Critical roles of arginine in growth and biofilm development by Streptococcus gordonii

A synthetic antibiotic class with a deeply-optimized design for overcoming bacterial resistance

The lack of new drugs that are effective against antibiotic-resistant bacteria has caused increasing concern in global public health. Based on this study, we report development of a modified antimicrobial drug through structure-based drug design (SBDD) and modular synthesis. The optimal modified compound, F8, was identified, which demonstrated in vitro and in vivo broad-spectrum antibacterial activity against drug-resistant bacteria and effectively mitigated the development of resistance. F8 exhibits significant bactericidal activity against bacteria resistant to antibiotics such as methicillin, polymyxin B, florfenicol (FLO), doxycycline, ampicillin and sulfamethoxazole. In a mouse model of drug-resistant bacteremia, F8 was found to increase survival and significantly reduce bacterial load in infected mice. Multi-omics analysis (transcriptomics, proteomics, and metabolomics) have indicated that ornithine carbamoyl transferase (arcB) is a antimicrobial target of F8. Further molecular docking, Isothermal Titration Calorimetry (ITC), and Differential Scanning Fluorimetry (DSF) studies verified arcB as a effective target for F8. Finally, mechanistic studies suggest that F8 competitively binds to arcB, disrupting the bacterial cell membrane and inducing a certain degree of oxidative damage. Here, we report F8 as a promising candidate drug for the development of antibiotic formulations to combat antibiotic-resistant bacteria-associated infections.

Formulation of arginine-loaded mesoporous silica nanoparticles (Arg@MSNs) modified orthodontic adhesive

OBJECTIVES

The objective of this study was to synthesize arginine loaded mesoporous silica nanoparticles (Arg@MSNs), develop a novel orthodontic adhesive using Arg@MSNs as modifiers, and investigate the adhesive performance, antibacterial activity, and biocompatibility.

METHODS

Arg@MSNs were synthesized by immobilizing arginine into MSNs and characterized using transmission electron microscope (TEM), dynamic light scattering (DLS), and Fourier Transform Infrared Spectrometer (FT-IR). Arg@MSNs were incorporated into Transbond XT adhesive with different mass fraction to form functional adhesives. The degree of conversion (DC), arginine release behavior, adhesive performance, antibacterial activity against Streptococcus mutans biofilm, and cytotoxicity were comprehensively evaluated.

RESULTS

TEM, DLS, and FT-IR characterizations confirmed the successful preparation of Arg@MSNs. The incorporation of Arg@MSNs did not significantly affect DC and exhibited clinically acceptable bonding strength. Compared to the commercial control, the Arg@MSNs modified adhesives greatly suppressed the metabolic activity and polysaccharide production while increased the biofilm pH values. The cell counting kit (CCK)-8 test indicated no cytotoxicity.

CONCLUSIONS

The novel orthodontic adhesive containing Arg@MSNs exhibited significantly enhanced antibacterial activities and inhibitory effects on acid production compared to the commercial adhesive without compromising their bonding strength or biocompatibility.

CLINICAL SIGNIFICANCE

The novel orthodontic adhesive containing Arg@MSNs exhibits potential clinical benefits in preventing demineralization of enamel surfaces around or beneath orthodontic brackets due to its enhanced antibacterial activities and acid-producing inhibitory effects.

Arginine impacts aggregation, biofilm formation, and antibiotic susceptibility in Enterococcus faecalis

Enterococcus faecalis is a commensal bacterium in the gastrointestinal tract (GIT) of humans and other organisms. E. faecalis also causes infections in root canals, wounds, the urinary tract, and on heart valves. E. faecalis metabolizes arginine through the arginine deiminase (ADI) pathway, which converts arginine to ornithine and releases ATP, ammonia, and CO2. E. faecalis arginine metabolism also affects virulence of other pathogens during co-culture. E. faecalis may encounter elevated levels of arginine in the GIT or the oral cavity, where arginine is used as a dental therapeutic. Little is known about how E. faecalis responds to growth in arginine in the absence of other bacteria. To address this, we used RNAseq and additional assays to measure growth, gene expression, and biofilm formation in E. faecalis OG1RF grown in arginine. We demonstrate that arginine decreases E. faecalis biofilm production and causes widespread differential expression of genes related to metabolism, quorum sensing, and polysaccharide synthesis. Growth in arginine also increases aggregation of E. faecalis and promotes decreased susceptibility to the antibiotics ampicillin and ceftriaxone. This work provides a platform for understanding of how the presence of arginine in biological niches affects E. faecalis physiology and virulence of surrounding microbes.

Acetylation of xenogeneic silencer H-NS regulates biofilm development through the nitrogen homeostasis regulator in Shewanella

Adjusting intracellular metabolic pathways and adopting suitable live state such as biofilms, are crucial for bacteria to survive environmental changes. Although substantial progress has been made in understanding how the histone-like nucleoid-structuring (H-NS) protein modulates the expression of the genes involved in biofilm formation, the precise modification that the H-NS protein undergoes to alter its DNA binding activity is still largely uncharacterized. This study revealed that acetylation of H-NS at Lys19 inhibits biofilm development in Shewanella oneidensis MR-1 by downregulating the expression of glutamine synthetase, a critical enzyme in glutamine synthesis. We further found that nitrogen starvation, a likely condition in biofilm development, induces deacetylation of H-NS and the trimerization of nitrogen assimilation regulator GlnB. The acetylated H-NS strain exhibits significantly lower cellular glutamine concentration, emphasizing the requirement of H-NS deacetylation in Shewanella biofilm development. Moreover, we discovered in vivo that the activation of glutamine biosynthesis pathway and the concurrent suppression of the arginine synthesis pathway during both pellicle and attached biofilms development, further suggesting the importance of fine tune nitrogen assimilation by H-NS acetylation in Shewanella. In summary, posttranslational modification of H-NS endows Shewanella with the ability to respond to environmental needs by adjusting the intracellular metabolism pathways.

Preparation of arginine-loaded mesoporous silica nanoparticles (Arg@MSNs) to improve the mechanical and antibacterial properties of denture base resin

STATEMENT OF PROBLEM

Whether the incorporation of arginine-loaded mesoporous silica nanoparticles (Arg@MSNs) into denture base resin can improve the mechanical and antibacterial properties is unclear.

PURPOSE

The purpose of this in vitro study was to synthesis Arg@MSNs and explore how Arg@MSNs incorporation affects the mechanical and antibacterial properties of denture base resin.

MATERIAL AND METHODS

Arg@MSNs were synthesized via a sol-gel process and characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), and X-ray diffraction (XRD). The prepared Arg@MSNs at different weight ratio concentrations were added into denture base resin as the experimental group, and unmodified denture base resin was the control. The fracture surface and arginine release behavior of each specimen were detected using scanning electron microscopy (SEM) and ultra-high-performance liquid chromatography-tandem mass spectrometer (UHPLC-HESI-MS/MS), respectively. Three-point bend tests were applied using a universal testing machine for evaluation of the mechanical properties of each group (n=5). Antibacterial efficiency (n=3) was evaluated by both quantitative and qualitative analysis using Streptococcus mutans. The cytotoxic effect of the Arg@MSN-modified denture base resin was investigated using a cell counting kit (CCK)-8 test. Data were subjected to 1-way analysis of variance followed by the post hoc Tukey honestly significant difference test (ɑ=.05).

RESULTS

The prepared Arg@MSNs had good monodispersity and spherical morphology. Arg@MSN concentration at 0.5 wt%, 1 wt%, and 2.5 wt% resulted in enhanced mechanical properties, while those at 5 wt% were adversely impacted. Biofilm pH values increased with the incorporation of Arg@MSNs, and the antibacterial performance was improved. The CCK-8 test revealed that all formulations were not cytotoxic.

CONCLUSIONS

The addition of Arg@MSNs into denture base resin can enhance its mechanical properties and improve its antibacterial performance without any apparent cytotoxic effect.

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.

