Evolution of resistance to cationic biocides in Streptococcus mutans and Enterococcus faecalis

Insights towards the impact of subinhibitory chlorhexidine on antimicrobial susceptibility and horizontal gene transfer in Enterococcus faecium

Enterococcus faecium, a human and animal commensal broadly distributed in the environment, is currently one of the most challenging multidrug-resistant (MDR) healthcare-associated pathogens worldwide. It is often exposed to chlorhexidine (CHX), a broad-spectrum antiseptic, extensively used in healthcare, domestic, and food production settings, and a diffused polluter. However, the impact of gradients of CHX concentrations, including at subinhibitory levels, on E. faecium adaptation to various antimicrobials remains unclear. Our study aimed to explore the effects of subinhibitory CHX concentrations on biocides and antibiotics susceptibility as well as in the transfer of clinically relevant antibiotic resistance genes among E. faecium (n = 11) from diverse sources and clonal backgrounds. Serial exposure to increasing CHX concentrations resulted in strain-specific MICCHX and MBCCHX changes among six E. faecium studied. These strains presented different CHX genotypes, namely the P102H mutation in DNA-binding response regulator ChtR in two strains showing twofold increased MICCHX and/or MBCCHX, and an absent EfrEF transporter in a strain exhibiting increased CHX susceptibility after exposure. Whole-genome comparison between parental and CHX-adapted strains found no alterations in genes with a recognized role in CHX reduced susceptibility. Additionally, in a different assay, subinhibitory CHX exposure enhanced the transfer (up to 12.5-fold) of vancomycin or linezolid resistance genes among most E. faecium strains tested, except one lacking a functional EfrEF transporter. Our data suggest that subinhibitory CHX concentrations could have a role in Enterococcus adaptation to CHX and in the spread of antibiotic resistance through horizontal transfer events. Further investigation is warranted to elucidate the underlying mechanisms driving these phenomena in E. faecium, ensuring the continued effectiveness of both CHX and antibiotics, and safeguarding Public Health.

Challenges of quaternary ammonium antimicrobial agents: Mechanisms, resistance, persistence and impacts on the microecology

Quaternary ammonium compounds (QACs) served as broad spectrum antimicrobial agents are widely applied for surface disinfection, skin and mucous disinfection, and mouthwash. The daily applications of QACs have significantly increased, especially during the COVID-19 pandemic. However, the environmental residues of QACs have demonstrated harmful impacts on the environment, leading to an increase in environmental contamination, resistant microbes and disruption of microecology. The actions of QACs were related to their cationic character, which can impact the negatively charged cell membranes, but the details are still unclear. Moreover, bacteria with lower sensitivity and resistant pathogens have been detected in clinics and environments, while QACs were also reported to induce the formation of bacterial persisters. Even worse, the resistant bacteria even showed co-resistance and cross-resistance with traditional antibiotics, decreasing therapeutic effectiveness, and disrupting the microecology homeostasis. Unfortunately, the resistance and persistence mechanisms of QACs and the effects of QACs on microecology are still not clear, which even neglected during their daily usages. Therefore, we summarized and discussed current understandings on the antimicrobial actions, resistance, persistence and impacts on the microecology to highlight the challenges in the QACs applications and discuss the possible strategies for overcoming their drawbacks.

Current insights into the effects of cationic biocides exposure on Enterococcus spp

Cationic biocides (CBs), such as quaternary ammonium compounds and biguanides, are critical for controlling the spread of bacterial pathogens like Enterococcus spp., a leading cause of multidrug-resistant healthcare-associated infections. The widespread use of CBs in recent decades has prompted concerns about the potential emergence of Enterococcus spp. populations exhibiting resistance to both biocides and antibiotics. Such concerns arise from their frequent exposure to subinhibitory concentrations of CBs in clinical, food chain and diverse environmental settings. This comprehensive narrative review aimed to explore the complexity of the Enterococcus' response to CBs and of their possible evolution toward resistance. To that end, CBs' activity against diverse Enterococcus spp. collections, the prevalence and roles of genes associated with decreased susceptibility to CBs, and the potential for co- and cross-resistance between CBs and antibiotics are reviewed. Significant methodological and knowledge gaps are identified, highlighting areas that future studies should address to enhance our comprehension of the impact of exposure to CBs on Enterococcus spp. populations' epidemiology. This knowledge is essential for developing effective One Health strategies that ensure the continued efficacy of these critical agents in safeguarding Public Health.

Efficacy of biofilm disrupters against Candida auris and other Candida species in monomicrobial and polymicrobial biofilms

Candida species are now considered global threats by the CDC and WHO. Candida auris specifically is on the critical pathogen threat list along with Candida albicans. In addition, it is not uncommon to find Candida spp. in a mixed culture with bacterial organisms, especially Staphylococcus aureus producing polymicrobial infections. To eradicate these organisms from the environment and from patient surfaces, surface agents such as chlorhexidine (CHD) and Puracyn are used. Biofilm disrupters (BDs) are novel agents with a broad spectrum of antimicrobial activity and have been used in the management of chronic wounds and to sterilise environmental surfaces for the past several years. The goal of this study was to evaluate BDs (BlastX, Torrent, NSSD) and CHD against Candida spp. and S. aureus using zone of inhibition assays, biofilm and time-kill assays. All BDs and CHD inhibited C. auris growth effectively in a concentration-dependent manner. Additionally, CHD and the BDs showed excellent antimicrobial activity within polymicrobial biofilms. A comparative analysis of the BDs and CHD against C. auris and C. albicans using biofilm kill-curves showed at least 99.999% killing. All three BDs and CHD have excellent activity against different Candida species, including C. auris. However, one isolate of C. auris in a polymicrobial biofilm assay showed resistance/tolerance to CHD, but not to the BDs. The fungicidal activity of these novel agents will be valuable in eradicating surface colonisation of Candida spp, especially C. auris from colonised environmental surfaces and from wounds in colonised patients.

Mouthwash Effects on the Oral Microbiome: Are They Good, Bad, or Balanced?

This narrative review describes the oral microbiome, and its role in oral health and disease, before considering the impact of commonly used over-the-counter (OTC) mouthwashes on oral bacteria, viruses, bacteriophages, and fungi that make up these microbial communities in different niches of the mouth. Whilst certain mouthwashes have proven antimicrobial actions and clinical effectiveness supported by robust evidence, this review reports more recent metagenomics evidence, suggesting that mouthwashes such as chlorhexidine may cause "dysbiosis," whereby certain species of bacteria are killed, leaving others, sometimes unwanted, to predominate. There is little known about the effects of mouthwashes on fungi and viruses in the context of the oral microbiome (virome) in vivo, despite evidence that they "kill" certain viral pathogens ex vivo. Evidence for mouthwashes, much like antibiotics, is also emerging with regards to antimicrobial resistance, and this should further be considered in the context of their widespread use by clinicians and patients. Therefore, considering the potential of currently available OTC mouthwashes to alter the oral microbiome, this article finally proposes that the ideal mouthwash, whilst combatting oral disease, should "balance" antimicrobial communities, especially those associated with health. Which antimicrobial mouthwash best fits this ideal remains uncertain.

Enhanced in vitro antibacterial effect against Enterococcus faecalis by using both low-dose cetylpyridinium chloride and silver ions

BACKGROUND

Enterococcus faecalis (E. faecalis) is frequently isolated from root canals with failed root canal treatments. Due to the strong ability of E. faecalis to resist many often-used antimicrobials, coping with E. faecalis infections remains a challenge. The aim of this study was to investigate the synergistic antibacterial effect of low-dose cetylpyridinium chloride (CPC) and silver ions (Ag⁺) against E. faecalis in vitro.

METHODS

The minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and the fractional inhibitory concentration index (FICI) were used to confirm the existence of the synergic antibacterial activity between low-dose CPC and Ag⁺. Colony-forming unit (CFU) counting, time-killing curve and dynamic growth curve were used to evaluate the antimicrobial effects of CPC and Ag⁺ combinations against planktonic E. faecalis. Four weeks biofilms were treated with drug-contained gels to determine the antimicrobial effect on biofilm-resident E.faecalis, and the integrity of E.faecalis and its biofilms were observed by FE-SEM. CCK-8 assays was used to test the cytotoxicity of CPC and Ag⁺ combinations on MC3T3-E1 cells.

RESULTS

The results confirmed the synergistic antibacterial effect of low-dose CPC and Ag⁺ against both planktonic and 4-week biofilm E. faecalis. After the addition of CPC, the sensitivity of both planktonic and biofilm-resident E. faecalis to Ag⁺ improved, and the combination showed good biocompatibility on MC3T3-E1 cells.

CONCLUSIONS

Low-dose CPC enhanced the antibacterial ability of Ag⁺ against both planktonic and biofilm E.faecalis with good biocompatibility. It may be developed into a novel and potent antibacterial agent against E.faecalis, with low toxicity for root canal disinfection or other related medical applications.

