Controlled cellular redox, repressive hemin utilization and adaptive stress responses are crucial to metronidazole tolerance of Porphyromonas gingivalis persisters

Microbial persisters and host: recent advances and future perspectives

Microbial persisters are defined as the tiny sub-population of microorganisms that develop intrinsic strategies for survival with high tolerance to various antimicrobials. Currently, persister research remains in its infancy, and it is indeed a great challenge to precisely distinguish persister cells from other drug tolerant ones. Notably, the existence of persisters crucially contributes to prolonged antibiotic exposure time and treatment failure, yet there is the formation of antibiotic-resistant mutants. Further understanding on persisters is of profound importance for effective prevention and control of chronic infections/inflammation. The past two decades have witnessed rapid advances on the science, technologies and methodologies for persister investigations, along with deep knowledge about persisters and numerous anti-persister approaches developed. Whereas, various critical issues remain unsolved, such as what are the potential interaction profiles of persisters and host cells, and how to apply what we know about persisters to translational studies and clinical practice. Importantly, it is highly essential to better understand the multifaceted and complex cross-talk of microbial persisters with the host to develop novel tackling strategies for precision healthcare in the near future.

Weak direct current exerts synergistic effect with antibiotics and reduces the antibiotic resistance: An in vitro subgingival plaque biofilm model

BACKGROUND AND OBJECTIVE

Weak direct current (DC) exerts killing effect and synergistic killing effect with antibiotics in some specific bacteria biofilms. However, the potential of weak DC alone or combined with periodontal antibiotics in controlling periodontal pathogens and plaque biofilms remains unclear. The objective of this study was to investigate whether weak DC could exert the anti-biofilm effect or enhance the killing effect of metronidazole (MTZ) and/or amoxicillin-clavulanate potassium (AMC) on subgingival plaque biofilms, by constructing an in vitro subgingival plaque biofilm model.

METHODS

The pooled subgingival plaque and saliva of patients with periodontitis (n = 10) were collected and cultured anaerobically on hydroxyapatite disks in vitro for 48 h to construct the subgingival plaque biofilm model. Then such models were stimulated with 0 μA DC alone (20 min/12 h), 1000 μA DC alone (20 min/12 h), 16 μg/ml MTZ, 16 μg/ml AMC or their combination, respectively. Through viable bacteria counting, metabolic activity assay, quantitative real-time PCR absolute quantification and 16S rDNA sequencing analysis, the anti-biofilm effect of 1000 μA DC and enhanced killing effects of 1000 μA DC combined with antibiotics (MTZ, AMC or MTZ+AMC) were explored.

RESULTS

The old subgingival plaque model (48 h) had no significant difference in total bacterial loads from subgingival plaque in situ, which achieved a similarity of 80%. The 1000 μA DC plus MTZ or AMC for 12 h showed a stronger synergistic killing effect than the same combination for 20 min. The metabolic activity was reduced to the lowest by DC plus MTZ+AMC, as 37.4% of that in the control group, while average synergistic killing effect reached 1.06 log units and average total bacterial loads decreased to 0.87 log units. Furthermore, the relative abundance of the genera Porphyromonas, Prevotella, Treponema_2, and Tannerella were decreased significantly.

CONCLUSION

The presence of weak DC (1000 μA) improved the killing effect of antibiotics on subgingival plaque biofilms, which might provide a novel strategy to reduce their antibiotic resistance.

Metronidazole enhances killing of Porphyromonas gingivalis by human PMNs

Background and objectives

Periodontitis affects the supporting structures of the teeth as a result of the interactions between the subgingival biofilm and the host immune system. Periodontal therapy in severe forms of periodontitis often utilizes antimicrobial agents with some potential to improve host defense responses. In the present study, we investigated the in vitro effect of metronidazole (MTZ) at concentrations achievable in the periodontal pocket on PMN activation and PMN mediated killing of Porphyromonas gingivalis.

Materials and methods

Flow cytometry based assays were used to measure the impact of MTZ on PMN degranulation, neutrophil extracellular trap (NET) formation and myeloperoxidase (MPO) release and phagocytosis in response to the keystone oral pathogen P. gingivalis. Functional assays for PMN mediated killing of P. gingivalis and reactive oxygen species (ROS) production in PMN were also carried out.