Contrasting genes conferring short- and long-term biofilm adaptation in Listeria

Listeria monocytogenes is an opportunistic food-borne bacterium that is capable of infecting humans with high rates of hospitalization and mortality. Natural populations are genotypically and phenotypically variable, with some lineages being responsible for most human infections. The success of L. monocytogenes is linked to its capacity to persist on food and in the environment. Biofilms are an important feature that allow these bacteria to persist and infect humans, so understanding the genetic basis of biofilm formation is key to understanding transmission. We sought to investigate the biofilm-forming ability of L. monocytogenes by identifying genetic variation that underlies biofilm formation in natural populations using genome-wide association studies (GWAS). Changes in gene expression of specific strains during biofilm formation were then investigated using RNA sequencing (RNA-seq). Genetic variation associated with enhanced biofilm formation was identified in 273 genes by GWAS and differential expression in 220 genes by RNA-seq. Statistical analyses show that the number of overlapping genes flagged by either type of experiment is less than expected by random sampling. This novel finding is consistent with an evolutionary scenario where rapid adaptation is driven by variation in gene expression of pioneer genes, and this is followed by slower adaptation driven by nucleotide changes within the core genome.

Natural phenolic compounds as biofilm inhibitors of multidrug-resistant Escherichia coli - the role of similar biological processes despite structural diversity

Multidrug-resistant gram-negative pathogens such as Escherichia coli have become increasingly difficult to treat and therefore alternative treatment options are needed. Targeting virulence factors like biofilm formation could be one such option. Inhibition of biofilm-related structures like curli and cellulose formation in E. coli has been shown for different phenolic natural compounds like epigallocatechin gallate. This study demonstrates this effect for other structurally unrelated phenolics, namely octyl gallate, scutellarein and wedelolactone. To verify whether these structurally different compounds influence identical pathways of biofilm formation in E. coli a broad comparative RNA-sequencing approach was chosen with additional RT-qPCR to gain initial insights into the pathways affected at the transcriptomic level. Bioinformatical analysis of the RNA-Seq data was performed using DESeq2, BioCyc and KEGG Mapper. The comparative bioinformatics analysis on the pathways revealed that, irrespective of their structure, all compounds mainly influenced similar biological processes. These pathways included bacterial motility, chemotaxis, biofilm formation as well as metabolic processes like arginine biosynthesis and tricarboxylic acid cycle. Overall, this work provides the first insights into the potential mechanisms of action of novel phenolic biofilm inhibitors and highlights the complex regulatory processes of biofilm formation in E. coli.

Effects of metformin on Streptococcus suis LuxS/AI-2 quorum sensing system and biofilm formation

Streptococcus suis (S. suis) regulates biofilm formation through LuxS/AI-2 quorum sensing system, increasing drug resistance and exacerbating infection. The anti-hyperglycaemic agent metformin has anti-bacterial and anti-biofilm activities. This study aimed to investigate the anti-biofilm and anti-quorum sensing activity of metformin in S. suis. We first determined the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of metformin on S. suis. The results indicated that metformin showed no obvious inhibitory or bactericidal effect. Crystal violet staining showed that metformin significantly inhibited the formation of S. suis biofilm at sub-MIC concentration, which was also confirmed by scanning electron microscopy. Then, we quantified the AI-2 signal molecules in S. suis, and the results showed that metformin had a significant inhibitory effect on the production of AI-2 signal in S. suis. Inhibition of enzyme activity and molecular docking experiments showed that metformin has a significant binding activity to LuxS protein. In addition, qRT-PCR results showed that metformin significantly down-regulated the expression of AI-2 synthesis-related genes luxS and pfs, and adhesion-related genes luxS, pfs, gapdh, sly, fbps, and ef. Western blotting also showed that metformin significantly reduced the expression of LuxS protein. Our study suggests that metformin seems to be a suitable candidate for the inhibition of S. suis LuxS/AI-2 QS system and prevention of biofilm formation, which provided a new idea for the prevention and control of S. suis.

A Combination of Zinc and Arginine Disrupt the Mechanical Integrity of Dental Biofilms

Mechanical cleaning remains the standard of care for maintaining oral hygiene. However, mechanical cleaning is often augmented with active therapeutics that further promote oral health. A dentifrice, consisting of the "Dual Zinc plus Arginine" (DZA) technology, was found to be effective at controlling bacteria using in vitro laboratory studies, translating to clinical efficacy to deliver plaque and gingivitis reduction benefits. Here, we used biophysical analyses and confocal laser scanning microscopy to understand how a DZA dentifrice impacted the mechanical properties of dental plaque biofilms and determine if changes to biofilm rheology enhanced the removal of dental plaque. Using both uniaxial mechanical indentation and an adapted rotating-disc rheometry assay, it was found that DZA treatment compromised biofilm mechanical integrity, resulting in the biofilm being more susceptible to removal by shear forces compared to treatment with either arginine or zinc alone. Confocal laser scanning microscopy revealed that DZA treatment reduced the amount of extracellular polymeric slime within the biofilm, likely accounting for the reduced mechanical properties. We propose a model where arginine facilitates the entry of zinc into the biofilm, resulting in additive effects of the two activities toward dental plaque biofilms. Together, our results support the use of a dentifrice containing Dual Zinc plus Arginine as part of daily oral hygiene regimens. IMPORTANCE Mechanical removal of dental plaque is augmented with therapeutic compounds to promote oral health. A dentifrice containing the ingredients zinc and arginine has shown efficacy at reducing dental plaque both in vitro and in vivo. However, how these active compounds interact together to facilitate dental plaque removal is unclear. Here, we used a combination of biophysical analyses and microscopy to demonstrate that combined treatment with zinc and arginine targets the matrix of dental plaque biofilms, which destabilized the mechanical integrity of these microbial communities, making them more susceptible to removal by shear forces.

Interspecies competition in oral biofilms mediated by Streptococcus gordonii extracellular deoxyribonuclease SsnA

Extracellular DNA (eDNA) is a key component of many microbial biofilms including dental plaque. However, the roles of extracellular deoxyribonuclease (DNase) enzymes within biofilms are poorly understood. Streptococcus gordonii is a pioneer colonizer of dental plaque. Here, we identified and characterised SsnA, a cell wall-associated protein responsible for extracellular DNase activity of S. gordonii. The SsnA-mediated extracellular DNase activity of S. gordonii was suppressed following growth in sugars. SsnA was purified as a recombinant protein and shown to be inactive below pH 6.5. SsnA inhibited biofilm formation by Streptococcus mutans in a pH-dependent manner. Further, SsnA inhibited the growth of oral microcosm biofilms in human saliva. However, inhibition was ameliorated by the addition of sucrose. Together, these data indicate that S. gordonii SsnA plays a key role in interspecies competition within oral biofilms. Acidification of the medium through sugar catabolism could be a strategy for cariogenic species such as S. mutans to prevent SsnA-mediated exclusion from biofilms.

Teg58, a small regulatory RNA, is involved in regulating arginine biosynthesis and biofilm formation in Staphylococcus aureus

Staphylococcus aureus adapts to different environments by sensing and responding to diverse environmental cues. The responses are coordinately regulated by regulatory proteins, and small regulatory RNAs at the transcriptional and translational levels. Here, we characterized teg58, a SarA repressed sRNA, using ChIP-Seq and RNA-Seq analysis of a sarA mutant. Phenotypic and genetic analyses indicated that inactivation of teg58 led to reduced biofilm formation in a process that is independent of SarA, agr, PIA, and PSMs. RNA-Seq analysis of teg58 mutant revealed up-regulation of arginine biosynthesis genes (i.e., argGH) as well as the ability of the mutant to grow in a chemical defined medium (CDM) lacking L-arginine. Exogenous L-arginine or endogenous induction of argGH led to decreased biofilm formation in parental strains. Further analysis in vitro and in vivo demonstrated that the specific interaction between teg58 and the argGH occurred at the post-transcriptional level to repress arginine synthesis. Biochemical and genetic analyses of various arginine catabolic pathway genes demonstrated that the catabolic pathway did not play a significant role in reduced biofilm formation in the teg58 mutant. Overall, results suggest that teg58 is a regulatory sRNA that plays an important role in modulating arginine biosynthesis and biofilm formation in S. aureus.

