Poly-N-acetylglucosamine mediates biofilm formation and detergent resistance in Aggregatibacter actinomycetemcomitans

Understanding the biofilm development of Acinetobacter baumannii and novel strategies to combat infection

Acinetobacter baumannii (A. baumannii) is a Gram-negative, nonmotile, and aerobic bacillus emerged as a superbug, due to increasing the possibility of infection and accelerating rates of antimicrobial agents. It is recognized as a nosocomial pathogen due to its ability to form biofilms. These biofilms serve as a defensive barrier, increase antibiotic resistance, and make treatment more difficult. As a result, the current situation necessitates the rapid emergence of novel therapeutic approaches to ensure successful treatment outcomes. This review explores the intricate relationship between biofilm formation and antibiotic resistance in A. baumannii, emphasizing the role of key virulence factors and quorum sensing (QS) mechanisms that will lead to infections and facilitate insight into developing innovative method to control A. baumannii infections. Furthermore, the review article looks into promising approaches for preventing biofilm formation on medically important surfaces and potential therapeutic methods for eliminating preformed biofilms, which can address biofilm-associated A. baumannii infections. Modern advances in emerging therapeutic options such as antimicrobial peptide (AMPs), nanoparticles (NPs), bacteriophage therapy, photodynamic therapy (PDT), and other biofilm inhibitors can assist readers understand the current landscape and future prospects for effectively treating A. baumannii biofilm infections.

Biofilm-mediated infections by multidrug-resistant microbes: a comprehensive exploration and forward perspectives

A biofilm is a collection of microorganisms organized in a matrix of extracellular polymeric material. Biofilms consist of microbial cells that attach to both surfaces and each other, whether they are living or non-living. These microbial biofilms can lead to hospital-acquired infections and are generally detrimental. They possess the ability to resist the human immune system and antibiotics. The National Institute of Health (NIH) states that biofilm formation is associated with 65% of all microbial illnesses and 80% of chronic illnesses. Additionally, non-device-related microbial biofilm infections include conditions like cystic fibrosis, otitis media, infective endocarditis, and chronic inflammatory disorders. This review aims to provide an overview of research on chronic infections caused by microbial biofilms, methods used for biofilm detection, recent approaches to combat biofilms, and future perspectives, including the development of innovative antimicrobial strategies such as antimicrobial peptides, bacteriophages, and agents that disrupt biofilms.

Strategy to combat biofilms: a focus on biofilm dispersal enzymes

Bacterial biofilms, which consist of three-dimensional extracellular polymeric substance (EPS), not only function as signaling networks, provide nutritional support, and facilitate surface adhesion, but also serve as a protective shield for the residing bacterial inhabitants against external stress, such as antibiotics, antimicrobials, and host immune responses. Biofilm-associated infections account for 65-80% of all human microbial infections that lead to serious mortality and morbidity. Tremendous effort has been spent to address the problem by developing biofilm-dispersing agents to discharge colonized microbial cells to a more vulnerable planktonic state. Here, we discuss the recent progress of enzymatic eradicating strategies against medical biofilms, with a focus on dispersal mechanisms. Particularly, we review three enzyme classes that have been extensively investigated, namely glycoside hydrolases, proteases, and deoxyribonucleases.

Bacterial biofilms in the human body: prevalence and impacts on health and disease

Bacterial biofilms can be found in most environments on our planet, and the human body is no exception. Consisting of microbial cells encased in a matrix of extracellular polymers, biofilms enable bacteria to sequester themselves in favorable niches, while also increasing their ability to resist numerous stresses and survive under hostile circumstances. In recent decades, biofilms have increasingly been recognized as a major contributor to the pathogenesis of chronic infections. However, biofilms also occur in or on certain tissues in healthy individuals, and their constituent species are not restricted to canonical pathogens. In this review, we discuss the evidence for where, when, and what types of biofilms occur in the human body, as well as the diverse ways in which they can impact host health under homeostatic and dysbiotic states.