Results

We demonstrate that PMNs pretreated with MTZ (2 μg/ml or 50 μg/ml) displayed enhanced killing of P. gingivalis compared to untreated PMNs. At concentrations achieved physiologically in the periodontal pocket, MTZ induced PMN surface expression of two activation markers (CD66 and CD63). MTZ did not alter P. gingivalis-induced NETosis, but suppressed P. gingivalis-induced ROS production and phagocytosis.

Conclusion

MTZ displays a positive interaction with PMNs to potentiate PMN mediated killing of P. gingivalis and may therefore contribute to its beneficial effects in the treatment of periodontitis initiated by P. gingivalis infections including those refractory to conventional treatment.

Antimicrobial, Polarizing Light, and Paired Helical Filament Properties of Fragmented Tau Peptides of Selected Putative Gingipains

BACKGROUND

Tau is an established substrate for gingipains secreted by Porphyromonas gingivalis. Hyperphosphorylation of tau and neurofibrillary tangle (NFT) formation is a defining lesion of Alzheimer's disease (AD) where NFT distribution is related to Braak stage and disease severity.

OBJECTIVE

To assess gingipains'-fragmented tau peptides for their antimicrobial properties and for the likelihood of paired helical/straight filament (PHF/SF) formation with implications for the NFT lesion.

METHODS

Seven non-phosphorylated (A-G) and three phosphorylated (A-C) tau peptides, were tested for antimicrobial properties against P. gingivalis. Polarizing light properties were determined using Congo Red staining. Secondary and tertiary structures of peptides B-F were determined using transmission electron microscopy (TEM) and circular dichroism (CD) was undertaken for the soluble peptides A in phosphorylated and non-phosphorylated states.

RESULTS

Phosphorylated tau peptide A displayed a significant effect against planktonic P. gingivalis. The CD results demonstrated that both peptides A, in phosphorylated and non-phosphorylated states, in aqueous solution, adopted mainly β-type structures. Non-phosphorylated peptides B-F and phosphorylated peptides B-C were insoluble and fibrillar under the TEM. The secondary and tertiary structures of the non-phosphorylated peptide B demonstrated fewer helical twists, whereas peptide C displayed significantly more helical twists along the whole fiber(s) length following its phosphorylation.

CONCLUSION

Phosphorylated peptide A reduced P. gingivalis viability. CD spectroscopy demonstrated the phosphorylated and the non-phosphorylated peptide A predominantly formed from β-sheet structures in aqueous solution with potential antimicrobial activity. Phosphorylation of tau peptides physically changed their tertiary structure into PHFs with potential for self-aggregation and binding to the NFT lesion.

RNA Sequencing Reveals the Upregulation of FOXO Signaling Pathway in Porphyromonas gingivalis Persister-Treated Human Gingival Epithelial Cells

Porphyromonas gingivalis as the keystone periodontopathogen plays a critical role in the pathogenesis of periodontitis, and crucially accounts for inflammatory comorbidities such as cardiovascular disease and Alzheimer's disease. We recently identified the existence of P. gingivalis persisters and revealed the unforeseen perturbation of innate response in human gingival epithelial cells (HGECs) due to these noxious persisters. Herein, RNA sequencing revealed how P. gingivalis persisters affected the expression profile of cytokine genes and related signaling pathways in HGECs. Results showed that metronidazole-treated P. gingivalis persisters (M-PgPs) impaired the innate host defense of HGECs, in a similar fashion to P. gingivalis. Notably, over one thousand differentially expressed genes were identified in HGECs treated with M-PgPs or P. gingivalis with reference to the controls. Gene Ontology and KEGG pathway analysis demonstrated significantly enriched signaling pathways, such as FOXO. Importantly, the FOXO1 inhibitor rescued the M-PgP-induced disruption of cytokine expression. This study suggests that P. gingivalis persisters may perturb innate host defense, through the upregulation of the FOXO signaling pathway. Thus, the current findings could contribute to developing new approaches to tackling P. gingivalis persisters for the effective control of periodontitis and P. gingivalis-related inflammatory comorbidities.