A Chinese herb preparation, honokiol, inhibits Streptococcus mutans biofilm formation

OBJECTIVE

This study aimed to investigate the antibiofilm and anticariogenic effects of honokiol, a traditional Chinese medicine, on the cariogenic bacterium Streptococcus mutans (S. mutans).

DESIGN

The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of honokiol on S. mutans UA159 were measured. Then, S. mutans were treated with honokiol at concentrations of 1/2 MIC and 1/4 MIC. Extracellular polysaccharide (EPS) synthesis was assessed with confocal laser scanning microscopy (CLSM) and the anthrone-sulfuric method. Crystal violet staining and scanning electron microscopy (SEM) were used to demonstrate the characteristics and morphology of S. mutans biofilms. Colony-forming unit (CFU) assay was performed to observe the antibacterial effect of honokiol. Lactic acid production of 24-h biofilms was measured by the lactic acid assay. The expression level of caries-related genes (gtfB/C/D, comD/E and ldh) was identified by quantitative real-time PCR (qRTPCR) to explore the relevant mechanism. And the cytotoxic effect on human gingival fibroblasts (HGFs) was evaluated by the Cell Counting Kit-8 (CCK-8) assay.

RESULTS

The MIC and MBC of honokiol on S. mutans were 30 μg/mL and 60 μg/mL, respectively. Honokiol inhibited biofilm formation, EPS synthesis and lactic acid production. It also decreased the expression of glucosyltransferases (Gtfs) and quorum sensing (QS) system encoding genes. Moreover, honokiol showed favorable biocompatibility with HGFs.

CONCLUSIONS

Honokiol has an inhibitory effect on S. mutans and favorable biocompatibility, with application potential as a novel anticaries agent.

Antibiotic Resistance of Selected Bacteria after Treatment of the Supragingival Biofilm with Subinhibitory Chlorhexidine Concentrations

Due to increasing rates of antibiotic resistance and very few novel developments of antibiotics, it is crucial to understand the mechanisms of resistance development. The aim of the present study was to investigate the adaptation of oral bacteria to the frequently used oral antiseptic chlorhexidine digluconate (CHX) and potential cross-adaptation to antibiotics after repeated exposure of supragingival plaque samples to subinhibitory concentrations of CHX. Plaque samples from six healthy donors were passaged for 10 days in subinhibitory concentrations of CHX, while passaging of plaque samples without CHX served as control. The surviving bacteria were cultured on agar plates and identified with Matrix-assisted Laser Desorption/Ionization-Time of Flight-Mass spectrometry (MALDI-TOF). Subsequently, the minimum inhibitory concentrations (MIC) of these isolates toward CHX were determined using a broth-microdilution method, and phenotypic antibiotic resistance was evaluated using the epsilometertest. Furthermore, biofilm-forming capacities were determined. Repeated exposure of supragingival plaque samples to subinhibitory concentrations of CHX led to the selection of oral bacteria with 2-fold up to 4-fold increased MICs toward CHX. Furthermore, these isolates showed up to 12-fold increased MICs towards some antibiotics such as erythromycin and clindamycin. Conversely, biofilm-forming capacity was decreased. In summary, this study shows that oral bacteria are able to adapt to CHX, while also decreasing their susceptibility to antibiotics.

Antimicrobial effects of bacterial binding to a dialkylcarbamoyl chloride-coated wound dressing: an in vitro study

OBJECTIVE

Wound dressings that inactivate or sequestrate microorganisms, such as those with a hydrophobic, bacteria-binding dialkylcarbamoyl chloride (DACC) surface, can reduce the risk of clinical infections. This 'passive' bioburden control, avoiding bacterial cell wall disruption with associated release of bacterial endotoxins aggravating inflammation, is advantageous in hard-to-heal wounds. Hence, the full scope of DACC dressings, including the potential impact of higher inoculum densities, increased protein load and different pH on antibacterial activity, needs to be evaluated.

METHOD

The Japanese Industrial Standard (JIS) L 1902 challenge test was used to evaluate the antimicrobial activity of the DACC-coated dressing against several World Health Organization (WHO)-prioritised wound pathogens (e.g., meticillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus, microorganisms with extended-spectrum beta-lactamases and Acinetobacter baumannii), the effect of repeated bacterial challenge in an adverse wound environment, and antimicrobial performance at wound-related pH.

RESULTS

High antibacterial activity of the DACC-coated dressing against the WHO-prioritised bacteria strains by its irreversible binding and inhibition of growth of bound bacteria was confirmed using JIS L 1902. At increased inoculation densities, compared to standard conditions, the DACC-coated dressing still achieved strong-to-significant antibacterial effects. Augmenting the media protein content also affected antibacterial performance; a 0.5-1 log reduction in antibacterial activity was observed upon addition of 10% fetal calf serum. The pH did not influence antibacterial performance. The DACC-coated dressing also sustained antibacterial activity over subsequent reinfection steps.

CONCLUSION

It can be assumed that the DACC-coated dressing exerts beneficial effects in controlling the wound bioburden, reducing the overall demand placed on antibiotics, without using antimicrobial substances.

Ecological Effects of Daily Antiseptic Treatment on Microbial Composition of Saliva-Grown Microcosm Biofilms and Selection of Resistant Phenotypes

Antiseptics are widely used in dental practice and included in numerous over-the-counter oral care products. However, the effects of routine antiseptic use on microbial composition of oral biofilms and on the emergence of resistant phenotypes remain unclear. Microcosm biofilms were inoculated from saliva samples of four donors and cultured in the Amsterdam Active Attachment biofilm model for 3 days. Then, they were treated two times daily with chlorhexidine digluconate (CHX) or cetylpyridinium chloride (CPC) for a period of 7 days. Ecological changes upon these multiple antiseptic treatments were evaluated by semiconductor-based sequencing of bacterial 16S rRNA genes and identification of amplicon sequence variants (ASVs). Furthermore, culture-based approaches were used for colony-forming units (CFU) assay, identification of antiseptic-resistant phenotypes using an agar dilution method, and evaluation of their antibiotic susceptibilities. Both CHX and CPC showed only slight effects on CFU and could not inhibit biofilm growth despite the two times daily treatment for 7 days. Both antiseptics showed significant ecological effects on the microbial compositions of the surviving microbiota, whereby CHX led to enrichment of rather caries-associated saccharolytic taxa and CPC led to enrichment of rather gingivitis-associated proteolytic taxa. Antiseptic-resistant phenotypes were isolated on antiseptic-containing agar plates, which also exhibited phenotypic resistance to various antibiotics. Our results highlight the need for further research into potential detrimental effects of antiseptics on the microbial composition of oral biofilms and on the spread of antimicrobial resistance in the context of their frequent use in oral healthcare.

Phenotypic Adaptation to Antiseptics and Effects on Biofilm Formation Capacity and Antibiotic Resistance in Clinical Isolates of Early Colonizers in Dental Plaque

Despite the wide-spread use of antiseptics in dental practice and oral care products, there is little public awareness of potential risks associated with antiseptic resistance and potentially concomitant cross-resistance. Therefore, the aim of this study was to investigate potential phenotypic adaptation in 177 clinical isolates of early colonizers of dental plaque (Streptococcus, Actinomyces, Rothia and Veillonella spp.) upon repeated exposure to subinhibitory concentrations of chlorhexidine digluconate (CHX) or cetylpyridinium chloride (CPC) over 10 passages using a modified microdilution method. Stability of phenotypic adaptation was re-evaluated after culture in antiseptic-free nutrient broth for 24 or 72 h. Strains showing 8-fold minimal inhibitory concentration (MIC)-increase were further examined regarding their biofilm formation capacity, phenotypic antibiotic resistance and presence of antibiotic resistance genes (ARGs). Eight-fold MIC-increases to CHX were detected in four Streptococcus isolates. These strains mostly exhibited significantly increased biofilm formation capacity compared to their respective wild-type strains. Phenotypic antibiotic resistance was detected to tetracycline and erythromycin, consistent with the detected ARGs. In conclusion, this study shows that clinical isolates of early colonizers of dental plaque can phenotypically adapt toward antiseptics such as CHX upon repeated exposure. The underlying mechanisms at genomic and transcriptomic levels need to be investigated in future studies.

Novel Rechargeable Nanostructured Calcium Phosphate Crown Cement with Long-Term Ion Release and Antibacterial Activity to Suppress Saliva Microcosm Biofilms

OBJECTIVE

Resin cements with remineralizing and antibacterial properties are favorable for inhibition of caries. The objectives of this study were: (1) to investigate the capability of the novel dimethylaminohexadecyl-methacrylate (DMAHDM) and nano-sized amorphous calcium phosphate (NACP) containing cement to reduce saliva microcosm biofilm, and (2) to investigate the long-term ion release, recharge, and re-release of DMAHDM-NACP cement.