Molecularly imprinted nanoparticles for pathogen visualisation

Saccharides displayed on the cell surface of pathogens play critical roles in many activities such as adhesion, recognition and pathogenesis, as well as in prokaryotic development. In this work, we report the synthesis of molecularly imprinted nanoparticles (nanoMIPs) against pathogen surface monosaccharides using an innovative solid-phase approach. These nanoMIPs can serve as robust and selective artificial lectins specific to one particular monosaccharide. The evaluation of their binding capabilities has been implemented against bacterial cells (E. coli and S. pneumoniae) as model pathogens. The nanoMIPs were produced against two different monosaccharides: mannose (Man), which is present mainly on the surface of Gram-negative bacteria, and N-acetylglucosamine (GlcNAc) exposed on the surface of the majority of bacteria. Herein, we assessed the potential use of nanoMIPs for pathogen cell imaging and detection via flow cytometry and confocal microscopy.

The deficiency of poly-β-1,6-N-acetyl-glucosamine deacetylase trigger A. baumannii to convert to biofilm-independent colistin-tolerant cells

Acinetobacter baumannii is a nosocomial pathogen that can be resistant to antibiotics by rapidly modulating its anti-drug mechanisms. The multidrug-resistant A. baumannii has been considered one of the most threatening pathogens to our society. Biofilm formation and persistent cells within the biofilm matrix are recognized as intractable problems, especially in hospital-acquired infections. Poly-β-1,6-N-acetyl-glucosamine (PNAG) is one of the important building blocks in A. baumannii's biofilm. Here, we discover a protein phosphoryl-regulation on PNAG deacetylase, AbPgaB1, in which residue Ser411 was phosphorylated. The phosphoryl-regulation on AbPgaB1 modulates the product turnover rate in which deacetylated PNAG is produced and reflected in biofilm production. We further uncovered the PgaB deficient A. baumannii strain shows the lowest level of biofilm production but has a high minimal inhibition concentration to antibiotic colistin and tetracycline. Based on bactericidal post-antibiotic effects and time-dependent killing assays with antibacterial drugs, we claim that the PgaB-deficient A. baumannii converts to colistin-tolerant cells. This study utilizes a biofilm-independent colistin-tolerant model of A. baumannii to further investigate its characteristics and mechanisms to better understand clinical outcomes.

Understanding the Predictive Potential of the Oral Microbiome in the Development and Progression of Early Childhood Caries

BACKGROUND

Early childhood caries (ECC) is the most common chronic disease in young children and a public health problem worldwide. It is characterized by the presence of atypical and fast progressive caries lesions. The aggressive form of ECC, severe early childhood caries (S-ECC), can lead to the destruction of the whole crown of most of the deciduous teeth and cause pain and sepsis, affecting the child's quality of life. Although the multifactorial etiology of ECC is known, including social, environmental, behavioral, and genetic determinants, there is a consensus that this disease is driven by an imbalance between the oral microbiome and host, or dysbiosis, mediated by high sugar consumption and poor oral hygiene. Knowledge of the microbiome in healthy and caries status is crucial for risk monitoring, prevention, and development of therapies to revert dysbiosis and restore oral health. Molecular biology tools, including next-generation sequencing methods and proteomic approaches, have led to the discovery of new species and microbial biomarkers that could reveal potential risk profiles for the development of ECC and new targets for anti-caries therapies. This narrative review summarized some general aspects of ECC, such as definition, epidemiology, and etiology, the influence of oral microbiota in the development and progression of ECC based on the current evidence from genomics, transcriptomic, proteomic, and metabolomic studies and the effect of antimicrobial intervention on oral microbiota associated with ECC.

CONCLUSION

The evaluation of genetic and proteomic markers represents a promising approach to predict the risk of ECC before its clinical manifestation and plan efficient therapeutic interventions for ECC in its initial stages, avoiding irreversible dental cavitation.

3-Phenyllactic acid generated in medicinal plant extracts fermented with plant-derived lactic acid bacteria inhibits the biofilm synthesis of Aggregatibacter actinomycetemcomitans