Proteomics in antibiotic resistance and tolerance research: Mapping the resistome and the tolerome of bacterial pathogens

Antibiotic resistance, the ability of a microbial pathogen to evade the effects of antibiotics thereby allowing them to grow under elevated drug concentrations, is an alarming health problem worldwide and has attracted the attention of scientists for decades. On the other hand, the clinical importance of persistence and tolerance as alternative mechanisms for pathogens to survive prolonged lethal antibiotic doses has recently become increasingly appreciated. Persisters and high-tolerance populations are thought to cause the relapse of infectious diseases, and provide opportunities for the pathogens to evolve resistance during the course of antibiotic therapy. Although proteomics and other omics methodology have long been employed to study resistance, its applications in studying persistence and tolerance are still limited. However, due to the growing interest in the topic and recent progress in method developments to study them, there have been some proteomic studies that yield fresh insights into the phenomenon of persistence and tolerance. Combined with the studies on resistance, these collectively guide us to novel molecular targets for the potential drugs for the control of these dangerous pathogens. In this review, we surveyed previous proteomic studies to investigate resistance, persistence, and tolerance mechanisms, and discussed emerging experimental strategies for studying these phenotypes with a combination of adaptive laboratory evolution and high-throughput proteomics. This article is protected by copyright. All rights reserved.

Direct current exerts electricidal and bioelectric effects on Porphyromonas gingivalis biofilms partially via promoting oxidative stress and antibiotic transport

Low electric current can inhibit certain microbial biofilms and enhance the efficacy of antimicrobials against them. This study investigated the electricidal and bioelectric effects of direct current (DC) against Porphyromonas gingivalis biofilms as well as the underlying mechanisms. Here, we firstly showed that DC significantly suppressed biofilm formation of P. gingivalis in time- and intensity-dependent manners, and markedly inhibited preformed P. gingivalis biofilms. Moreover, DC enhanced the killing efficacy of metronidazole (MTZ) and amoxicillin with clavulanate potassium (AMC) against the biofilms. Notably, DC-treated biofilms displayed upregulated intracellular ROS and expression of ROS related genes (sod, feoB, and oxyR) as well as porin gene. Interestingly, DC-induced killing of biofilms was partially reversed by ROS scavenger N-dimethylthiourea (DMTU), and the synergistic effect of DC with MTZ/AMC was weakened by small interfering RNA of porin gene (si-Porin). In conclusion, DC can exert electricidal and bioelectric effects against P. gingivalis biofilms partially via promotion of oxidative stress and antibiotic transport, which offers a promising approach for effective management of periodontitis.

In vitro evaluation of the antibacterial effect of colloidal bismuth subcitrate on Porphyromonas gingivalis and its biofilm

OBJECTIVE

To investigate the antibacterial and anti-biofilm effects of colloidal bismuth subcitrate (CBS) on Porphyromonas gingivalis (P. gingivalis) in its planktonic and biofilm forms and also compare it with that of 0.2% chlorhexidine (CHX).

DESIGN

The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of CBS were determined by the microdilution method; the bacteriostatic rate of CBS was determined by the MTT assay; the effect of CBS on the membrane integrity of P. gingivalis was investigated by the flow cytometric methods. The effects of CBS on the biomass and bacterial activity of biofilm were investigated. Confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) were used to investigate the activity and structure of biofilms.

RESULTS

The MIC and MBC values were 18.75 µg/mL and 37.5 µg/mL. CBS could damage the cell membrane of P. gingivalis. CBS effectively inhibited biofilm formation and promoted dissociation at higher concentrations of 37.5 µg/mL and 75 µg/mL, respectively. The results also indicated an altered biofilm structure and reduced biofilm thickness and bacterial aggregation.

CONCLUSIONS

CBS affected the metabolic and physiological processes of P. gingivalis, inhibited the formation of biofilm, and disrupted the mature biofilm.

Eradication of Porphyromonas gingivalis Persisters Through Colloidal Bismuth Subcitrate Synergistically Combined With Metronidazole

Microbial persisters enable the development of certain intrinsic strategies for survival with extreme tolerance to multiple antimicrobials. Porphyromonas gingivalis is considered to be the "keystone" periodontopathogen. Indeed, periodontitis, as a highly common inflammatory disease, is the major cause of severe tooth loss and edentulism in adults globally, and yet it is crucially involved in various systemic comorbidities like diabetes. We have recently revealed P. gingivalis persisters-induced perturbation of immuno-inflammatory responses and effective suppression of this key pathogen by bismuth drugs. This study further explored novel approaches to eradicating P. gingivalis persisters through synergistic combination of colloidal bismuth subcitrate (CBS) with traditional antibiotics. P. gingivalis (ATCC 33277) cells in planktonic and biofilm states were cultured to stationary phase, and then treated with metronidazole (100 mg/L), amoxicillin (100 mg/L), CBS, (100 μM) and combinations of these medications, respectively. Persister survival rate was calculated by colony-forming unit. Cell viability and cytotoxicity of CBS were assessed in human gingival epithelial cells (HGECs). Notably, CBS combined with metronidazole enabled the effective eradication of P. gingivalis persisters in planktonic mode, and nearly eliminated their existence in biofilm mode. Importantly, CBS exhibited no effects on the viability of HGECs, along with minimal cytotoxicity (<5%) even at a high concentration (400 μM). This pioneering study shows that P. gingivalis persisters could be well eliminated via the synergistic combination of CBS with metronidazole. Our findings may contribute to developing novel approaches to tackling periodontitis and inflammatory systemic comorbidities.