METHODS

Pyromellitic glycerol dimethacrylate (PMGDM) and ethoxylated bisphenol-A-dimethacrylate (EBPADMA) were used to make PEHB monomer. Five cements were fabricated: (1) PEHB+0%NACP+0%DMAHDM (experimental control); (2) PEHB+25%NACP+0%DMAHDM, (3) PEHB+25%NACP+0%DMAHDM; (4) PEHB+25%NACP+3%DMAHDM; (5) PEHB+25%NACP+5%DMAHDM. RelyX luting cement was used as commercial control. Colony-forming units (CFU), lactic acid production, metabolic activities, and minimum inhibitory concentration (MIC) were performed. Long-term Calcium (Ca) and phosphate (P) ion release, recharge, and re-release were assessed.

RESULTS

Compared to experimental and commercial controls, the NACP-DMAHDM cement significantly reduced CFU biofilm by 2-3 orders of magnitude, metabolic activities from 0.24±0.06 A₅₄₀/cm² to 0.03±0.01 A₅₄₀/cm², and lactic acid production from 27.7±2.5 mmol/L to 5.4±2.1 mmol/L (n=6) (p<0.05). The DMAHDM showed an MIC value of 0.03 mg/L. However, when the DMAHDM was combined with PMGDM monomer, the MIC was greater than DMAHDM alone. The ion concentrations for the experimental groups significantly increased over time (1-84 days), indicating continuous ion release (n=3) (p<0.05). Increasing the DMAHDM mass fraction from 0% to 5% and 3% to 5% significantly enhanced ion recharge and re-release at the third cycle (p<0.05).

CONCLUSIONS

Incorporating DMAHDM and NACP into resin-based crown cement provides strong antibacterial action against saliva microcosm biofilm and presents a high level of Ca and P ion recharge abilities, exhibiting long-term Ca and P ion release and remineralization potential.

CLINICAL SIGNIFICANCE

Resin based cement containing NACP and DMAHDM were developed with remineralizing and potent antibacterial effects. This cement formulation showed ion release and remineralization potential and are promising formulations to inhibit the incidence of recurrent caries and could promote remineralization and be sustainable for the long term.

Transcriptomic Stress Response in Streptococcus mutans following Treatment with a Sublethal Concentration of Chlorhexidine Digluconate

Despite the widespread use of antiseptics such as chlorhexidine digluconate (CHX) in dental practice and oral care, the risks of potential resistance toward these antimicrobial compounds in oral bacteria have only been highlighted very recently. Since the molecular mechanisms behind antiseptic resistance or adaptation are not entirely clear and the bacterial stress response has not been investigated systematically so far, the aim of the present study was to investigate the transcriptomic stress response in Streptococcus mutans after treatment with CHX using RNA sequencing (RNA-seq). Planktonic cultures of stationary-phase S. mutans were treated with a sublethal dose of CHX (125 µg/mL) for 5 min. After treatment, RNA was extracted, and RNA-seq was performed on an Illumina NextSeq 500. Differentially expressed genes were analyzed and validated by qRT-PCR. Analysis of differential gene expression following pathway analysis revealed a considerable number of genes and pathways significantly up- or downregulated in S. mutans after sublethal treatment with CHX. In summary, the expression of 404 genes was upregulated, and that of 271 genes was downregulated after sublethal CHX treatment. Analysis of differentially expressed genes and significantly regulated pathways showed regulation of genes involved in purine nucleotide synthesis, biofilm formation, transport systems and stress responses. In conclusion, the results show a transcriptomic stress response in S. mutans upon exposure to CHX and offer insight into potential mechanisms that may result in development of resistances.

Sub-Inhibitory Concentrations of Chlorhexidine Induce Resistance to Chlorhexidine and Decrease Antibiotic Susceptibility in Neisseria gonorrhoeae

Objectives: Chlorhexidine digluconate (chlorhexidine) and Listerine^® mouthwashes are being promoted as alternative treatment options to prevent the emergence of antimicrobial resistance in Neisseria gonorrhoeae. We performed in vitro challenge experiments to assess induction and evolution of resistance to these two mouthwashes and potential cross-resistance to other antimicrobials.

Methods: A customized morbidostat was used to subject N. gonorrhoeae reference strain WHO-F to dynamically sustained Listerine^® or chlorhexidine pressure for 18 days and 40 days, respectively. Cultures were sampled twice a week and minimal inhibitory concentrations (MICs) of Listerine^®, chlorhexidine, ceftriaxone, ciprofloxacin, cefixime and azithromycin were determined using the agar dilution method. Isolates with an increased MIC for Listerine^® or chlorhexidine were subjected to whole genome sequencing to track the evolution of resistance.

Results: We were unable to increase MICs for Listerine^®. Three out of five cultures developed a 10-fold increase in chlorhexidine MIC within 40 days compared to baseline (from 2 to 20 mg/L). Increases in chlorhexidine MIC were positively associated with increases in the MICs of azithromycin and ciprofloxacin. Low-to-higher-level chlorhexidine resistance (2–20 mg/L) was associated with mutations in NorM. Higher-level resistance (20 mg/L) was temporally associated with mutations upstream of the MtrCDE efflux pump repressor (mtrR) and the mlaA gene, part of the maintenance of lipid asymmetry (Mla) system.

Conclusion: Exposure to sub-lethal chlorhexidine concentrations may not only enhance resistance to chlorhexidine itself but also cross-resistance to other antibiotics in N. gonorrhoeae. This raises concern regarding the widespread use of chlorhexidine as an oral antiseptic, for example in the field of dentistry.

Reduced Susceptibility and Increased Resistance of Bacteria against Disinfectants: A Systematic Review

Disinfectants are used to reduce the concentration of pathogenic microorganisms to a safe level and help to prevent the transmission of infectious diseases. However, bacteria have a tremendous ability to respond to chemical stress caused by biocides, where overuse and improper use of disinfectants can be reflected in a reduced susceptibility of microorganisms. This review aims to describe whether mutations and thus decreased susceptibility to disinfectants occur in bacteria during disinfectant exposure. A systematic literature review following PRISMA guidelines was conducted with the databases PubMed, Science Direct and Web of Science. For the final analysis, 28 sources that remained of interest were included. Articles describing reduced susceptibility or the resistance of bacteria against seven different disinfectants were identified. The important deviation of the minimum inhibitory concentration was observed in multiple studies for disinfectants based on triclosan and chlorhexidine. A reduced susceptibility to disinfectants and potentially related problems with antibiotic resistance in clinically important bacterial strains are increasing. Since the use of disinfectants in the community is rising, it is clear that reasonable use of available and effective disinfectants is needed. It is necessary to develop and adopt strategies to control disinfectant resistance.

Evaluation of the antibacterial effect of tea tree oil on Enterococcus faecalis and biofilm in vitro

ETHNOPHARMACOLOGICAL RELEVANCE

Tea tree essential oil (TTO) is extracted from the leaves of Melaleuca alternifolia by steam distillation. It is well known for its traditional medicinal uses, particularly for the treatment of bruises, insect bites, skin infections, vertigo, convulsions, toothache, and rheumatism. Earlier research has shown that TTO can effectively inhibit oral microorganisms in the root canals. Enterococcus faecalis (E. faecalis) has been considered to be associated with persistent root canal infections and root canal treatment failure. The biofilm of E. faecalis makes it more vigorous, toxic, and resistant to antibiotics.

AIM OF THE STUDY

In this study, our aim was to evaluate the antimicrobial effects of TTO on planktonic E. faecalis and biofilms compared with 0.2% CHX.

MATERIALS AND METHODS

We explored the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC), the bacteriostatic rate by MTT assay, the antimicrobial time by time-kill assay, and the effects on cell integrity, the biomass, and bacterial activity of E. faecalis biofilms. Finally, we investigated the microstructure changes of E. faecalis biofilms using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM).

RESULTS

The MIC and MBC values were 0.25% and 0.5%, the bacterial inhibition rate, time-kill was dosage dependent, and TTO can effectively destroy membrane integrity. SEM CLSM images revealed that TTO could reduce bacterial aggregation, biofilm thickness and inhibited biofilm formation. The effect of TTO was the same as that of 0.2% CHX at some specific concentrations. In summary, TTO has the potential to be effective against E. faecalis infections.

CONCLUSIONS

TTO was able to inhibit E. faecalis by destroying cell membrane, inhibiting the formation of E. faecalis biofilms, and eliminating mature formed biofilms. In this study, TTO has the potential to be further developed as a novel antibacterial drug.

Small Molecule Compounds, A Novel Strategy against Streptococcus mutans

Dental caries, as a common oral infectious disease, is a worldwide public health issue. Oral biofilms are the main cause of dental caries. Streptococcus mutans (S. mutans) is well recognized as the major causative factor of dental caries within oral biofilms. In addition to mechanical removal such as tooth brushing and flossing, the topical application of antimicrobial agents is necessarily adjuvant to the control of caries particularly for high-risk populations. The mainstay antimicrobial agents for caries such as chlorhexidine have limitations including taste confusions, mucosal soreness, tooth discoloration, and disruption of an oral microbial equilibrium. Antimicrobial small molecules are promising in the control of S. mutans due to good antimicrobial activity, good selectivity, and low toxicity. In this paper, we discussed the application of antimicrobial small molecules to the control of S. mutans, with a particular focus on the identification and development of active compounds and their modes of action against the growth and virulence of S. mutans.