In the present study, the effect of PLA on a periodontic pathogen, Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans), the biofilm, and virulence-related genes was investigated. We confirmed that two lactic acid bacteria (LAB) strains isolated from plant sources, Lactiplantibacillus plantarum MSC-C2 and Pediococcus pentosaceus K40, secrete PLA into the de Man, Rogosa & Sharpe (MRS) broth when supplemented with phenyl pyruvic acid (PPA) as a precursor to PLA. Moreover, PLA was generated in the fermentation broths of two medicinal plant extracts, Paeonia lactiflora Pall (PR) and Carthamus tinctorius (CT), when used by each LAB strain and each extract supplemented with PPA. We determined that the minimum inhibitory concentration (MIC) of PLA against A. actinomycetemcomitans was 20 mM. PLA significantly decreased biofilm formation and suppressed the transcription of pgA, ltxA, and cdtB genes, which encode the poly-N-acetylglucosamine (PGA) polysaccharide of biofilm matrix and exotoxins leukotoxin and cytolethal distending toxin (CDT), respectively. The PLA produced by the MSC-C2 and K40 strains was increased several times by the addition of PPA to the MRS broth. The anti-biofilm effect of the extracts from the fermentation broth was proportional to the increasing PLA concentration, while a cumulatively higher effect than that of PLA alone suggested a combinational effect of PLA and the other metabolites, such as lactic acid (LA). Among the two medicinal plants, PLA, produced after the addition of PPA, was higher in PR extract in case of both the LAB strains. PLA production by the MSC-C2 strain in the PR extract reached 4.8 ± 0.23 mM, which was obviously higher than that in the MRS broth (3.88 ± 0.12 mM) supplemented with 1 mg/ml PPA. The activity to inhibit biofilm formation in the fermented PR extract was clearly high. PLA formed in the fermented PR extract downregulated the dispersin B encoding the dspB gene together with pgA, ltxA, and cdtB. In conclusion, this study shows a promising activity of PLA against the A. actinomycetemcomitans biofilm and virulence genes. In addition, the combinational effect of PLA and the medicinal plant extract can be achieved by fermentation with a specific plant-derived LAB strain.

Polymicrobial biofilms related to dental implant diseases: unravelling the critical role of extracellular biofilm matrix

Biofilms are complex tri-dimensional structures that encase microbial cells in an extracellular matrix comprising self-produced polymeric substances. The matrix rich in extracellular polymeric substance (EPS) contributes to the unique features of biofilm lifestyle and structure, enhancing microbial accretion, biofilm virulence, and antimicrobial resistance. The role of the EPS matrix of biofilms growing on biotic surfaces, especially dental surfaces, is largely unravelled. To date, there is a lack of a broad overview of existing literature concerning the relationship between the EPS matrix and the dental implant environment and its role in implant-related infections. Here, we discuss recent advances in the critical role of the EPS matrix on biofilm growth and virulence on the dental implant surface and its effect on the etiopathogenesis and progression of implant-related infections. Similar to other biofilms associated with human diseases/conditions, EPS-enriched biofilms on implant surfaces promote microbial accumulation, microbiological shift, cross-kingdom interaction, antimicrobial resistance, biofilm virulence, and, consequently, peri-implant tissue damage. But intriguingly, the protagonism of EPS role on implant-related infections and the development of matrix-target therapeutic strategies has been neglected. Finally, we highlight the need for more in-depth analyses of polymicrobial interactions within EPS matrix and EPS-targeting technologies' rationale for disrupting the complex biofilm microenvironment with more outstanding translation to implant applications in the near future.

The effect of N-acetylcysteine in a combined antibiofilm treatment against antibiotic-resistant Staphylococcus aureus

BACKGROUND

The WHO declared Staphylococcus aureus as a 'pathogen of high importance' in 2017. One-fifth of all bloodstream-related infections in Australia and 12 000 cases of bacteraemia in the UK (2017-18) were caused by the MRSA variant. To address the need for novel therapies, we investigated several permutations of an innovative combination therapy containing N-acetylcysteine (NAC), an antibiotic and an enzyme of choice in eradicating MRSA and MSSA biofilms.

METHODS

Biofilm viability (resazurin assay) and colony count methods were used to investigate the effect of NAC, antibiotics and enzymes on S. aureus biofilm disruption and killing. The effects of NAC and enzymes on the polysaccharide content of biofilm matrices were analysed using the phenol/sulphuric acid method and the effect of NAC on DNA cleavage was determined using the Qubit fluorometer technique. Changes in biofilm architecture when subjected to NAC and enzymes were visualized using confocal laser scanning microscopy (CLSM).

RESULTS

NAC alone displayed bacteriostatic effects when tested on planktonic bacterial growth. Combination treatments containing 30 mM NAC resulted in ≥90% disruption of biofilms across all MRSA and MSSA strains with a 2-3 log10 decrease in cfu/mL in treated biofilms. CLSM showed that NAC treatment drastically disrupted S. aureus biofilm architecture. There was also reduced polysaccharide production in MRSA biofilms in the presence of NAC.