The Impacts of Microgravity on Bacterial Metabolism

The inside of a space-faring vehicle provides a set of conditions unlike anything experienced by bacteria on Earth. The low-shear, diffusion-limited microenvironment with accompanying high levels of ionizing radiation create high stress in bacterial cells, and results in many physiological adaptations. This review gives an overview of the effect spaceflight in general, and real or simulated microgravity in particular, has on primary and secondary metabolism. Some broad trends in primary metabolic responses can be identified. These include increases in carbohydrate metabolism, changes in carbon substrate utilization range, and changes in amino acid metabolism that reflect increased oxidative stress. However, another important trend is that there is no universal bacterial response to microgravity, as different bacteria often have contradictory responses to the same stress. This is exemplified in many of the observed secondary metabolite responses where secondary metabolites may have increased, decreased, or unchanged production in microgravity. Different secondary metabolites in the same organism can even show drastically different production responses. Microgravity can also impact the production profile and localization of secondary metabolites. The inconsistency of bacterial responses to real or simulated microgravity underscores the importance of further research in this area to better understand how microbes can impact the people and systems aboard spacecraft.

Catabolic Ornithine Carbamoyltransferase Activity Facilitates Growth of Staphylococcus aureus in Defined Medium Lacking Glucose and Arginine

Previous studies have found that arginine biosynthesis in Staphylococcus aureus is repressed via carbon catabolite repression (CcpA), and proline is used as a precursor. Unexpectedly, however, robust growth of S. aureus is not observed in complete defined medium lacking both glucose and arginine (CDM-R). Mutants able to grow on agar-containing defined medium lacking arginine (CDM-R) were selected and found to contain mutations within ahrC, encoding the canonical arginine biosynthesis pathway repressor (AhrC), or single nucleotide polymorphisms (SNPs) upstream of the native arginine deiminase (ADI) operon arcA1B1D1C1. Reverse transcription-PCR (RT-PCR) studies found that mutations within ccpA or ahrC or SNPs identified upstream of arcA1B1D1C1 increased the transcription of both arcB1 and argGH, encoding ornithine carbamoyltransferase and argininosuccinate synthase/lyase, respectively, facilitating arginine biosynthesis. Furthermore, mutations within the AhrC homologue argR2 facilitated robust growth within CDM-R. Complementation with arcB1 or arcA1B1D1C1, but not argGH, rescued growth in CDM-R. Finally, supplementation of CDM-R with ornithine stimulated growth, as did mutations in genes (proC and rocA) that presumably increased the pyrroline-5-carboxylate and ornithine pools. Collectively, these data suggest that the transcriptional regulation of ornithine carbamoyltransferase and, in addition, the availability of intracellular ornithine pools regulate arginine biosynthesis in S. aureus in the absence of glucose. Surprisingly, ~50% of clinical S. aureus isolates were able to grow in CDM-R. These data suggest that S. aureus is selected to repress arginine biosynthesis in environments with or without glucose; however, mutants may be readily selected that facilitate arginine biosynthesis and growth in specific environments lacking arginine.

The arginine deaminase system plays distinct roles in Borrelia burgdorferi and Borrelia hermsii

Borrelia species are amino acid auxotrophs that utilize di- and tri- peptides obtained through their oligopeptide transport system to supply amino acids for replicative growth during their enzootic cycles. However, Borrelia species from both the Lyme disease (LD) and relapsing fever (RF) groups harbor an amino acid transport and catabolism system, the Arginine Deiminase System (ADI), that could potentially augment intracellular L-arginine required for growth. RF spirochetes contain a “complete”, four gene ADI (arcA, B, D, and C) while LD spirochetes harbor arcA, B, and sometimes D but lack arcC (encoding carbamate kinase). In this study, we evaluated the role of the ADI system in bacterial survival and virulence and discovered important differences in RF and LD ADIs. Both in vitro and in a murine model of infection, B. hermsii cells significantly reduced extracellular L-arginine levels and that reduction was dependent on arginine deiminase expression. Conversely, B. burgdorferi did not reduce the concentration of L-arginine during in vitro growth experiments nor during infection of the mammalian host, suggesting a fundamental difference in the ability to directly utilize L-arginine compared to B. hermsii. Further experiments using a panel of mutants generated in both B. burgdorferi and B. hermsii, identified important differences in growth characteristics and ADI transcription and protein expression. We also found that the ADI system plays a key role in blood and spleen colonization in RF spirochetes. In this study we have identified divergent metabolic strategies in two closely related human pathogens, that ultimately impacts the host-pathogen interface during infection.

Reports of tick-borne diseases have been steadily increasing in the US and the number of Lyme disease cases caused by B. burgdorferi have tripled since the late 1990’s. Although less common, cases of tick-borne relapsing fever, caused by B. hermsii and B. turicatae in the US, have increased as well. While transmitted by different ticks and maintained in unique enzootic cycles, the closely related spirochetes B. burgdorferi and B. hermsii share numerous genetic features including a truncated and streamlined capacity for metabolic activity. In this study we combine genetic and biochemical assays to define the role of the ADI in the infective cycles of B. burgdorferi and B. hermsii. When we compared B. burgdorferi and B. hermsii, we identified important differences in their respective ADI’s including operon arrangement, sensitivity to L-arginine and L-ornithine levels, as well as gene and protein expression. In addition, we show that arginine deiminase is required to reduce host L-arginine levels during murine infection with B. hermsii. This study provides new insights into the metabolic activities of two medically relevant spirochetes and highlights the dynamic nature of host-pathogen interactions.

Interactions Between Streptococcus gordonii and Fusobacterium nucleatum Altered Bacterial Transcriptional Profiling and Attenuated the Immune Responses of Macrophages

Interspecies coaggregation promotes transcriptional changes in oral bacteria, affecting bacterial pathogenicity. Streptococcus gordonii (S. gordonii) and Fusobacterium nucleatum (F. nucleatum) are common oral inhabitants. The present study investigated the transcriptional profiling of S. gordonii and F. nucleatum subsp. polymorphum in response to the dual-species coaggregation using RNA-seq. Macrophages were infected with both species to explore the influence of bacterial coaggregation on both species' abilities to survive within macrophages and induce inflammatory responses. Results indicated that, after the 30-min dual-species coaggregation, 116 genes were significantly up-regulated, and 151 genes were significantly down-regulated in S. gordonii; 97 genes were significantly down-regulated, and 114 genes were significantly up-regulated in F. nucleatum subsp. polymorphum. Multiple S. gordonii genes were involved in the biosynthesis and export of cell-wall proteins and carbohydrate metabolism. F. nucleatum subsp. polymorphum genes were mostly associated with translation and protein export. The coaggregation led to decreased expression levels of genes associated with lipopolysaccharide and peptidoglycan biosynthesis. Coaggregation between S. gordonii and F. nucleatum subsp. polymorphum significantly promoted both species' intracellular survival within macrophages and attenuated the production of pro-inflammatory cytokines IL-6 and IL-1β. Physical interactions between these two species promoted a symbiotic lifestyle and repressed macrophage's killing and pro-inflammatory responses.

Arginine Induced Streptococcus gordonii Biofilm Detachment Using a Novel Rotating-Disc Rheometry Method

Oral diseases are one of the most common pathologies affecting human health. These diseases are typically associated with dental plaque-biofilms, through either build-up of the biofilm or dysbiosis of the microbial community. Arginine can disrupt dental plaque-biofilms, and maintain plaque homeostasis, making it an ideal therapeutic to combat the development of oral disease. Despite our understanding of the actions of arginine towards dental plaque-biofilms, it is still unclear how or if arginine effects the mechanical integrity of the dental plaque-biofilm. Here we adapted a rotating-disc rheometry assay, a method used to quantify marine biofilm fouling, to study how arginine treatment of Streptococcus gordonii biofilms influences biofilm detachment from surfaces. We demonstrate that the assay is highly sensitive at quantifying the presence of biofilm and the detachment or rearrangement of the biofilm structure as a function of shear stress. We demonstrate that arginine treatment leads to earlier detachment of the biofilm, indicating that arginine treatment weakens the biofilm, making it more susceptible to removal by shear stresses. Finally, we demonstrate that the biofilm disrupting affect is specific to arginine, and not a general property of amino acids, as S. gordonii biofilms treated with either glycine or lysine had mechanical properties similar to untreated biofilms. Our results add to the understanding that arginine targets biofilms by multifaceted mechanisms, both metabolic and physical, further promoting the potential of arginine as an active compound in dentifrices to maintain oral health.