Controlled release, antimicrobial activity, and oral mucosa irritation of cetylpyridinium chloride-montmorillonite incorporated in a tissue conditioner

This study examined the controlled release of cetylpyridinium chloride (CPC) from a tissue conditioner (TC) containing CPC-montmorillonite (CPC-Mont), the associated antimicrobial activity, and oral mucosa irritation. The CPC release test was performed daily for 28 days in three test solutions: distilled water, 0.2 M NaCl, and 0.2 M HCl. The antimicrobial activities for 7, 14, 21, and 28 days against Candida albicans, Staphylococcus aureus, and Streptococcus mutans were assessed according to the JIS Z 2801/ISO 22196 standard. An oral mucosa irritation test was conducted using cheek pouches in five male hamsters according to the ISO 10993-10:2010 standard. The amount of CPC released each day and the cumulative amount released over 28 days (6.12 mg) were less than the daily safe maximum of sore throat medicines (8 mg). Additionally, TC with CPC-Mont could sustain antimicrobial activity against adherent bacteria for 14 days and has no oral mucosa irritation potential.

Broad-Spectrum Bactericidal Activity and Remarkable Selectivity of Main-Chain Sulfonium-Containing Polymers with Alternating Sequences

Incorporation of cationic groups into polymers represents one of the most widely used strategies to prepare antibacterial materials. Sulfonium, as a typical cationic moiety, displays potent antibacterial efficacy in the form of small molecules, however, has long underperformed in polymeric systems. Herein, we developed a series of alternating polysulfoniums, where the hydrophobicity of each alternating unit can be accurately tuned by altering the monomer precursors. Excellent antibacterial activity against a broad spectrum of clinically relevant bacteria, including Methicillin-resistant Staphylococcus aureus, can be obtained in the optimal compositions with minimum bactericidal concentrations in the range of 1.25-10 μg/mL, as well as negligible hemolytic effect at polymer concentrations even up to 10000 μg/mL. Bacteria do not readily develop resistance to polysulfoniums due to the antibacterial action is possibly the membrane disrupting mechanism. This work demonstrates sulfonium-based polymers with well-defined sequences can function as a promising candidate to combat drug-resistant bacterial infection.

Cytotoxicity and effects of curcumin and cinnamaldehyde hybrids on biofilms of oral pathogens

The objective of the study was to evaluate the cytotoxicity and effect of curcumin-cinnamaldehyde hybrids (CCHs) on the biofilm of oral pathogens. Of the 18 hybrids tested, nine had an inhibitory effect on at least one of the bacterial species tested, with minimal inhibitory and bactericidal concentrations ranging from 9 to 625 μg ml^-1. CCH 7 promoted a potent inhibitory effect against all the bacterial species tested and better compatibility than chlorhexidine (CHX). CCH 7 also presented a similar or improved effect over that of CHX, causing a reduction in bacterial metabolism and viability in single and dual-species biofilms. CCH 7 reduced by 86% and 34% the viability of multispecies biofilms formed by collection and clinical strains. It can be concluded that CCH 7 was cytocompatible at the minimal inhibitory concentration, presented anti-biofilm action against oral pathogens, and could act as an antimicrobial agent for application in endodontics.

Trans-Cinnamaldehyde Attenuates Enterococcus faecalis Virulence and Inhibits Biofilm Formation

Enterococcus faecalis as an important nosocomial pathogen is critically implicated in the pathogenesis of endocarditis, urinary tract, and persistent root canal infections. Its major virulence attributes (biofilm formation, production of proteases, and hemolytic toxins) enable it to cause extensive host tissue damage. With the alarming increase in enterococcal resistance to antibiotics, novel therapeutics are required to inhibit E. faecalis biofilm formation and virulence. Trans-cinnamaldehyde (TC), the main phytochemical in cinnamon essential oils, has demonstrated promising activity against a wide range of pathogens. Here, we comprehensively investigated the effect of TC on planktonic growth, biofilm formation, proteolytic and hemolytic activities, as well as gene regulation in E. faecalis. Our findings revealed that sub-inhibitory concentrations of TC reduced biofilm formation, biofilm exopolysaccharides, as well as its proteolytic and hemolytic activities. Mechanistic studies revealed significant downregulation of the quorum sensing fsr locus and downstream gelE, which are major virulence regulators in E. faecalis. Taken together, our study highlights the potential of TC to inhibit E. faecalis biofilm formation and its virulence.

Recent Progress in Antimicrobial Strategies for Resin-Based Restoratives

Repairing tooth defects with dental resin composites is currently the most commonly used method due to their tooth-colored esthetics and photocuring properties. However, the higher than desirable failure rate and moderate service life are the biggest challenges the composites currently face. Secondary caries is one of the most common reasons leading to repair failure. Therefore, many attempts have been carried out on the development of a new generation of antimicrobial and therapeutic dental polymer composite materials to inhibit dental caries and prolong the lifespan of restorations. These new antimicrobial materials can inhibit the formation of biofilms, reduce acid production from bacteria and the occurrence of secondary caries. These results are encouraging and open the doors to future clinical studies on the therapeutic value of antimicrobial dental resin-based restoratives. However, antimicrobial resins still face challenges such as biocompatibility, drug resistance and uncontrolled release of antimicrobial agents. In the future, we should focus on the development of more efficient, durable and smart antimicrobial dental resins. This article focuses on the most recent 5 years of research, reviews the current antimicrobial strategies of composite resins, and introduces representative antimicrobial agents and their antimicrobial mechanisms.

Development of endodontic sealers containing antimicrobial-loaded polymer particles with long-term antibacterial effects

OBJECTIVE

The objective of this study is to prepare new dental resins with a long-lasting antimicrobial activity. Specifically, this study evaluates an approach for controlling infection in root canals using sealers containing polyhydroxyethyl methacrylate trimethylolpropane trimethacrylate (polyHEMA/TMPT) particles loaded with cetylpyridinium chloride (CPC). In addition, the physical properties of sealers containing CPC-loaded polyHEMA/TMPT particles (CLP) are determined.

METHODS

PolyHEMA/TMPT particles with 10 (10%-CLP) and 25wt.% CPC (25%-CLP) with different particle sizes were fabricated and incorporated in HEMA-based sealers. CPC-release profiles were evaluated over 14 days of immersion in water, followed by 14 days of storage and 14 days of water immersion. The antibacterial activity of these sealers against Enterococcus faecalis in dentinal tubules was assessed using a root-canal-infection model. Their sealing abilities were evaluated by fluid filtration and physical properties were tested according to the ISO 6876 standard. The long-term antibacterial activity of the cured sealer containing 25%-CLP (∼21μm particle diameter) was re-assessed after 1 year of storage.

RESULTS

After 28 days of immersion, 25%-CLP exhibited a higher and sustained CPC release unlike 10%-CLP. Residual bacteria in root dentinal tubules were eradicated by obturation with 25%-CLP-containing sealers. The incorporation of 25%-CLP (∼21μm) had no adverse effects on the sealing ability and physical properties of the sealer and resulted in long-term antibacterial activity.

SIGNIFICANCE

The incorporation of CPC-loaded particles in HEMA resins yielded endodontic sealers with long-term bactericidal activity against E. faecalis in root canals. These sealers can potentially be used to prevent recurrent apical periodontitis.

Potential prebiotic substrates modulate composition, metabolism, virulence and inflammatory potential of an in vitro multi-species oral biofilm

Background: Modulation of the commensal oral microbiota constitutes a promising preventive/therapeutic approach in oral healthcare. The use of prebiotics for maintaining/restoring the health-associated homeostasis of the oral microbiota has become an important research topic.

Aims: This study hypothesised that in vitro 14-species oral biofilms can be modulated by (in)direct stimulation of beneficial/commensal bacteria with new potential prebiotic substrates tested at 1 M and 1%_(w/v), resulting in more host-compatible biofilms with fewer pathogens, decreased virulence and less inflammatory potential.

Methods: Established biofilms were repeatedly rinsed with N-acetyl-D-glucosamine, α-D-lactose, D-(+)-trehalose or D-(+)-raffinose at 1 M or 1%_(w/v). Biofilm composition, metabolic profile, virulence and inflammatory potential were eventually determined.

Results: Repeated rinsing caused a shift towards a more health-associated microbiological composition, an altered metabolic profile, often downregulated virulence gene expression and decreased the inflammatory potential on oral keratinocytes. At 1 M, the substrates had pronounced effects on all biofilm aspects, whereas at 1%_(w/v) they had a pronounced effect on virulence gene expression and a limited effect on inflammatory potential.

Conclusion: Overall, this study identified four new potential prebiotic substrates that exhibit different modulatory effects at two different concentrations that cause in vitro multi-species oral biofilms to become more host-compatible.