Proteomic Analysis of Caspofungin-Induced Responses in Planktonic Cells and Biofilms of Candida albicans

Candida albicans biofilms display markedly increased antifungal resistance, and the underlying mechanisms remain unclear. This study investigated the signature profiles of C. albicans planktonic cells and biofilms in response to caspofungin (CAS) by mass spectrometry-based shotgun proteomics. We found that C. albicans biofilms were twofold more resistant to CAS with reference to planktonic cells. Notably, 9.6% of C. albicans biofilm cells survived the lethal treatment of CAS (128 μg/ml), confirmed by LIVE/DEAD staining, confocal laser scanning microscopy (CLSM) and scanning electron microscopy analyses. The responses of C. albicans planktonic cells and biofilms to CAS treatment at respective minimum inhibitory concentrations (MICs) were assessed by high-throughput proteomics and bioinformatics approaches. There were 148 and 224 proteins with >twofold difference identified from the planktonic cells and biofilms, respectively. CAS treatment downregulated several cell wall- and oxidative stress-related proteins. Whereas, CAS-induced action was compensated by markedly increased expression of many other proteins involved in cell wall integrity and stress response (e.g., heat shock proteins). Moreover, considerable expression changes were identified in metabolism-associated proteins like glycolysis, tricarboxylic acid (TCA) cycle and ATP biosynthesis. Importantly, various key proteins for cell wall integrity, stress response and metabolic regulation (e.g., PIL1, LSP1, HSP90, ICL1, and MLS1) were exclusively enriched and implicated in C. albicans biofilms. This study demonstrates that C. albicans biofilms undergo highly complicated yet complex regulation of multiple cellular pathways in response to CAS. Signature proteins essential for modulating cell wall integrity, stress response and metabolic activities may account for the antifungal resistance of C. albicans biofilms.

MazEF Toxin-Antitoxin System-Mediated DNA Damage Stress Response in Deinococcus radiodurans

Deinococcus radiodurans shows marked resistance to various types of DNA-damaging agents, including mitomycin C (MMC). A type II toxin-antitoxin (TA) system that responds to DNA damage stress was identified in D. radiodurans, comprising the toxin MazF-dr and the antitoxin MazE-dr. The cleavage specificity of MazF-dr, an endoribonuclease, was previously characterized. Here, we further investigated the regulatory role of the MazEF system in the response to DNA damage stress in D. radiodurans. The crystal structure of D. radiodurans MazF (MazF-dr) was determined at a resolution of 1.3 Å and is the first structure of the toxin of the TA system of D. radiodurans. MazF-dr forms a dimer mediated by the presence of interlocked loops. Transcriptional analysis revealed 650 downregulated genes in the wild-type (WT) strain, but not in the mazEF mutant strain, which are potentially regulated by MazEF-dr in response to MMC treatment. Some of these genes are involved in membrane trafficking and metal ion transportation. Subsequently, compared with the WT strain, the mazEF mutant strain exhibited much lower MMC-induced intracellular iron concentrations, reactive oxygen species (ROS), and protein carbonylation levels. These results provide evidence that MazEF-mediated cell death in D. radiodurans might be caused by an increase in ROS accumulation upon DNA damage stress.

Analyzing Persister Proteomes with SILAC and Label-Free Methods

State-of-the-art mass spectrometry enables in-depth analysis of proteomes in virtually all organisms. This chapter describes methods for the analysis of persister proteomes by mass spectrometry. Stable isotope labeling by amino acids in cell culture (SILAC) is applied to assess protein biosynthesis in persister cells, which are isolated by treatment with beta-lactam antibiotics. Furthermore, persister proteomes during the postantibiotic recovery phase are analyzed by label-free quantification. The presented methods are valuable tools to shed light on persister physiology.