The Effect of Impaired Polyamine Transport on Pneumococcal Transcriptome

Infections due to Streptococcus pneumoniae, a commensal in the nasopharynx, still claim a significant number of lives worldwide. Genome plasticity, antibiotic resistance, and limited serotype coverage of the available polysaccharide-based conjugate vaccines confounds therapeutic interventions to limit the spread of this pathogen. Pathogenic mechanisms that allow successful adaption and persistence in the host could be potential innovative therapeutic targets. Polyamines are ubiquitous polycationic molecules that regulate many cellular processes. We previously reported that deletion of polyamine transport operon potABCD, which encodes a putrescine/spermidine transporter (ΔpotABCD), resulted in an unencapsulated attenuated phenotype. Here, we characterize the transcriptome, metabolome, and stress responses of polyamine transport-deficient S. pneumoniae. Compared with the wild-type strain, the expression of genes involved in oxidative stress responses and the nucleotide sugar metabolism was reduced, while expression of genes involved in the Leloir, tagatose, and pentose phosphate pathways was higher in ΔpotABCD. A metabolic shift towards the pentose phosphate pathway will limit the synthesis of precursors of capsule polysaccharides. Metabolomics results show reduced levels of glutathione and pyruvate in the mutant. Our results also show that the potABCD operon protects pneumococci against hydrogen peroxide and nitrosative stress. Our findings demonstrate the importance of polyamine transport in pneumococcal physiology that could impact in vivo fitness. Thus, polyamine transport in pneumococci represents a novel target for therapeutic interventions.

Polymicrobial communities in periodontal disease: Their quasi-organismal nature and dialogue with the host

In health, indigenous polymicrobial communities at mucosal surfaces maintain an ecological balance via both inter-microbial and host-microbial interactions that promote their own and the host's fitness, while preventing invasion by exogenous pathogens. However, genetic and acquired destabilizing factors (including immune deficiencies, immunoregulatory defects, smoking, diet, obesity, diabetes and other systemic diseases, and aging) may disrupt this homeostatic balance, leading to selective outgrowth of species with the potential for destructive inflammation. This process, known as dysbiosis, underlies the development of periodontitis in susceptible hosts. The pathogenic process is not linear but involves a positive-feedback loop between dysbiosis and the host inflammatory response. The dysbiotic community is essentially a quasi-organismal entity, where constituent organisms communicate via sophisticated physical and chemical signals and display functional specialization (eg, accessory pathogens, keystone pathogens, pathobionts), which enables polymicrobial synergy and dictates the community's pathogenic potential or nososymbiocity. In this review, we discuss early and recent studies in support of the polymicrobial synergy and dysbiosis model of periodontal disease pathogenesis. According to this concept, disease is not caused by individual "causative pathogens" but rather by reciprocally reinforced interactions between physically and metabolically integrated polymicrobial communities and a dysregulated host inflammatory response.

Tooth brushing using toothpaste containing theaflavins reduces the oral pathogenic bacteria in healthy adults

Theaflavins (TFs) are the main bioactive component in black tea. At present, little effort has been done to evaluate the influence of TFs when included in the toothpaste on the diversity of oral microbiota. In this study, eighty samples collected from the saliva and supragingival plaque of 20 healthy adults using toothpaste with the absence or presence of TFs for a period of 4 weeks were used for the oral microbiome analysis by 16S rRNA gene sequencing. Alpha and beta diversity analysis showed that tooth brushing using the toothpaste with TFs significantly increased the microbial abundance in the saliva samples, and altered the oral microbiota obtained from the saliva and supragingival plaque. The linear discriminant analysis revealed that the use of toothpaste with TFs significantly reduced the abundance of oral pathogens (e.g., Prevotella, Selenomonas, and Atopobium) while increased the abundance of oral-health associated bacteria (e.g., Streptococcus and Rothia). In addition, using toothpaste with TFs reduced the functional pathways abundance relevance to the extracellular polymeric substance (EPS) synthesis while enriched the functions in transporters, ABC transporters, two-component system, and amino acid metabolism. Collectively, our results provide evidence for the application of toothpaste containing TFs as a promising oral care product.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02699-7.

The otc gene of Streptococcus suis plays an important role in biofilm formation, adhesion, and virulence in a murine model

Streptococcus suis (S. suis) is an emerging zoonotic pathogen that can cause meningitis, arthritis, pneumonia, and sepsis. It poses a serious threat to the swine industry and public health worldwide. Ornithine carbamoyltransferase (OTC) is involved in the arginine deiminase system. OTC, which is a widely distributed enzyme in microorganisms, mammals, and higher plants, catalyzes the conversion of ornithine to citrulline. The present study showed that the otc gene plays an important role in the pathogenesis of S. suis infections. The ability of an otc-deficient mutant (Δotc) to form a biofilm was significantly reduced compared to the wild-type (WT) strain, as determined by crystal violet staining. Confocal laser scanning microscopy and scanning electron microscopy observations showed that the weakening of biofilm formation by the Δotc strain is related to a decrease in the extracellular matrix. In addition, compared to the WT strain, the Δotc strain had a reduced capacity to adhere to human laryngeal epidermoid carcinoma (HEp-2) cells compared to the WT strain. A real-time PCR analysis showed that the expression of adhesion-related genes by the Δotc strain was also lower than that of the WT strain. The virulence of the Δotc strain was significantly lower than that of the WT strain in a murine infection model. In addition, a histological analysis showed that the pathogenicity of the Δotc strain was lower than that of the WT strain, causing only slight inflammatory lesions in lung, liver, spleen, and kidney tissues. No significant differences were observed between the complemented mutant (CΔotc) and WT strains with respect to biofilm formation, adhesion, gene expression, and virulence. The present study provided evidence that the otc gene plays a pivotal role in the regulation of S. suis adhesion and biofilm formation. It also suggested that the otc gene is indirectly involved in the pathogenesis of S. suis serotype 2 infections.

Metabolomics profiling during biofilm development of Bacillus licheniformis isolated from milk powder

Bacillus licheniformis is a major source of microbial contamination to dairy industry, and biofilm formation by this spoilage bacterium aggravates the safety issues. Especially for milk powder manufactures, the evaporation process at temperatures between 50 °C and 70 °C before spray drying, is a critical control point against thermophilic bacteria multiplication. In our study, metabolomics analysis was performed to investigate dynamic changes of the metabolites and their roles during process of biofilm development of B. licheniformis at 55 °C for 24 h. Amino acid metabolism was quite active, with cooperation from lipid metabolism, carbohydrate metabolism and nucleotide metabolism. Amino acid biosynthesis provided significant contributions especially during early biofilm development from 8 to 12 h. Metabolites involved in specific pathways of arginine biosynthetic, galactose metabolism and sphingolipid metabolism played a crucial role in building biofilm. This work provided new insights into dynamic metabolic alternations and a comprehensive network during B. licheniformis biofilm development, which will extend the knowledge on the metabolic process of biofilm formation by B. licheniformis. The results are helpful in creating better environmental hygiene in dairy processing and new strategies for ensuring quality of dairy products.

Arginine as an environmental and metabolic cue for cyclic diguanylate signalling and biofilm formation in Pseudomonas putida

Cyclic diguanylate (c-di-GMP) is a broadly conserved intracellular second messenger that influences different bacterial processes, including virulence, stress tolerance or social behaviours and biofilm development. Although in most cases the environmental cue that initiates the signal transduction cascade leading to changes in cellular c-di-GMP levels remains unknown, certain l- and d-amino acids have been described to modulate c-di-GMP turnover in some bacteria. In this work, we have analysed the influence of l-amino acids on c-di-GMP levels in the plant-beneficial bacterium Pseudomonas putida KT2440, identifying l-arginine as the main one causing a significant increase in c-di-GMP. Both exogenous (environmental) and endogenous (biosynthetic) l-arginine influence biofilm formation by P. putida through changes in c-di-GMP content and altered expression of structural elements of the biofilm extracellular matrix. The contribution of periplasmic binding proteins forming part of amino acid transport systems to the response to environmental l-arginine was also studied. Contrary to what has been described in other bacteria, in P. putida these proteins seem not to be directly responsible for signal transduction. Rather, their contribution to global l-arginine pools appears to determine changes in c-di-GMP turnover. We propose that arginine plays a connecting role between cellular metabolism and c-di-GMP signalling in P. putida.