CONCLUSIONS

Our results indicate that inclusion of NAC in a combination treatment is a promising strategy for S. aureus biofilm eradication. The intrinsic acidity of NAC was identified as key to maximum biofilm disruption and degradation of matrix components.

Amylases: Biofilm Inducer or Biofilm Inhibitor?

Biofilm is a syntrophic association of sessile groups of microbial cells that adhere to biotic and abiotic surfaces with the help of pili and extracellular polymeric substances (EPS). EPSs also prevent penetration of antimicrobials/antibiotics into the sessile groups of cells. Hence, methods and agents to avoid or remove biofilms are urgently needed. Enzymes play important roles in the removal of biofilm in natural environments and may be promising agents for this purpose. As the major component of the EPS is polysaccharide, amylase has inhibited EPS by preventing the adherence of the microbial cells, thus making amylase a suitable antimicrobial agent. On the other hand, salivary amylase binds to amylase-binding protein of plaque-forming Streptococci and initiates the formation of biofilm. This review investigates the contradictory actions and microbe-associated genes of amylases, with emphasis on their structural and functional characteristics.

Extracellular DNA, cell surface proteins and c-di-GMP promote biofilm formation in Clostridioides difficile

Clostridioides difficile is the leading cause of nosocomial antibiotic-associated diarrhoea worldwide, yet there is little insight into intestinal tract colonisation and relapse. In many bacterial species, the secondary messenger cyclic-di-GMP mediates switching between planktonic phase, sessile growth and biofilm formation. We demonstrate that c-di-GMP promotes early biofilm formation in C. difficile and that four cell surface proteins contribute to biofilm formation, including two c-di-GMP regulated; CD2831 and CD3246, and two c-di-GMP-independent; CD3392 and CD0183. We demonstrate that C. difficile biofilms are composed of extracellular DNA (eDNA), cell surface and intracellular proteins, which form a protective matrix around C. difficile vegetative cells and spores, as shown by a protective effect against the antibiotic vancomycin. We demonstrate a positive correlation between biofilm biomass, sporulation frequency and eDNA abundance in all five C. difficile lineages. Strains 630 (RT012), CD305 (RT023) and M120 (RT078) contain significantly more eDNA in their biofilm matrix than strains R20291 (RT027) and M68 (RT017). DNase has a profound effect on biofilm integrity, resulting in complete disassembly of the biofilm matrix, inhibition of biofilm formation and reduced spore germination. The addition of exogenous DNase could be exploited in treatment of C. difficile infection and relapse, to improve antibiotic efficacy.

Biofilms as Promoters of Bacterial Antibiotic Resistance and Tolerance

Multidrug resistant bacteria are a global threat for human and animal health. However, they are only part of the problem of antibiotic failure. Another bacterial strategy that contributes to their capacity to withstand antimicrobials is the formation of biofilms. Biofilms are associations of microorganisms embedded a self-produced extracellular matrix. They create particular environments that confer bacterial tolerance and resistance to antibiotics by different mechanisms that depend upon factors such as biofilm composition, architecture, the stage of biofilm development, and growth conditions. The biofilm structure hinders the penetration of antibiotics and may prevent the accumulation of bactericidal concentrations throughout the entire biofilm. In addition, gradients of dispersion of nutrients and oxygen within the biofilm generate different metabolic states of individual cells and favor the development of antibiotic tolerance and bacterial persistence. Furthermore, antimicrobial resistance may develop within biofilms through a variety of mechanisms. The expression of efflux pumps may be induced in various parts of the biofilm and the mutation frequency is induced, while the presence of extracellular DNA and the close contact between cells favor horizontal gene transfer. A deep understanding of the mechanisms by which biofilms cause tolerance/resistance to antibiotics helps to develop novel strategies to fight these infections.

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.

Synthesis of C6-substituted UDP-GlcNAc derivatives

UDP-sugar analogs are useful for the study of glycosyltransferases and the production of unnatural glycans. The preparation of five UDP-GlcNAc derivatives is reported with 6-deoxy, 6-azido, 6-amino, 6-mercapto, or 6-fluoro substitutions. A concise chemoenzymatic synthesis was developed using the kinase NahK (B. longum JCM1217) and the uridyl transferase GlmU (E. coli K12).