Disinfectant resistance in bacteria: Mechanisms, spread, and resolution strategies

Disinfectants are widely acknowledged for removing microorganisms from the surface of the objects and transmission media. However, the emergence of disinfectant resistance has become a severe threat to the safety of life and health and the rational allocation of resources due to the reduced disinfectant effectiveness. The horizontal gene transfer (HGT) of disinfectant resistance genes has also expanded the resistant flora, making the situation worse. This review focused on the resistance mechanisms of disinfectant resistant bacteria on biofilms, cell membrane permeability, efflux pumps, degradable enzymes, and disinfectant targets. Efflux can be the fastest and most effective resistance mechanism for bacteria to respond to stress. The qac genes, located on some plasmids which can transmit resistance through conjugative transfer, are the most commonly reported in the study of disinfectant resistance genes. Whether the qac genes can be transferred through transformation or transduction is still unclear. Studying the factors affecting the resistance of bacteria to disinfectants can find breakthrough methods to more adequately deal with the problem of reduced disinfectant effectiveness. It has been confirmed that the interaction of probiotics and bacteria or the addition of 4-oxazolidinone can inhibit the formation of biofilms. Chemicals such as eugenol and indole derivatives can increase bacterial sensitivity by reducing the expression of efflux pumps. The role of these findings in anti-disinfectant resistance has proved invaluable.

Antimicrobial activities of a small molecule compound II-6s against oral streptococci

Background: The side effects of present antimicrobials like chlorhexidine (CHX) and the emergence of drug resistance necessitate the development of alternative agents to control dental caries. Aim: This study developed a novel small molecule, namely II-6s, and investigated its antimicrobial activities against common oral streptococci associated with dental caries. Methods: The susceptibility of streptococci to II-6s was evaluated by the microdilution method, time-kill assay and scanning electron microscopy. The exopolysaccharides, dead/live bacteria and bacterial composition of the II-6s-treated Streptococcus mutans/Streptococcus gordonii/Streptococcus sanguinis 3-species biofilms were analyzed by confocal laser scanning microscopy, fluorescent in situ hybridization and quantitative PCR. The anti-demineralization effect and cytotoxicity of II-6s were evaluated by transverse microradiography and CCK-8 assay, respectively. Repeated exposure of S. mutans to II-6s was performed to assess if II-6s could induce drug resistance. Results: II-6s exhibited antimicrobial activity similar to CHX against S. mutans, S. gordonii and S. sanguinis and significantly inhibited exopolysaccharides production, live bacteria and the demineralizing capability of the 3-species streptococcal biofilms. Besides, II-6s showed reduced cytotoxicity relative to CHX and did not induce drug resistance in S. mutans after 15 passages. Conclusion: - II-6s may serve as a promising part of a successful caries management plan.

Effect of a chlorhexidine-encapsulated nanotube modified pit-and-fissure sealant on oral biofilm

The purpose of this study was to characterize a chlorhexidine-encapsulated nanotube modified pit-and-fissure sealant for biofilm development prevention. HS (commercial control); HNT (HS+15wt%Halloysite^®-clay-nanotube); CHX10% (HS+15wt% HNT-encapsulated with chlorhexidine 10%); and CHX20% (HS+15wt% HNT-encapsulated with CHX20%) were tested. Degree-of-conversion (DC%), Knoop hardness (KHN), and viscosity were analyzed. The ability of the sealant to wet the fissures was evaluated. Specimens were tested for zones of inhibition of microbial growth. S. mutans biofilm was tested by measuring recovered viability. Data were statistically analyzed (p<0.05). DC% was significantly higher for the HNT-CHX groups. For KHN, CHX10% presented a lower mean value than the other groups. Adding HNT resulted in higher viscosity values. The biofilm on CHX10% and CHX20% sealants presented remarkable CFU/mL reduction in comparison to the HS. The experimental material was able to reduce the biofilm development in S. mutans biofilm without compromising the sealant properties.

Resin-based dental materials containing 3-aminopropyltriethoxysilane modified halloysite-clay nanotubes for extended drug delivery

OBJECTIVE

To synthesize and characterize a novel resin-based dental material containing 3-aminopropyltriethoxysilane (APTES) surface-modified halloysite-clay nanotubes (HNTs) for long-term delivery of guest molecules.

METHODS

The optimal concentrations of HNT (10, 15, 20 wt.%) and silane (0, 2, 4 vol.%sil) to be incorporated into the resin-based materials were determined (15 wt.%HNT, 4 vol.%sil) after assessment of the mechanical properties (DC%, degree of conversion; FS, flexural strength; FM, flexural modulus; and UTS, ultimate tensile strength). The HNTsil-powder was loaded with chlorhexidine (CHX) to evaluate the effect of the silanization on drug release. Resin-discs were prepared for the following groups: RES (resin), HNT (resin+15 wt.%HNT), HNTsil (resin+15 wt.%HNT silanized), HNT-CHX (resin+15 wt.%HNT loaded with chlorhexidine), HNTsil-CHX (resin+15 wt.%HNTsil-CHX), and 0.2 vol.%CHX (resin+0.2 vol.%CHX solution). Specimens were stored in water for 1, 3, 5, 10, and 15 days at 37 °C. Aliquots from each time point and the final 15-day specimens were evaluated for the zone of inhibition (ZOI) against Streptococcus mutans. CHX release was analyzed using spectrophotometry at absorbance of 300 nm. Data were statistically analyzed (α = 0.05).

RESULTS

All materials presented similar DC%. Reduced FS but increased FM was detected for 20 wt.%HNT-4%APTES. Groups with 15 wt.% and 20 wt.%HNT with/without APTES presented higher values of UTS. Agar diffusion data indicates that the HNTsil-CHX had a greater ZOI than all other groups over 15 days. HNTsil-CHX had the highest absorbance for day 1 but presented similar values to other groups every time point after.

SIGNIFICANCE

Silanization of nanotubes followed by encapsulation of chlorhexidine is a promising technique for long-term delivery of guest molecules.

Repurposing Napabucasin as an Antimicrobial Agent against Oral Streptococcal Biofilms

Objectives

Disruption of microbial biofilms is an effective way to control dental caries. Drug resistance and side effects of the existing antimicrobials necessitate the development of novel antibacterial agents. The current study was aimed at investigating the antibacterial activities of the repurposed natural compound napabucasin against oral streptococci.

Methods

The minimum inhibitory concentration, minimum bactericidal concentration, minimum biofilm inhibition concentration, and minimum biofilm reduction concentration of Streptococcus mutans, Streptococcus gordonii, and Streptococcus sanguinis were examined by a microdilution method. Cytotoxicity of napabucasin against human oral keratinocytes, human gingival epithelia, and macrophage RAW264.7 was evaluated by CCK8 assays. The dead/live bacterium and exopolysaccharide in the napabucasin-treated multispecies biofilms were evaluated by confocal laser scanning microscopy. Microbial composition within the napabucasin-treated biofilms was further visualized by fluorescent in situ hybridization and qPCR. And the cariogenicity of napabucasin-treated biofilms was evaluated by transverse microradiography.

Results

Napabucasin exhibited good antimicrobial activity against oral streptococcal planktonic cultures and biofilms but with lessened cytotoxicity as compared to chlorhexidine. Napabucasin reduced the cariogenic S. mutans and increased the proportion of the commensal S. gordonii in the multispecies biofilms. More importantly, napabucasin significantly reduced the demineralization capability of biofilms on tooth enamels.

Conclusion

Napabucasin shows lessened cytotoxicity and comparable antimicrobial effects to chlorhexidine. Repurposing napabucasin may represent a promising adjuvant for the management of dental caries.

Limited antimicrobial efficacy of oral care antiseptics in microcosm biofilms and phenotypic adaptation of bacteria upon repeated exposure

Objectives

The aims of this study were to investigate the antimicrobial efficacy of antiseptics in saliva-derived microcosm biofilms, and to examine phenotypic adaption of bacteria upon repeated exposure to sub-inhibitory antiseptic concentrations.

Methods

Saliva-derived biofilms were formed mimicking caries- or gingivitis-associated conditions, respectively. Microbial compositions were analyzed by semiconductor-based 16S rRNA sequencing. Biofilms were treated with CHX, CPC, BAC, ALX, and DQC for 1 or 10 min, and colony forming units (CFU) were evaluated. Phenotypic adaptation of six selected bacterial reference strains toward CHX, CPC, and BAC was assessed by measuring minimum inhibitory concentrations (MICs) over 10 passages of sub-inhibitory exposure. Protein expression profiles were investigated by SDS-PAGE.

Results

Both biofilms showed outgrowth of streptococci and Veillonella spp., while gingivitis biofilms also showed increased relative abundances of Actinomyces, Granulicatella, and Gemella spp. Antiseptic treatment for 1 min led to no relevant CFU-reductions despite for CPC. When treated for 10 min, CPC was most effective followed by BAC, ALX, CHX, and DQC. Stable adaptations with up to fourfold MIC increases were found in E. coli toward all tested antiseptics, in E. faecalis toward CHX and BAC, and in S. aureus toward CPC. Adapted E. coli strains showed different protein expression as compared with the wildtype strain.