Targeting Biofilms Therapy: Current Research Strategies and Development Hurdles

Biofilms are aggregate of microorganisms in which cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) and adhere to each other and/or to a surface. The development of biofilm affords pathogens significantly increased tolerances to antibiotics and antimicrobials. Up to 80% of human bacterial infections are biofilm-associated. Dispersal of biofilms can turn microbial cells into their more vulnerable planktonic phenotype and improve the therapeutic effect of antimicrobials. In this review, we focus on multiple therapeutic strategies that are currently being developed to target important structural and functional characteristics and drug resistance mechanisms of biofilms. We thoroughly discuss the current biofilm targeting strategies from four major aspects—targeting EPS, dispersal molecules, targeting quorum sensing, and targeting dormant cells. We explain each aspect with examples and discuss the main hurdles in the development of biofilm dispersal agents in order to provide a rationale for multi-targeted therapy strategies that target the complicated biofilms. Biofilm dispersal is a promising research direction to treat biofilm-associated infections in the future, and more in vivo experiments should be performed to ensure the efficacy of these therapeutic agents before being used in clinic.

Effect of arginine on microorganisms involved in dental caries: a systematic literature review of in vitro studies

This systematic review aimed to discuss the effects of arginine on caries-related microorganisms in different in vitro biofilm models. The eligibility criteria were in vitro studies that evaluated the effect of arginine at different concentrations on caries-related microorganisms using biofilm models. Eighteen studies published between 2012 and 2019 were included. Different bacterial species were studied. Seventeen studies (94.4%) achieved a low risk of bias and only one showed a medium risk of bias. The studies showed that arginine is a promising approach for the ecological management of dental caries. The focus of this review was to evaluate the effects of arginine on microorganisms involved in the mechanism of dental caries.

Casein phosphopeptide combined with fluoride enhances the inhibitory effect on initial adhesion of Streptococcus mutans to the saliva-coated hydroxyapatite disc

Background

Recent preventive strategies for dental caries focus on targeting the mechanisms underlying biofilm formation, including the inhibition of bacterial adhesion. A promising approach to prevent bacterial adhesion is to modify the composition of acquired salivary pellicle. This in vitro study investigated the effect and possible underlying mechanism of pellicle modification by casein phosphopeptide (CPP) on Streptococcus mutans (S. mutans) initial adhesion, and the impact of fluoride on the efficacy of CPP.

Methods

The salivary pellicle-coated hydroxyapatite (s-HA) discs were treated with phosphate buffered saline (negative control), heat-inactivated 2.5% CPP (heat-inactivated CPP), 2.5% CPP (CPP) or 2.5% CPP supplemented with 900 ppm fluoride (CPP + F). After cultivation of S. mutans for 30 min and 2 h, the adherent bacteria were visualized by scanning electron microscopy (SEM) and quantitatively evaluated using the plate count method. Confocal laser scanning microscopy (CLSM) was used to evaluate the proportions of total and dead S. mutans. The concentrations of total, free, and bound calcium and fluoride in the CPP and fluoride-doped CPP solutions were determined. The water contact angle and zeta potential of s-HA with and without modification were measured. The data were statistically analyzed using one-way ANOVA followed by a Turkey post hoc multiple comparison test.

Results

Compared to the negative control group, the amount of adherent S. mutans significantly reduced in the CPP and CPP + F groups, and was lowest in the CPP + F group. CLSM analysis showed that there was no statistically significant difference in the proportion of dead S. mutans between the four groups. Water contact angle and zeta potential of s-HA surface significantly decreased in the CPP and CPP + F groups as compared to the negative control group, and both were lowest in the CPP + F group.

Conclusions

Pellicle modification by CPP inhibited S. mutans initial adhesion to s-HA, possibly by reducing hydrophobicity and negative charge of the s-HA surface, and incorporating fluoride into CPP further enhanced the anti-adhesion effect.

Nanoparticles for the Treatment of Oral Biofilms: Current State, Mechanisms, Influencing Factors, and Prospects

Due to their excellent size, designability, and outstanding targeted antibacterial effects, nanoparticles have become a potential option for controlling oral biofilm-related infections. However, the formation of an oral biofilm is a dynamic process, and factors affecting the performance of antibiofilm treatments are complex. As such, when examining the existing literature on the antibiofilm effects of nanoparticles, attention should be paid to the specific mechanisms of action at different stages of oral biofilm formation, as well as relevant influencing factors, in order to achieve an objective and comprehensive evaluation. This review is intended to detail the antibacterial mechanisms of nanoparticles during the four stages of the formation of oral biofilms: 1) acquired film formation; 2) bacterial adhesion; 3) early biofilm development; and 4) biofilm maturation. In addition, factors influencing the antibiofilm properties of nanoparticles are summarized from the aspects of nanoparticles themselves, biofilm models, and host factors. The limitations of current research and possible trends for future research are also discussed. In summary, nanoparticles are a promising antioral biofilm strategy. It is hoped that this review can serve as a reference and inspire ideas for further research on the application of nanoparticles for effectively targeting and treating oral biofilms.

Optimal environmental and culture conditions allow the in vitro coexistence of Pseudomonas aeruginosa and Staphylococcus aureus in stable biofilms

The coexistence between species that occurs in some infections remains hard to achieve in vitro since bacterial fitness differences eventually lead to a single organism dominating the mixed culture. Pseudomonas aeruginosa and Staphylococcus aureus are major pathogens found growing together in biofilms in disease-affected lungs or wounds. Herein, we tested and analyzed different culture media, additives and environmental conditions to support P. aeruginosa and S. aureus coexistence in vitro. We have unraveled the potential of DMEM to support the growth of these two organisms in mature cocultured biofilms (three days old) in an environment that dampens the pH rise. Our conditions use equal initial inoculation ratios of both strains and allow the stable formation of separate S. aureus microcolonies that grow embedded in a P. aeruginosa biofilm, as well as S. aureus biofilm overgrowth when bovine serum albumin is added to the system. Remarkably, we also found that S. aureus survival is strictly dependent on a well-characterized phenomenon of oxygen stratification present in the coculture biofilm. An analysis of differential tolerance to gentamicin and ciprofloxacin treatment, depending on whether P. aeruginosa and S. aureus were growing in mono- or coculture biofilms, was used to validate our in vitro coculture conditions.

The Role of a Host-Induced Arginase of Xanthomonas oryzae pv. oryzae in Promoting Virulence on Rice

The plant bacterial pathogen Xanthomonas oryzae pv. oryzae causes bacterial blight of rice, which is one of the most destructive rice diseases prevalent in Asia and parts of Africa. Despite many years of research, how X. oryzae pv. oryzae causes bacterial blight of rice is still not completely understood. Here, we show that the loss of the rocF gene caused a significant decrease in the virulence of X. oryzae pv. oryzae in the susceptible rice cultivar IR24. Bioinformatics analysis demonstrated that rocF encodes arginase. Quantitative real-time PCR and Western blot assays revealed that rocF expression was significantly induced by rice and arginine. The rocF deletion mutant strain showed elevated sensitivity to hydrogen peroxide, reduced extracellular polysaccharide (EPS) production, and reduced biofilm formation, all of which are important determinants for the full virulence of X. oryzae pv. oryzae, compared with the wild-type strain. Taken together, the results of this study revealed a mechanism by which a bacterial arginase is required for the full virulence of X. oryzae pv. oryzae on rice because of its contribution to tolerance to reactive oxygen species, EPS production, and biofilm formation.

Exploiting biofilm phenotypes for functional characterization of hypothetical genes in Enterococcus faecalis

Enterococcus faecalis is a commensal organism as well as an important nosocomial pathogen, and its infections are typically linked to biofilm formation. Nearly 25% of the E. faecalis OG1RF genome encodes hypothetical genes or genes of unknown function. Elucidating their function and how these gene products influence biofilm formation is critical for understanding E. faecalis biology. To identify uncharacterized early biofilm determinants, we performed a genetic screen using an arrayed transposon (Tn) library containing ~2000 mutants in hypothetical genes/intergenic regions and identified eight uncharacterized predicted protein-coding genes required for biofilm formation. We demonstrate that OG1RF_10435 encodes a phosphatase that modulates global protein expression and arginine catabolism and propose renaming this gene bph (biofilm phosphatase). We present a workflow for combining phenotype-driven experimental and computational evaluation of hypothetical gene products in E. faecalis, which can be used to study hypothetical genes required for biofilm formation and other phenotypes of diverse bacteria.