Glycoside hydrolase (PelAh) immobilization prevents Pseudomonas aeruginosa biofilm formation on cellulose-based wound dressing

Bacterial cellulose (BC) is recognized as a wound dressing material well-suited for chronic wounds; however, it has no intrinsic antimicrobial activity. Further, the formation of biofilms can limit the effectiveness of the pre-saturation of BC with antimicrobial agents. Here, to hinder biofilm formation by P. aeruginosa, we immobilized the hydrolytic domain of PelA (a glycohydrolase involved in the synthesis of biofilm polysaccharide Pel) on the surface of BC. The immobilization of 32.35 ± 1.05 mg PelA_h per g BC membrane resulted in an eight-fold higher P. aeruginosa cell detachment from BC membrane, indicating reduced biofilm matrix stability. Further, 1D and 2D infrared spectroscopy analysis indicated systematic reduction of polysaccharide biofilm elements, confirming the specificity of immobilized PelA_h. Importantly, BC-PelA_h was not cytotoxic towards L929 fibroblast cells. Thus, we conclude that PelA_h can be used in BC wound dressings for safe and specific protection against biofilm formation by P. aeruginosa.

Synthesis of defined mono-de-N-acetylated β-(1→6)-N-acetyl-d-glucosamine oligosaccharides to characterize PgaB hydrolase activity

Many clinically-relevant biofilm-forming bacterial strains produce partially de-N-acetylated poly-β-(1→6)-N-acetyl-d-glucosamine (dPNAG) as an exopolysaccharide. In Gram-negative bacteria, the periplasmic protein PgaB is responsible for partial de-N-acetylation of PNAG prior to its export to the extracellular space. In addition to de-N-acetylase activity found in the N-terminal domain, PgaB contains a C-terminal hydrolase domain that can disrupt dPNAG-dependent biofilms and hydrolyzes dPNAG but not fully acetylated PNAG. The role of this C-terminal domain in biofilm formation has yet to be determined in vivo. Further characterization of the enzyme's hydrolase activity has been hampered by a lack of specific dPNAG oligosaccharides. Here, we report the synthesis of a defined mono de-N-acetylated dPNAG penta- and hepta-saccharide. Using mass spectrometry analysis and a fluorescence-based thin-layer chromatography (TLC) assay, we found that our defined dPNAG oligosaccharides are hydrolase substrates. In addition to the expected cleavage site, two residues to the reducing side of the de-N-acetylated residue, minor cleavage products on the non-reducing side of the de-N-acetylation site were observed. These findings provide quantitative data to support how PNAG is processed in Gram-negative bacteria.

Lysocins: Bioengineered Antimicrobials That Deliver Lysins across the Outer Membrane of Gram-Negative Bacteria

The prevalence of multidrug-resistant Pseudomonas aeruginosa has stimulated development of alternative therapeutics. Bacteriophage peptidoglycan hydrolases, termed lysins, represent an emerging antimicrobial option for targeting Gram-positive bacteria. However, lysins against Gram-negatives are generally deterred by the outer membrane and their inability to work in serum. One solution involves exploiting evolved delivery systems used by colicin-like bacteriocins (e.g., S-type pyocins of P. aeruginosa) to translocate through the outer membrane. Following surface receptor binding, colicin-like bacteriocins form Tol- or TonB-dependent translocons to actively import bactericidal domains through outer membrane protein channels. With this understanding, we developed lysocins, which are bioengineered lysin-bacteriocin fusion molecules capable of periplasmic import. In our proof-of-concept studies, components from the P. aeruginosa bacteriocin pyocin S2 (PyS2) responsible for surface receptor binding and outer membrane translocation were fused to the GN4 lysin to generate the PyS2-GN4 lysocin. PyS2-GN4 delivered the GN4 lysin to the periplasm to induce peptidoglycan cleavage and log-fold killing of P. aeruginosa with minimal endotoxin release. While displaying narrow-spectrum antipseudomonal activity in human serum, PyS2-GN4 also efficiently disrupted biofilms, outperformed standard-of-care antibiotics, exhibited no cytotoxicity toward eukaryotic cells, and protected mice from P. aeruginosa challenge in a bacteremia model. In addition to targeting P. aeruginosa, lysocins can be constructed to target other prominent Gram-negative bacterial pathogens.