Conclusion

Antiseptics showed limited antimicrobial efficacy toward mature biofilms when applied for clinically relevant treatment periods. Bacteria showed phenotypic adaptation upon repeated sub-inhibitory exposure.

Clinical relevance

Clinicians should be aware that wide-spread use of antiseptics may pose the risk of inducing resistances in oral bacteria.

Antimicrobial, antibiofilm and cytotoxic effects of a colloidal nanocarrier composed by chitosan-coated iron oxide nanoparticles loaded with chlorhexidine

OBJECTIVES

This study evaluated the antimicrobial and antibiofilm effects of a colloidal nanocarrier for chlorhexidine (CHX) on Candida glabrata and Enterococcus faecalis, as well as tested its cytotoxic effect on murine fibroblasts.

METHODS

Iron oxide nanoparticles (IONPs) were coated with chitosan (CS) and loaded with CHX at 31.2, 78 and 156 μg/mL. Antimicrobial effects were assessed by determining the minimum inhibitory concentration (MIC), using the broth microdilution method, and fractional inhibitory concentration index (FICI). Preformed biofilms (48 h) were treated with different concentrations of the nanocarrier (24 h) and quantified by colony-forming units (CFUs), total biomass and metabolic activity. For cytotoxicity, the viability of L929 cells was evaluated by MTT assay after 24 and 48 h of exposure to the nanocarrier. Data were submitted to ANOVA and Fisher LSD or Tukey post-hoc tests (α = 0.05).

RESULTS

MIC and FICI results showed an indifferent interaction among the components of the nanocarrier for all strains evaluated. CHX alone and nanocarrier containing 156 μg/mL CHX did not differ from each other in reducing the number of CFUs. However, the nanocarrier containing 156 μg/mL CHX promoted the highest reductions in total biofilm biomass and metabolism, surpassing the effect of CHX alone. After 24 and 48 h of exposure, the nanocarrier reduced CHX toxicity to the L929 cell at low concentrations.

CONCLUSION

These findings suggest that the CHX nanocarrier has potential to be used in the control of oral diseases associated with C. glabrata and E. faecalis.

CLINICAL RELEVANCE

CHX has improved the antibiofilm effect and reduced the cytotoxicity (at low concentrations) when conjugated to CS-coated IONPs. This new colloidal formulation has potential as an alternative antimicrobial agent to pure CHX for the control of biofilm-related oral diseases, such as oral candidiasis and endodontic infections.

Emerging Contact-Killing Antibacterial Strategies for Developing Anti-Biofilm Dental Polymeric Restorative Materials

Polymeric materials are the first choice for restoring tooth cavities, bonding tooth-colored fillings, sealing root canal systems, and many other dental restorative applications. However, polymeric materials are highly susceptible to bacterial attachment and colonization, leading to dental diseases. Many approaches have been investigated to minimize the formation of biofilms over polymeric restorative materials and at the tooth/material interfaces. Among them, contact-killing compounds have shown promising results to inhibit dental biofilms. Contact-killing compounds can be immobilized within the polymer structure, delivering a long-lasting effect with no leaching or release, thus providing advantages compared to release-based materials. This review discusses cutting-edge research on the development of contact-killing compounds in dental restorative materials to target oral pathogens. Contact-killing compounds in resin composite restorations, dental adhesives, root canal sealers, denture-based materials, and crown cements have all demonstrated promising antibacterial properties. Contact-killing restorative materials have been found to effectively inhibit the growth and activities of several oral pathogens related to dental caries, periodontal diseases, endodontic, and fungal infections. Further laboratory optimization and clinical trials using translational models are needed to confirm the clinical applicability of this new generation of contact-killing dental restorative materials.

Cetylpyridinium Chloride: Mechanism of Action, Antimicrobial Efficacy in Biofilms, and Potential Risks of Resistance

Antimicrobial resistance is a serious issue for public health care all over the world. While resistance toward antibiotics has attracted strong interest among researchers and the general public over the last 2 decades, the directly related problem of resistance toward antiseptics and biocides has been somewhat left untended. In the field of dentistry, antiseptics are routinely used in professional care, but they are also included in lots of oral care products such as mouthwashes or dentifrices, which are easily available for consumers over-the-counter. Despite this fact, there is little awareness among the dental community about potential risks of the widespread, unreflected, and potentially even needless use of antiseptics in oral care. Cetylpyridinium chloride (CPC), a quaternary ammonium compound, which was first described in 1939, is one of the most commonly used antiseptics in oral care products and included in a wide range of over-the-counter products such as mouthwashes and dentifrices. The aim of the present review is to summarize the current literature on CPC, particularly focusing on its mechanism of action, its antimicrobial efficacy toward biofilms, and on potential risks of resistance toward this antiseptic as well as underlying mechanisms. Furthermore, this work aims to raise awareness among the dental community about the risk of resistance toward antiseptics in general.

Novel antibacterial calcium phosphate nanocomposite with long-term ion recharge and re-release to inhibit caries

Short-term studies on calcium-phosphate (CaP) ion-rechargeable composites were reported. The long-term rechargeability is important but unknown. The objectives of this study were to investigate nanocomposite with strong antibacterial and ion-recharge capabilities containing dimethylaminododecyl methacrylate (DMAHDM) and nanoparticles of amorphous calcium phosphate (NACP), and evaluate long-term ion-recharge by testing for 12 cycles (taking 6 months to complete) for the first time. Three groups were tested: (1) Heliomolar control; (2) Resin+20%NACP+50%glass; (3) Resin+3%DMAHDM+20%NACP+50%glass. Biofilm acid and colony-forming units (CFU) were measured. Ion-recharge was tested for 12 cycles. NACP-DMAHDM composite reduced biofilm acid, and reduced CFU by 4 logs. High levels of ion releases were maintained throughout 12 cycles of recharge, maintaining steady-state releases without reduction in 6 months (p>0.1), representing long-term remineralization potential. Bioactive nanocomposite demonstrated long-term ion-rechargeability for the first time, showed remineralization and potent anti-biofilm functions, with promise for tooth restorations to combat caries.

The Essential Oil and Hydrolats from Myristica fragrans Seeds with Magnesium Aluminometasilicate as Excipient: Antioxidant, Antibacterial, and Anti-inflammatory Activity

Nutmeg (Myristica fragrans) essential oil has antimicrobial, antiseptic, antiparasitic, anti-inflammatory, and antioxidant properties. We have recently demonstrated that hydrodistillation of nutmeg essential oil by applying magnesium aluminometasilicate as an excipient significantly increases both the content and amount of bioactive substances in the oil and hydrolats. In this study, we aimed to compare the antioxidant, antimicrobial, and anti-inflammatory activity of hydrolats and essential oil obtained by hydrodistillation in the presence and absence of magnesium aluminometasilicate as an excipient. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging method revealed that magnesium aluminometasilicate did not significantly improved antioxidant activity of both essential oil and hydrolat. Antibacterial efficiency was evaluated by monitoring growth of 15 bacterial strains treated by a range of dilutions of the essential oil and the hydrolats. Essential oil with an excipient completely inhibited the growth of E. faecalis, S. mutans (referent), and P. multocida, whereas the pure oil was only efficient against the latter strain. Finally, the anti-inflammatory properties of the substances were assessed in a fibroblast cell culture treated with viral dsRNR mimetic Poly I:C. The essential oil with an excipient protected cells against Poly I:C-induced necrosis more efficiently compared to pure essential oil. Also, both the oil and the hydrolats with aluminometasilicate were more efficient in preventing IL-6 release in the presence of Poly I:C. Our results show that the use of magnesium aluminometasilicate as an excipient might change and in some cases improve the biological activities of nutmeg essential oil and hydrolats.