Streptococcus gordonii Challisin protease is required for sensing cell--cell contact with Actinomyces oris

The ability of microorganisms to regulate gene expression is thought to be critical for survival and growth during the development of polymicrobial biofilms such as dental plaque. The commensal dental plaque colonizer, Streptococcus gordonii, responds to cell--cell contact (coaggregation) with Actinomyces oris by regulating >20 genes, including those involved in arginine biosynthesis. We hypothesized that an S. gordonii extracellular protease is critical for sensing by providing amino acids that modulate gene expression. S. gordonii coaggregated strongly with A. oris in buffer, saliva or chemically defined medium (CDM). In wild-type S. gordonii, expression of arginine biosynthesis genes argC and argG increased within two hours' growth in CDM in monocultures, but not following coaggregation with A. oris. By contrast, coaggregation of A. oris with an S. gordonii mutant lacking sgc, encoding the extracellular protease Challisin, resulted in increases in argC and argG gene expression that were similar to monocultures. Genetic complementation of sgc restored the ability of S. gordonii to sense coaggregation with A. oris. Coaggregation enabled growth of S. gordonii in low/no arginine and disruption of sgc did not affect this ability. We propose that extracellular bacterial proteases may be key mediators of cell--cell contact sensing by diverse microbial species.

Analysis of proteomic responses of freeze-dried Oenococcus oeni to access the molecular mechanism of acid acclimation on cell freeze-drying resistance

Malolactic fermentation (MLF), usually induced by Oenococcus oeni (O. oeni), is an important process to improve wine quality. Acid acclimation has been proven to be useful for enhancing the viability of lyophilized O. oeni. To explain the involved mechanisms, cell integrity, morphology and protein patterns of lyophilized O. oeni SD-2a were investigated with acid acclimation. After lyophilization, improvement of cell integrity and more extracellular polymeric substances (EPS) were observed in acid acclimated cells. Combined with GO and KEGG analysis, different abundant proteins were noticeably enriched in the carbohydrate metabolism process, especially amino sugar and nucleotide sugar metabolism. The most significant result was the over-expression of proteins participating in cell wall biosynthesis, EPS production, ATP binding and the bacterial secretion system. This result indicated the important role of acid acclimation on cell envelope properties. In addition, protein response to stress and arginine deiminase pathway were also proven to be over-expressed.

Expression of Adhesive Pili and the Collagen-Binding Adhesin Ace Is Activated by ArgR Family Transcription Factors in Enterococcus faecalis

It was shown previously that the disruption of the ahrC gene encoding a predicted ArgR family transcription factor results in a severe defect in biofilm formation in vitro, as well as a significant attenuation of virulence of Enterococcus faecalis strain OG1RF in multiple experimental infection models. Using transcriptome sequencing (RNA-seq), we observed ahrC-dependent changes in the expression of more than 20 genes. AhrC-repressed genes included predicted determinants of arginine catabolism and several other metabolic genes and predicted transporters, while AhrC-activated genes included determinants involved in the production of surface protein adhesins. Most notably, the structural and regulatory genes of the ebp locus encoding adhesive pili were positively regulated, as well as the ace gene, encoding a collagen-binding adhesin. Using lacZ transcription reporter fusions, we determined that ahrC and a second argR transcription factor gene, argR2, both function to activate the expression of ebpR, which directly activates the transcription of the pilus structural genes. Our data suggest that in the wild-type E. faecalis, the low levels of EbpR limit the expression of pili and that biofilm biomass is also limited by the amount of pili expressed by the bacteria. The expression of ace is similarly enhanced by AhrC and ArgR2, but ace expression is not dependent on EbpR. Our results demonstrate the existence of novel regulatory cascades controlled by a pair of ArgR family transcription factors that might function as a heteromeric protein complex.IMPORTANCE Cell surface adhesins play critical roles in the formation of biofilms, host colonization, and the pathogenesis of opportunistic infections by Enterococcus faecalis Here, we present new results showing that the expression of two major enterococcal surface adhesins, ebp pili, and the collagen-binding protein Ace is positively regulated at the transcription level by two argR family transcription factors, AhrC and ArgR2. In the case of pili, the direct target of regulation is the ebpR gene, previously shown to activate the transcription of the pilus structural genes, while the activation of ace transcription appears to be directly impacted by the two ArgR proteins. These transcription factors may represent new targets for blocking enterococcal infections.

Targeting microbial biofilms: current and prospective therapeutic strategies

Biofilm formation is a key virulence factor for a wide range of microorganisms that cause chronic infections. The multifactorial nature of biofilm development and drug tolerance imposes great challenges for the use of conventional antimicrobials and indicates the need for multi-targeted or combinatorial therapies. In this Review, we focus on current therapeutic strategies and those under development that target vital structural and functional traits of microbial biofilms and drug tolerance mechanisms, including the extracellular matrix and dormant cells. We emphasize strategies that are supported by in vivo or ex vivo studies, highlight emerging biofilm-targeting technologies and provide a rationale for multi-targeted therapies aimed at disrupting the complex biofilm microenvironment.

Hygrocin C from marine-derived Streptomyces sp. SCSGAA 0027 inhibits biofilm formation in Bacillus amyloliquefaciens SCSGAB0082 isolated from South China Sea gorgonian

Several ansamycins have been reported to inhibit bacterial biofilm formation and accelerate the eradication of developed biofilms, but little is known about the effect of hygrocin C, an ansamycin, on bacterial biofilm formation. Here, hygrocin C was isolated from the marine-derived Streptomyces sp. SCSGAA 0027 and reported for the first time to be capable of inhibiting the biofilm formation of Staphylococcus aureus and Bacillus amyloliquefaciens SCSGAB0082 with the production of anti-microbial lipopeptides from South China Sea gorgonian Subergorgia suberosa at concentrations of less than minimum inhibitory concentrations. Moreover, hygrocin C also promoted the eradication of developed biofilms, affected the biofilm architecture, and lowered the extracellular polymeric matrix formation, cell motility, and surface hydrophobicity in B. amyloliquefaciens, which was in accordance with the inhibition of biofilm formation. Furthermore, transcriptome analysis revealed that hygrocin C altered the transcripts of several genes associated with bacterial chemotaxis and flagellar, two-component system and the synthesis of arginine and histidine, which are important for bacterial biofilm formation. In conclusion, hygrocin C could be used as a potential biofilm inhibitor against S. aureus and B. amyloliquefaciens. But further genetic investigations are needed to provide more details for elucidation of the molecular mechanisms responsible for the effects of hygrocin C on B. amyloliquefaciens biofilm formation.

Arginine Exposure Decreases Acidogenesis in Long-Term Oral Biofilm Microcosms

Arginine is used in dental health formulations to help prevent dental cavities. This study assessed the effects of the long-term dosing of laboratory dental plaques with an arginine dentifrice. Data indicate that the addition of arginine dentifrice during sucrose challenge significantly increased plaque pH, thus potentially mitigating cariogenesis. Several functional groups of bacteria associated with tooth decay were significantly decreased in the laboratory plaques during exposure to the arginine dentifrice.