A Novel Small Molecule, 1,3-di-m-tolyl-urea, Inhibits and Disrupts Multispecies Oral Biofilms

An imbalance of homeostasis between the microbial communities and the host system leads to dysbiosis in oral micro flora. DMTU (1,3-di-m-tolyl-urea) is a biocompatible compound that was shown to inhibit Streptococcus mutans biofilm by inhibiting its communication system (quorum sensing). Here, we hypothesized that DMTU is able to inhibit multispecies biofilms. We developed a multispecies oral biofilm model, comprising an early colonizer Streptococcus gordonii, a bridge colonizer Fusobacterium nucleatum, and late colonizers Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans. We performed comprehensive investigations to demonstrate the effect of DMTU on planktonic cells and biofilms. Our findings showed that DMTU inhibits and disrupts multispecies biofilms without bactericidal effects. Mechanistic studies revealed a significant down regulation of biofilm and virulence-related genes in P. gingivalis. Taken together, our study highlights the potential of DMTU to inhibit polymicrobial biofilm communities and their virulence.

Bismuth drugs tackle Porphyromonas gingivalis and attune cytokine response in human cells

Periodontitis is the leading cause of severe tooth loss and edentulism in adults worldwide and is closely linked to systemic conditions such as diabetes and cardiovascular disease. Porphyromonas gingivalis is the key pathogen in periodontitis. Herein, we provided the first evidence that bismuth drugs suppress P. gingivalis in its planktonic, biofilm, and intracellular states. In total, 42 bismuth-associated proteins were identified including its major virulent factors (e.g., gingipains, hemagglutinin HagA, and fimbriae). Bismuth perturbed its iron acquisition, disturbed the energy metabolism and virulence, and deactivated multiple key enzymes (e.g., superoxide dismutase and thioredoxins). Moreover, bismuth inhibited its biofilm formation and disrupted the 3-day matured biofilms. Notably, the internalized P. gingivalis in various human cells (e.g., human gingival epithelium progenitors, HGEPs) was oppressed by bismuth but not the commonly used antibiotic metronidazole. Importantly, bismuth drugs enabled the counteraction of immuno-inflammatory responses in different host cells perturbed by P. gingivalis. The production of IL-6 and IL-8 attenuated by P. gingivalis in both of native and IL-1β-stimulated HGEPs was restored, while the bacterium-enhanced expression of IL-6, IL-1β, and TNFα in THP-1 macrophages was alleviated. This proof-of-concept study brings prospects for the potential reposition of the routinely used anti-Helicobacter pylori bismuth drugs to better manage inflammatory diseases such as periodontitis and P. gingivalis-related complex systemic disorders.

Metronidazole-Treated Porphyromonas gingivalis Persisters Invade Human Gingival Epithelial Cells and Perturb Innate Responses

Periodontitis as a biofilm-associated inflammatory disease is highly prevalent worldwide. It severely affects oral health and yet closely links to systemic diseases like diabetes and cardiovascular disease. Porphyromonas gingivalis as a "keystone" periodontopathogen drives the shift of microbe-host symbiosis to dysbiosis and critically contributes to the pathogenesis of periodontitis. Persisters represent a tiny subset of biofilm-associated microbes highly tolerant to lethal treatment of antimicrobials, and, notably, metronidazole-tolerant P. gingivalis persisters have recently been identified by our group. This study further explored the interactive profiles of metronidazole-treated P. gingivalis persisters (M-PgPs) with human gingival epithelial cells (HGECs). P. gingivalis cells (ATCC 33277) at stationary phase were treated with a lethal dosage of metronidazole (100 μg/ml, 6 h) for generating M-PgPs. The interaction of M-PgPs with HGECs was assessed by microscopy, flow cytometry, cytokine profiling, and quantitative PCR (qPCR). We demonstrated that the overall morphology and ultracellular structure of M-PgPs remained unchanged. Importantly, M-PgPs maintained the capabilities to adhere to and invade HGECs. Moreover, M-PgPs significantly suppressed proinflammatory cytokine expression in HGECs at a level comparable to that seen with the untreated P. gingivalis cells, through the thermosensitive components. The present report reveals that P. gingivalis persisters induced by lethal treatment of antibiotics were able to maintain their capabilities to adhere to and invade human gingival epithelial cells and to perturb the innate host responses. Novel strategies and approaches need to be developed for tackling P. gingivalis and favorably modulating the dysregulated immunoinflammatory responses for oral/periodontal health and general well-being.