative stress tolerance, and are regulated by oxygenation, glucose levels, and by the LytST two-component system. In this study, we sought to determine if LytST was involved in regulating lrgAB expression in response to glucose and oxygenation in S. mutans.

Results

Real-time PCR revealed that growth phase-dependent regulation of lrgAB expression in response to glucose metabolism is mediated by LytST under low-oxygen conditions. However, the effect of LytST on lrgAB expression was less pronounced when cells were grown with aeration. RNA expression profiles in the wild-type and lytS mutant strains were compared using microarrays in early exponential and late exponential phase cells. The expression of 40 and 136 genes in early-exponential and late exponential phase, respectively, was altered in the lytS mutant. Although expression of comYB, encoding a DNA binding-uptake protein, was substantially increased in the lytS mutant, this did not translate to an effect on competence. However, a lrgA mutant displayed a substantial decrease in transformation efficiency, suggestive of a previously-unknown link between LrgA and S. mutans competence development. Finally, increased expression of genes encoding antioxidant and DNA recombination/repair enzymes was observed in the lytS mutant, suggesting that the mutant may be subjected to increased oxidative stress during normal growth. Although the intracellular levels of reaction oxygen species (ROS) appeared similar between wild-type and lytS mutant strains after overnight growth, challenge of these strains with hydrogen peroxide (H₂O₂) resulted in increased intracellular ROS in the lytS mutant.

Conclusions

Overall, these results: (1) Reinforce the importance of LytST in governing lrgAB expression in response to glucose and oxygen, (2) Define a new role for LytST in global gene regulation and resistance to H₂O₂, and (3) Uncover a potential link between LrgAB and competence development in S. mutans.

The role of hydrogen peroxide in environmental adaptation of oral microbial communities

Oral streptococci are able to produce growth-inhibiting amounts of hydrogen peroxide (H(2)O(2)) as byproduct of aerobic metabolism. Several recent studies showed that the produced H(2)O(2) is not a simple byproduct of metabolism but functions in several aspects of oral bacterial biofilm ecology. First, the release of DNA from cells is closely associated to the production of H(2)O(2) in Streptococcus sanguinis and Streptococcus gordonii. Extracellular DNA is crucial for biofilm development and stabilization and can also serve as source for horizontal gene transfer between oral streptococci. Second, due to the growth inhibiting nature of H(2)O(2), H(2)O(2) compatible species associate with the producers. H(2)O(2) production therefore might help in structuring the initial biofilm development. On the other hand, the oral environment harbors salivary peroxidases that are potent in H(2)O(2) scavenging. Therefore, the effects of biofilm intrinsic H(2)O(2) production might be locally confined. However, taking into account that 80% of initial oral biofilm constituents are streptococci, the influence of H(2)O(2) on biofilm development and environmental adaptation might be under appreciated in current research.

Reactive oxygen species in the signaling and adaptation of multicellular microbial communities

One of the universal traits of microorganisms is their ability to form multicellular structures, the cells of which differentiate and communicate via various signaling molecules. Reactive oxygen species (ROS), and hydrogen peroxide in particular, have recently become well-established signaling molecules in higher eukaryotes, but still little is known about the regulatory functions of ROS in microbial structures. Here we summarize current knowledge on the possible roles of ROS during the development of colonies and biofilms, representatives of microbial multicellularity. In Saccharomyces cerevisiae colonies, ROS are predicted to participate in regulatory events involved in the induction of ammonia signaling and later on in programmed cell death in the colony center. While the latter process seems to be induced by the total ROS, the former event is likely to be regulated by ROS-homeostasis, possibly H₂O₂-homeostasis between the cytosol and mitochondria. In Candida albicans biofilms, the predicted signaling role of ROS is linked with quorum sensing molecule farnesol that significantly affects biofilm formation. In bacterial biofilms, ROS induce genetic variability, promote cell death in specific biofilm regions, and possibly regulate biofilm development. Thus, the number of examples suggesting ROS as signaling molecules and effectors in the development of microbial multicellularity is rapidly increasing.