Arginine is an important pH-elevating agent in the oral cavity. It has been incorporated in oral hygiene formulations to mitigate sensitivity and to prevent caries. In this investigation, the effects of sustained arginine dosing of dental plaque microcosms on bacteriological composition and pH were evaluated under controlled conditions. Plaque microcosms were established in constant-depth film fermentors (CDFFs) using salivary inocula and fed continuously with artificial saliva. To simulate resting and cariogenic states, the CDFFs were supplemented with sterile water or 5% sucrose, respectively. Plaques were then dosed twice daily with a dentifrice with 1.5% arginine arginine added (DA) or without arginine (DN). This regimen continued for over 3 weeks, after which fermentors were maintained without dosing. Microcosms were analyzed by differential viable counting, with a pH microelectrode, and by eubacterial DNA profiling. Sucrose dosing was associated with significantly (P < 0.001) decreased pH, significantly (P < 0.05) increased counts of total aerobes, Gram-negative anaerobes, aciduric species, acidogenic species, arginine utilizing species, bifidobacteria, lactobacilli and streptococci, and significant (P < 0.05) changes in DNA profiles. Plaques dosed with DA had a significantly (P < 0.001) higher pH than those dosed with DN, with or without sucrose supplementation. Dosing with DA but not DN significantly (P < 0.05) decreased counts of all functional bacterial groups apart from the total anaerobes in cariogenic plaques, and in resting plaques, dosing with DA significantly (P < 0.05) decreased counts of streptococci, lactobacilli, bifidobacteria, and acidogenic bacteria. In summary, sustained exposure of oral microcosms to arginine in formulation significantly increased plaque pH and significantly reduced the viability of cariogenic bacterial species.

IMPORTANCE Arginine is used in dental health formulations to help prevent dental cavities. This study assessed the effects of the long-term dosing of laboratory dental plaques with an arginine dentifrice. Data indicate that the addition of arginine dentifrice during sucrose challenge significantly increased plaque pH, thus potentially mitigating cariogenesis. Counts of several functional groups of bacteria associated with tooth decay were significantly decreased in the laboratory plaques during exposure to the arginine dentifrice.

Effects of Arginine on Streptococcus mutans Growth, Virulence Gene Expression, and Stress Tolerance

Streptococcus mutans is a common constituent of oral biofilms and a primary etiologic agent of human dental caries. The bacteria associated with dental caries have potent abilities to produce organic acids from dietary carbohydrates and to grow and metabolize in acidic conditions. By contrast, many commensal bacteria produce ammonia through the arginine deiminase system (ADS), which moderates the pH of oral biofilms. Arginine metabolism by the ADS is a significant deterrent to the initiation and progression of dental caries. In this study, we observed how exogenously provided l-arginine affects the growth, the virulence properties, and the tolerance of environmental stresses of S. mutans Supplementation with 1.5% arginine (final concentration) had an inhibitory effect on the growth of S. mutans in complex and chemically defined media, particularly when cells were exposed to acid or oxidative stress. The genes encoding virulence factors required for attachment/accumulation (gtfB and spaP), bacteriocins (nlmA, nlmB, nlmD, and cipB), and the sigma factor required for competence development (comX) were downregulated during growth with 1.5% arginine. Deep sequencing of RNA (RNA-Seq) comparing the transcriptomes of S. mutans growing in chemically defined media with and without 1.5% arginine revealed differential expression of genes encoding ATP-binding cassette transporters, metal transporters, and constituents required for survival, metabolism, and biofilm formation. Therefore, the mechanisms of action by which arginine inhibits dental caries include direct adverse effects on multiple virulence-related properties of the most common human dental caries pathogen.IMPORTANCE Metabolism of the amino acid arginine by the arginine deiminase system (ADS) of certain oral bacteria raises the pH of dental plaque and provides a selective advantage to health-associated bacteria, thereby protecting the host from dental caries (cavities). Here, we examine the effects of arginine on the cavity-causing bacterium Streptococcus mutans We find that arginine negatively impacts the growth, the pathogenic potential, and the tolerance of environmental stresses in a way that is likely to compromise the ability of S. mutans to cause disease. Using genetic and genomic techniques, multiple mechanisms by which arginine exerts its influence on virulence-related properties of S. mutans are discovered. This report demonstrates that a primary mechanism of action by which arginine inhibits the initiation and progression of dental caries may be by reducing the pathogenic potential of S. mutans.

Mutation of the Streptococcus gordonii Thiol-Disulfide Oxidoreductase SdbA Leads to Enhanced Biofilm Formation Mediated by the CiaRH Two-Component Signaling System

Streptococcus gordonii is a commensal inhabitant of human oral biofilms. Previously, we identified an enzyme called SdbA that played an important role in biofilm formation by S. gordonii. SdbA is thiol-disulfide oxidoreductase that catalyzes disulfide bonds in secreted proteins. Surprisingly, inactivation of SdbA results in enhanced biofilm formation. In this study we investigated the basis for biofilm formation by the ΔsdbA mutant. The results revealed that biofilm formation was mediated by the interaction between the CiaRH and ComDE two-component signalling systems. Although it did not affect biofilm formation by the S. gordonii parent strain, CiaRH was upregulated in the ΔsdbA mutant and it was essential for the enhanced biofilm phenotype. The biofilm phenotype was reversed by inactivation of CiaRH or by the addition of competence stimulating peptide, the production of which is blocked by CiaRH activity. Competition assays showed that the enhanced biofilm phenotype also corresponded to increased oral colonization in mice. Thus, the interaction between SdbA, CiaRH and ComDE affects biofilm formation both in vitro and in vivo.

l-Arginine Modifies the Exopolysaccharide Matrix and Thwarts Streptococcus mutans Outgrowth within Mixed-Species Oral Biofilms

l-Arginine, a ubiquitous amino acid in human saliva, serves as a substrate for alkali production by arginolytic bacteria. Recently, exogenous l-arginine has been shown to enhance the alkalinogenic potential of oral biofilm and destabilize its microbial community, which might help control dental caries. However, l-arginine exposure may inflict additional changes in the biofilm milieu when bacteria are growing under cariogenic conditions. Here, we investigated how exogenous l-arginine modulates biofilm development using a mixed-species model containing both cariogenic (Streptococcus mutans) and arginolytic (Streptococcus gordonii) bacteria in the presence of sucrose. We observed that 1.5% (wt/vol) l-arginine (also a clinically effective concentration) exposure suppressed the outgrowth of S. mutans, favored S. gordonii dominance, and maintained Actinomyces naeslundii growth within biofilms (versus vehicle control). In parallel, topical l-arginine treatments substantially reduced the amounts of insoluble exopolysaccharides (EPS) by >3-fold, which significantly altered the three-dimensional (3D) architecture of the biofilm. Intriguingly, l-arginine repressed S. mutans genes associated with insoluble EPS (gtfB) and bacteriocin (SMU.150) production, while spxB expression (H2O2 production) by S. gordonii increased sharply during biofilm development, which resulted in higher H2O2 levels in arginine-treated biofilms. These modifications resulted in a markedly defective EPS matrix and areas devoid of any bacterial clusters (microcolonies) on the apatitic surface, while the in situ pH values at the biofilm-apatite interface were nearly one unit higher in arginine-treated biofilms (versus the vehicle control). Our data reveal new biological properties of l-arginine that impact biofilm matrix assembly and the dynamic microbial interactions associated with pathogenic biofilm development, indicating the multiaction potency of this promising biofilm disruptor.

IMPORTANCE

Dental caries is one of the most prevalent and costly infectious diseases worldwide, caused by a biofilm formed on tooth surfaces. Novel strategies that compromise the ability of virulent species to assemble and maintain pathogenic biofilms could be an effective alternative to conventional antimicrobials that indiscriminately kill other oral species, including commensal bacteria. l-Arginine at 1.5% has been shown to be clinically effective in modulating cariogenic biofilms via alkali production by arginolytic bacteria. Using a mixed-species ecological model, we show new mechanisms by which l-arginine disrupts the process of biofilm matrix assembly and the dynamic microbial interactions that are associated with cariogenic biofilm development, without impacting the bacterial viability. These results may aid in the development of enhanced methods to control biofilms using l-arginine.