Antimicrobial Photoinactivation Using Visible Light Plus Water-Filtered Infrared-A (VIS + wIRA) and Hypericum Perforatum Modifies In Situ Oral Biofilms

Due to increasing antibiotic resistance, the application of antimicrobial photodynamic therapy (aPDT) is gaining increasing popularity in dentistry. The aim of this study was to investigate the antimicrobial effects of aPDT using visible light (VIS) and water-filtered infrared-A (wIRA) in combination with a Hypericum perforatum extract on in situ oral biofilms. The chemical composition of H. perforatum extract was analyzed using ultra-high-performance liquid chromatography coupled with high resolution mass spectrometry (UPLC-HRMS). To obtain initial and mature oral biofilms in situ, intraoral devices with fixed bovine enamel slabs (BES) were carried by six healthy volunteers for two hours and three days, respectively. The ex situ exposure of biofilms to VIS + wIRA (200 mWcm^-2) and H. perforatum (32 mg ml^-1, non-rinsed or rinsed prior to aPDT after 2-min preincubation) lasted for five minutes. Biofilm treatment with 0.2% chlorhexidine gluconate solution (CHX) served as a positive control, while untreated biofilms served as a negative control. The colony-forming units (CFU) of the aPDT-treated biofilms were quantified, and the surviving microorganisms were identified using MALDI-TOF biochemical tests as well as 16 S rDNA-sequencing. We could show that the H. perforatum extract had significant photoactivation potential at a concentration of 32 mg ml^-1. When aPDT was carried out in the presence of H. perforatum, all biofilms (100%) were completely eradicated (p = 0.0001). When H. perforatum was rinsed off prior to aPDT, more than 92% of the initial viable bacterial count and 13% of the mature oral biofilm were killed. Overall, the microbial composition in initial and mature biofilms was substantially altered after aPDT, inducing a shift in the synthesis of the microbial community. In conclusion, H. perforatum-mediated aPDT using VIS + wIRA interferes with oral biofilms, resulting in their elimination or the substantial alteration of microbial diversity and richness. The present results support the evaluation of H. perforatum-mediated aPDT for the adjunctive treatment of biofilm-associated oral diseases.

Effects of S. mutans gene-modification and antibacterial monomer dimethylaminohexadecyl methacrylate on biofilm growth and acid production

OBJECTIVES

Antibacterial quaternary ammonium monomers (QAMs) are used in resins. The rnc gene in Streptococcus mutans (S. mutans) plays a key role in resisting antibiotics. The objectives of this study were to investigate for the first time: (1) the effects of rnc deletion on S. mutans biofilms and acid production; (2) the combined effects of rnc deletion with dimethylaminohexadecyl methacrylate (DMAHDM) on biofilm-inhibition efficacy.

METHODS

Parent S. mutans strain UA159 (ATCC 700610) and the rnc-deleted S. mutans were used. Bacterial growth, minimum inhibitory concentration (MIC), and minimal bactericidal concentration (MBC) were measured to analyze the bacterial susceptibility of the parent and rnc-deleted S. mutans against DMAHDM, with the gold-standard chlorhexidine (CHX) as control. Biofilm biomass, polysaccharide and lactic acid production were measured.

RESULTS

The drug-susceptibility of the rnc-deleted S. mutans to DMAHDM or CHX was 2-fold higher than parent S. mutans. The drug-susceptibility did not increase after 10 passages (p < 0.05). Deleting the rnc gene increased the biofilm susceptibility to DMAHDM or CHX by 2-fold. The rnc-deletion in S. mutans reduced biofilm biomass, polysaccharide and lactic acid production, even at no drugs. DMAHDM was nearly 40 % more potent than the gold-standard CHX. The combination of rnc deletion+DMAHDM treatment achieved the greatest reduction in biofilm biomass, polysaccharide synthesis, and lactic acid production.

SIGNIFICANCE

Gene modification by deleting the rnc in S. mutans reduced the biofilm growth and acid production, and the rnc deletion+DMAHDM method showed the greatest biofilm-inhibition efficacy, for the first time. The dual strategy of antibacterial monomer+bacterial gene modification shows great potential to control biofilms and inhibit caries.

Oropharyngeal Bacterial Colonization after Chlorhexidine Mouthwash in Mechanically Ventilated Critically Ill Patients

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Oropharyngeal care with chlorhexidine to prevent ventilator-associated pneumonia is currently questioned, and exhaustive microbiologic data assessing its efficacy are lacking. The authors therefore aimed to study the effect of chlorhexidine mouthwash on oropharyngeal bacterial growth, to determine chlorhexidine susceptibility of these bacteria, and to measure chlorhexidine salivary concentration after an oropharyngeal care.

METHODS

This observational, prospective, single-center study enrolled 30 critically ill patients under mechanical ventilation for over 48 h. Oropharyngeal contamination was assessed by swabbing the gingivobuccal sulcus immediately before applying 0.12% chlorhexidine with soaked swabs, and subsequently at 15, 60, 120, 240, and 360 min after. Bacterial growth and identification were performed, and chlorhexidine minimal inhibitory concentration of recovered pathogens was determined. Saliva was collected in 10 patients, at every timepoint, with an additional timepoint after 30 min, to measure chlorhexidine concentration.

RESULTS

Two hundred fifty bacterial samples were analyzed and identified 48 pathogens including Streptococci (27.1%) and Enterobacteriaceae (20.8%). Oropharyngeal contamination before chlorhexidine mouthwash ranged from 10 to 10 colony-forming units (CFU)/ml in the 30 patients (median contamination level: 2.5·10 CFU/ml), and remained between 8·10 (lowest) and 3·10 CFU/ml (highest count) after chlorhexidine exposure. These bacterial counts did not decrease overtime after chlorhexidine mouthwash (each minute increase in time resulted in a multiplication of bacterial count by a coefficient of 1.001, P = 0.83). Viridans group streptococci isolates had the lowest chlorhexidine minimal inhibitory concentration (4 [4 to 8] mg/l); Enterobacteriaceae isolates had the highest ones (32 [16 to 32] mg/l). Chlorhexidine salivary concentration rapidly decreased, reaching 7.6 [1.8 to 31] mg/l as early as 60 min after mouthwash.

CONCLUSIONS

Chlorhexidine oropharyngeal care does not seem to reduce bacterial oropharyngeal colonization in critically ill ventilated patients. Variable chlorhexidine minimal inhibitory concentrations along with low chlorhexidine salivary concentrations after mouthwash could explain this ineffectiveness, and thus question the use of chlorhexidine for ventilator-associated pneumonia prevention.

Efficacy and potential use of novel sustained release fillers as intracanal medicaments against Enterococcus faecalis biofilm in vitro

Background

Enterococcus faecalis is a bacterium frequently isolated after failed root canal therapy. This study analyzed the antibacterial and antibiofilm effects in vitro of sustained-release fillers (SRF) containing cetylpyridinium chloride (CPC) against vancomycin resistant E. faecalis.

Methods

First, the solidification capability was tested by introducing liquid SRF into phosphate buffered saline, followed by 30 s of vortexing. The antimicrobial effects of SRF-CPC against static monospecies biofilms were analyzed with a metabolic assay. Inhibition of biofilm formation was tested by exposing daily refreshed E. faecalis suspensions to SRF-CPC for 9 weeks. To evaluate the effects of SRF-CPC against preformed biofilms, biofilms were grown for 1, 3 and 7 days, and then treated with SRF-CPC for 24 h. Biofilm kill time was tested by applying SRF-CPC to a 3-day-old biofilm and measuring its viability at different time points. All experiments were compared to Placebo SRFs and to untreated control biofilms. Data were analyzed with two-way ANOVA followed by Tukey’s test. Results were considered significant at P < 0.05.

Results

The liquid SRF solidified within seconds and no structural changes were observed after 30 s of vortexing at maximum speed. SRF-CPC inhibited E. faecalis biofilm formation for 7 weeks and significantly reduced its viability in weeks 8 and 9. Mature biofilms grown for 1, 3 and 7 days were destructed by SRF-CPC in less than 24 h. Fifty percent of a 3-day-old biofilm was destructed in 2 h and complete destruction occurred in less than 12 h. (P < 0.05 in all cases, compared to SRII-Placebo).

Conclusions

SRF-CPC’s physical properties and long-lasting anti-biofilm effects make it a promising coadjuvant medication for endodontic therapy.

Electronic supplementary material

The online version of this article (10.1186/s12903-019-0879-1) contains supplementary material, which is available to authorized users.

Sustained-Release Fillers for Dentin Disinfection: An Ex Vivo Study

Enterococcus faecalis is the most commonly recovered species from failed root canal treatments. In this study, we tested the capability of a novel intracanal sustained-release filler (SRF) containing cetylpyridinium chloride (CPC) to disinfect dentinal tubules of segmented human tooth specimens. Human dental root specimens were infected with E. faecalis V583 for 3 weeks in a static environment. The tested intracanal medicaments were SRF-CPC and calcium hydroxide (CH). Each medicament was introduced into the canal of the dental specimen and incubated for 7 days. The bacteriological samples were taken by shaving the dentine surrounding the root canal with dental burs ranging in size from ISO 014-020. The obtained dentine powder was collected in test tubes containing phosphate-buffered saline, sonicated, and plated on agar plates. Colony-forming units were counted after 48 h of incubation. Random specimens were also examined under confocal laser scanning microscopy and scanning electron microscopy. A statistical difference was found in the bacterial counts obtained from all layers of infected dentin between the control and the SRF-CPC groups. CH reduced bacterial viability significantly only in the first layer of the infected dentin, up to 150 μm into the dentinal tubules. CLSM images showed that SRF-CPC killed most bacteria throughout the infected dentin up to 700 μm of penetration. SEM images demonstrated the adhesion ability of SRF-CPC to the dentinal wall. In conclusion, SRF-CPC is a potential intracanal medicament for disinfecting dentinal tubules.