A Type I Signal Peptidase Is Required for Pilus Assembly in the Gram-Positive, Biofilm-Forming Bacterium Actinomyces oris

The Gram-positive bacterium Actinomyces oris, a key colonizer in the development of oral biofilms, contains 18 LPXTG motif-containing proteins, including fimbrillins that constitute two fimbrial types critical for adherence, biofilm formation, and polymicrobial interactions. Export of these protein precursors, which harbor a signal peptide, is thought to be mediated by the Sec machine and require cleavage of the signal peptide by type I signal peptidases (SPases). Like many Gram-positive bacteria, A. oris expresses two SPases, named LepB1 and LepB2. The latter has been linked to suppression of lethal "glyco-stress," caused by membrane accumulation of the LPXTG motif-containing glycoprotein GspA when the housekeeping sortase srtA is genetically disrupted. Consistent with this finding, we show here that a mutant lacking lepB2 and srtA was unable to produce high levels of glycosylated GspA and hence was viable. However, deletion of neither lepB1 nor lepB2 abrogated the signal peptide cleavage and glycosylation of GspA, indicating redundancy of SPases for GspA. In contrast, the lepB2 deletion mutant failed to assemble the wild-type levels of type 1 and 2 fimbriae, which are built by the shaft fimbrillins FimP and FimA, respectively; this phenotype was attributed to aberrant cleavage of the fimbrillin signal peptides. Furthermore, the lepB2 mutants, including the catalytically inactive S101A and K169A variants, exhibited significant defects in polymicrobial interactions and biofilm formation. Conversely, lepB1 was dispensable for the aforementioned processes. These results support the idea that LepB2 is specifically utilized for processing of fimbrial proteins, thus providing an experimental model with which to study the basis of type I SPase specificity.

IMPORTANCE

Sec-mediated translocation of bacterial protein precursors across the cytoplasmic membrane involves cleavage of their signal peptide by a signal peptidase (SPase). Like many Gram-positive bacteria, A. oris expresses two SPases, LepB1 and LepB2. The latter is a genetic suppressor of lethal "glyco-stress" caused by membrane accumulation of glycosylated GspA when the housekeeping sortase srtA is genetically disrupted. We show here that LepB1 and LepB2 are capable of processing GspA, whereas only LepB2 is required for cleavage of fimbrial signal peptides. This is the first example of a type I SPase dedicated to LPXTG motif-containing fimbrial proteins. Thus, A. oris provides an experimental model with which to investigate the specificity mechanism of type I SPases.

Low Concentrations of Nitric Oxide Modulate Streptococcus pneumoniae Biofilm Metabolism and Antibiotic Tolerance

Streptococcus pneumoniaeis one of the key pathogens responsible for otitis media (OM), the most common infection in children and the largest cause of childhood antibiotic prescription. Novel therapeutic strategies that reduce the overall antibiotic consumption due to OM are required because, although widespread pneumococcal conjugate immunization has controlled invasive pneumococcal disease, overall OM incidence has not decreased. Biofilm formation represents an important phenotype contributing to the antibiotic tolerance and persistence ofS. pneumoniaein chronic or recurrent OM. We investigated the treatment of pneumococcal biofilms with nitric oxide (NO), an endogenous signaling molecule and therapeutic agent that has been demonstrated to trigger biofilm dispersal in other bacterial species. We hypothesized that addition of low concentrations of NO to pneumococcal biofilms would improve antibiotic efficacy and that higher concentrations exert direct antibacterial effects. Unlike in many other bacterial species, low concentrations of NO did not result inS. pneumoniaebiofilm dispersal. Instead, treatment of bothin vitrobiofilms andex vivoadenoid tissue samples (a reservoir forS. pneumoniaebiofilms) with low concentrations of NO enhanced pneumococcal killing when combined with amoxicillin-clavulanic acid, an antibiotic commonly used to treat chronic OM. Quantitative proteomic analysis using iTRAQ (isobaric tag for relative and absolute quantitation) identified 13 proteins that were differentially expressed following low-concentration NO treatment, 85% of which function in metabolism or translation. Treatment with low-concentration NO, therefore, appears to modulate pneumococcal metabolism and may represent a novel therapeutic approach to reduce antibiotic tolerance in pneumococcal biofilms.

Metabolic engineering of acid resistance elements to improve acid resistance and propionic acid production of Propionibacterium jensenii

Propionic acid (PA) and its salts are widely used in the food, pharmaceutical, and chemical industries. Microbial production of PA by propionibacteria is a typical product-inhibited process, and acid resistance is crucial in the improvement of PA titers and productivity. We previously identified two key acid resistance elements-the arginine deaminase and glutamate decarboxylase systems-that protect propionibacteria against PA stress by maintaining intracellular pH homeostasis. In this study, we attempted to improve the acid resistance and PA production of Propionibacterium jensenii ATCC 4868 by engineering these elements. Specifically, five genes (arcA, arcC, gadB, gdh, and ybaS) encoding components of the arginine deaminase and glutamate decarboxylase systems were overexpressed in P. jensenii. The activities of the five enzymes in the engineered strains were 26.7-489.0% higher than those in wild-type P. jensenii. The growth rates of the engineered strains decreased, whereas specific PA production increased significantly compared with those of the wild-type strain. Among the overexpressed genes, gadB (encoding glutamate decarboxylase) increased PA resistance and yield most effectively; the PA resistance of P. jensenii-gadB was more than 10-fold higher than that of the wild-type strain, and the production titer, yield, and conversion ratio of PA reached 10.81 g/L, 5.92 g/g cells, and 0.56 g/g glycerol, representing increases of 22.0%, 23.8%, and 21.7%, respectively. We also investigated the effects of introducing these acid resistance elements on the transcript levels of related enzymes. The results showed that the expression of genes in the engineered pathways affected the expression of the other genes. Additionally, the intracellular pools of amino acids were altered as different genes were overexpressed, which may further contribute to the enhanced PA production. This study provides an effective strategy for improving PA production in propionibacteria; this strategy may be useful for the production of other organic acids. Biotechnol. Bioeng. 2016;113: 1294-1304. © 2015 Wiley Periodicals, Inc.

Dual lifestyle of Porphyromonas gingivalis in biofilm and gingival cells

Porphyromonas gingivalis is deeply involved in the pathogenesis of marginal periodontitis, and recent findings have consolidated its role as an important and unique pathogen. This bacterium has a unique dual lifestyle in periodontal sites including subgingival dental plaque (biofilm) and gingival cells, as it has been clearly shown that P. gingivalis is able to exert virulence using completely different tactics in each environment. Inter-bacterial cross-feeding enhances the virulence of periodontal microflora, and such metabolic and adhesive interplay creates a supportive environment for P. gingivalis and other species. Human oral epithelial cells harbor a large intracellular bacterial load, resembling the polymicrobial nature of periodontal biofilm. P. gingivalis can enter gingival epithelial cells and pass through the epithelial barrier into deeper tissues. Subsequently, from its intracellular position, the pathogen exploits cellular recycling pathways to exit invaded cells, by which it is able to control its population in infected tissues, allowing for persistent infection in gingival tissues. Here, we outline the dual lifestyle of P. gingivalis in subgingival areas and its effects on the pathogenesis of periodontitis.

Arginine-Ornithine Antiporter ArcD Controls Arginine Metabolism and Interspecies Biofilm Development of Streptococcus gordonii

Arginine is utilized by the oral inhabitant Streptococcus gordonii as a substrate of the arginine deiminase system (ADS), eventually producing ATP and NH₃, the latter of which is responsible for microbial resistance to pH stress. S. gordonii expresses a putative arginine-ornithine antiporter (ArcD) whose function has not been investigated despite relevance to the ADS and potential influence on inter-bacterial communication with periodontal pathogens that utilize amino acids as a main energy source. Here, we generated an S. gordonii ΔarcD mutant to explore the role of ArcD in physiological homeostasis and bacterial cross-feeding. First, we confirmed that S. gordonii ArcD plays crucial roles for mediating arginine uptake and promoting bacterial growth, particularly under arginine-limited conditions. Next, metabolomic profiling and transcriptional analysis of the ΔarcD mutant revealed that deletion of this gene caused intracellular accumulation of ornithine leading to malfunction of the ADS and suppression of de novo arginine biosynthesis. The mutant strain also showed increased susceptibility to low pH stress due to reduced production of ammonia. Finally, accumulation of Fusobacterium nucleatum was found to be significantly decreased in biofilm formed by the ΔarcD mutant as compared with the wild-type strain, although ornithine supplementation restored fusobacterium biovolume in dual-species biofilms with the ΔarcD mutant and also enhanced single species biofilm development by F. nucleatum. Our results are the first direct evidence showing that S. gordonii ArcD modulates not only alkali and energy production but also interspecies interaction with F. nucleatum, thus initiating a middle stage of periodontopathic biofilm formation, by metabolic cross-feeding.