Development of antiseptic adaptation and cross-adapatation in selected oral pathogens in vitro

There is evidence that pathogenic bacteria can adapt to antiseptics upon repeated exposure. More alarming is the concomitant increase in antibiotic resistance that has been described for some pathogens. Unfortunately, effects of adaptation and cross-adaptation are hardly known for oral pathogens, which are very frequently exposed to antiseptics. Therefore, this study aimed to determine the in vitro increase in minimum inhibitory concentrations (MICs) in oral pathogens after repeated exposure to chlorhexidine or cetylpyridinium chloride, to examine if (cross-)adaptation to antiseptics/antibiotics occurs, if (cross-)adaptation is reversible and what the potential underlying mechanisms are. When the pathogens were exposed to antiseptics, their MICs significantly increased. This increase was in general at least partially conserved after regrowth without antiseptics. Some of the adapted species also showed cross-adaptation, as shown by increased MICs of antibiotics and the other antiseptic. In most antiseptic-adapted bacteria, cell-surface hydrophobicity was increased and mass-spectrometry analysis revealed changes in expression of proteins involved in a wide range of functional domains. These in vitro data shows the adaptation and cross-adaptation of oral pathogens to antiseptics and antibiotics. This was related to changes in cell surface hydrophobicity and in expression of proteins involved in membrane transport, virulence, oxidative stress protection and metabolism.

Resistance Toward Chlorhexidine in Oral Bacteria - Is There Cause for Concern?

The threat of antibiotic resistance has attracted strong interest during the last two decades, thus stimulating stewardship programs and research on alternative antimicrobial therapies. Conversely, much less attention has been given to the directly related problem of resistance toward antiseptics and biocides. While bacterial resistances toward triclosan or quaternary ammonium compounds have been considered in this context, the bis-biguanide chlorhexidine (CHX) has been put into focus only very recently when its use was associated with emergence of stable resistance to the last-resort antibiotic colistin. The antimicrobial effect of CHX is based on damaging the bacterial cytoplasmic membrane and subsequent leakage of cytoplasmic material. Consequently, mechanisms conferring resistance toward CHX include multidrug efflux pumps and cell membrane changes. For instance, in staphylococci it has been shown that plasmid-borne qac ("quaternary ammonium compound") genes encode Qac efflux proteins that recognize cationic antiseptics as substrates. In Pseudomonas stutzeri, changes in the outer membrane protein and lipopolysaccharide profiles have been implicated in CHX resistance. However, little is known about the risk of resistance toward CHX in oral bacteria and potential mechanisms conferring this resistance or even cross-resistances toward antibiotics. Interestingly, there is also little awareness about the risk of CHX resistance in the dental community even though CHX has been widely used in dental practice as the gold-standard antiseptic for more than 40 years and is also included in a wide range of oral care consumer products. This review provides an overview of general resistance mechanisms toward CHX and the evidence for CHX resistance in oral bacteria. Furthermore, this work aims to raise awareness among the dental community about the risk of resistance toward CHX and accompanying cross-resistance to antibiotics. We propose new research directions related to the effects of CHX on bacteria in oral biofilms.

Adaptive bacterial response to low level chlorhexidine exposure and its implications for hand hygiene

Chlorhexidine digluconate (CHG) is commonly used in healthcare, e.g. in skin antiseptics, antimicrobial soaps, alcohol-based hand rubs and oral or wound antiseptics. Aim of the literature review was to evaluate the potential of bacteria to adapt to low level CHG exposure. A maximum 4fold MIC increase to CHG was found after low level exposure in most of the 71 evaluated bacterial species. A strong adaptive mostly stable MIC change was described in strains or isolates of the healthcare-associated species E. coli, S. marcescens and P. aeruginosa (up to 500fold, 128fold or 32fold, respectively). The highest MIC values after adaptation were 2,048 mg/l (S. marcescens) and 1,024 mg/l (P. aeruginosa). A new resistance to tetracycline, gentamicin, meropeneme or triclosan was found in some adapted isolates. In E. coli horizontal gene transfer was induced (sulfonamide resistance by conjugation), pointing out an additional risk of sublethal CHG. The use of CHG in patient care - but also all other settings such as consumer products and households - should therefore be critically assessed and restricted to indications with a proven health benefit or justifiable public health benefits. Additional CHG has no health benefit when used in alcohol-based hand rubs and is not recommended by the WHO. For routine hand washing of soiled hands the use of plain soap is sufficient, CHG in soaps has no health benefit. In surgical hand antisepsis alcohol-based hand rubs should be preferred to CHG soaps. Implementation of these principles will help to reduce avoidable selection pressure.

Novel bioactive root canal sealer with antibiofilm and remineralization properties

OBJECTIVES

(1) To develop a novel bioactive root canal sealer with antibiofilm and remineralization properties using dimethylaminohexadecyl methacrylate (DMAHDM) and nanoparticles of amorphous calcium phosphate (NACP); (2) investigate the effects on E. faecalis biofilm inhibition, sealer flow and sealing ability, compared with an epoxy-resin-based sealer AH Plus; and (3) investigate the calcium (Ca) and phosphate (P) ion release from the sealers.

METHODS

A series of dual-cure endodontic sealers were formulated with DMAHDM and NACP at 5% and 20% by mass, respectively. Flow properties and sealing ability of the sealers were measured. Colony-forming units (CFU), live/dead assay, and polysaccharide production of biofilms on sealers were determined. Ca and P ion releases from the sealers were measured.

RESULTS

The new sealer containing 20% NACP and 5% DMAHDM yielded a paste flow of (28.99 ± 0.69) mm, within the range of ISO recommendations. The sealing properties of the sealer with 5% DMAHDM and 20% NACP were similar to a commercial control (p > 0.05). The sealer with DMAHDM decreased E. faecalis biofilm CFU by more than 4 orders of magnitude, compared to AH plus and experimental controls. The sealer with 20% NACP and 5% DMAHDM had relatively high levels of Ca and P ion release necessary for remineralization.

CONCLUSIONS

A new bioactive endodontic sealer was developed with strong antibiofilm activity against E. faecalis biofilms and high levels of Ca and P ion release for remineralization, without compromising the paste flow and sealing properties.

CLINICAL SIGNIFICANCE

The bioactive antibacterial and remineralizing root canal sealer is promising to inhibit E. faecalis biofilms to prevent endodontic treatment failure and secondary endodontic infections, while releasing high levels of Ca and P ions that could remineralize and strengthen the tooth structures and potentially prevent future root fractures and teeth extractions.

The effect of antimicrobial additives on the properties of dental glass-ionomer cements: a review

Aim: The aim of this article is to review the literature on the use of antimicrobial additives in glass-ionomer dental cements. Method: An electronic search between 1987 and the end of 2017 was performed using PubMed, Web of Science and Google search engines with the terms glass-ionomer, glass polyalkenoate, antibacterial and antimicrobial as the key words. The search was refined by excluding the majority of references concerned with cement antimicrobial properties only. Extra papers already known to the authors were added to those considered. Results: A total of 92 relevant articles have been cited in the review of which 55 are specifically concerned with the enhancement of antibacterial properties of glass-ionomers, both conventional and resin-modified, with additives. In addition, information is included on the uses of glass-ionomers and the biological properties of the antibacterial additives employed. There are several reports that show that additives are typically released by diffusion, and that a high proportion is usually left behind, trapped in the cement. Additives generally increase setting times of cements, and reduce mechanical properties. However, smaller amounts of additive have only slight effects and the longer-term durability of cements appears unaffected. Conclusion: Modified glass-ionomer cements seem to be acceptable for clinical use, especially in the Atraumatic Restorative Treatment (ART) technique.

Exposure to Sublethal Concentrations of Benzalkonium Chloride Induces Antimicrobial Resistance and Cellular Changes in Klebsiellae pneumoniae Clinical Isolates

Benzalkonium chloride (BAC) is widely used as a disinfectant and preservative. This study investigated the effect on antimicrobial susceptibility and the cellular changes that occurred after exposure of Klebsiella pneumoniae clinical isolates to sublethal concentrations of BAC. Minimum inhibitory concentration and minimum bactericidal concentration of BAC were determined for the collected 50 K. pneumoniae clinical isolates by broth microdilution method, and the tested isolates were adapted to increasing sublethal concentrations of BAC. The effect of adaptation on MICs of the tested 16 antimicrobial agents, the cell ultrastructure, efflux, and membrane depolarization of the tested isolates were examined. Interestingly, most K. pneumoniae isolates that adapted to BAC showed increased antimicrobial resistance, various morphological and structural changes, increased membrane depolarization, and enhanced efflux activity. The findings of this study suggest that the extensive use of BAC at sublethal concentrations could contribute to the emergence of antibiotic resistance in K. pneumoniae clinical isolates that might complicate the therapy of infections caused by this pathogen. In conclusion, the hazard associated with the prolonged exposure to sublethal concentrations of BAC represents a public health risk and therefore it should be a focus in both hospital and community sanitation practices.