The Role of Exopolysaccharides in Oral Biofilms

The oral cavity contains a rich consortium of exopolysaccharide-producing microbes. These extracellular polysaccharides comprise a major component of the oral biofilm. Together with extracellular proteins, DNA, and lipids, they form the biofilm matrix, which contributes to bacterial colonization, biofilm formation and maintenance, and pathogenesis. While a number of oral microbes have been studied in detail with regard to biofilm formation and pathogenesis, the exopolysaccharides have been well characterized for only select organisms, namely Streptococcus mutans and Aggregatibacter actinomycetemcomitans. Studies on the exopolysaccharides of other oral organisms, however, are in their infancy. In this review, we present the current research on exopolysaccharides of oral microbes regarding their biosynthesis, regulation, contributions to biofilm formation and stability of the matrix, and immune evasion. In addition, insight into the role of exopolysaccharides in biofilms is highlighted through the evaluation of emerging techniques such as pH probing of biofilm colonies, solid-state nuclear magnetic resonance for macromolecular interactions within biofilms, and super-resolution microscopy analysis of biofilm development. Finally, exopolysaccharide as a potential nutrient source for species within a biofilm is discussed.

Aggregatibacter actinomycetemcomitans LPS binds human interleukin-8

Various gram-negative species sequester host cytokines using outer membrane proteins or surface modulation by sulfated polysaccharides. An outer membrane lipoprotein (BilRI) of the periodontal pathogen Aggregatibacter actinomycetemcomitans binds several cytokines, including interleukin (IL)-8. Because IL-8 is positively charged at physiological pH, we aimed to determine whether IL-8 interacts with negatively charged lipopolysaccharide (LPS). Binding was investigated using electrophoretic mobility shift assays and microwell-based time-resolved fluorometric immunoassay. LPS from each tested strain of A. actinomycetemcomitans (N = 13), Pseudomonas aeruginosa (N = 1) and Escherichia coli (N = 1) bound IL-8. The K_d value of the A. actinomycetemcomitans LPS-IL-8 interaction varied between 1.2–17 μM irrespective of the serotype and the amount of phosphorus in LPS and was significantly lower than that of the BilRI-IL-8 interaction. Moreover, IL-8 interacted with whole A. actinomycetemcomitans cells and outer membrane vesicles. Hence, LPS might be involved in binding of IL-8 to the outer membrane of A. actinomycetemcomitans. This raises an interesting question regarding whether other gram-negative periodontal pathogens use LPS for IL-8 sequestering in vivo.

Interactions between the Aggregatibacter actinomycetemcomitans secretin HofQ and host cytokines indicate a link between natural competence and interleukin-8 uptake

Naturally competent bacteria acquire DNA from their surroundings to survive in nutrient-poor environments and incorporate DNA into their genomes as new genes for improved survival. The secretin HofQ from the oral pathogen Aggregatibacter actinomycetemcomitans has been associated with DNA uptake. Cytokine sequestering is a potential virulence mechanism in various bacteria and may modulate both host defense and bacterial physiology. The objective of this study was to elucidate a possible connection between natural competence and cytokine uptake in A. actinomycetemcomitans. The extramembranous domain of HofQ (emHofQ) was shown to interact with various cytokines, of which IL-8 exhibited the strongest interaction. The dissociation constant between emHofQ and IL-8 was 43 nM in static settings and 2.4 μM in dynamic settings. The moderate binding affinity is consistent with the hypothesis that emHofQ recognizes cytokines before transporting them into the cells. The interaction site was identified via crosslinking and mutational analysis. By structural comparison, relateda type I KH domain with a similar interaction site was detected in the Neisseria meningitidis secretin PilQ, which has been shown to participate in IL-8 uptake. Deletion of hofQ from the A. actinomycetemcomitans genome decreased the overall biofilm formation of this organism, abolished the response to cytokines, i.e., decreased eDNA levels in the presence of cytokines, and increased the susceptibility of the biofilm to tested β-lactams. Moreover, we showed that recombinant IL-8 interacted with DNA. These results can be used in further studies on the specific role of cytokine uptake in bacterial virulence without interfering with natural-competence-related DNA uptake.

HldE Is Important for Virulence Phenotypes in Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is one of the most common causes of diarrheal illness in third world countries and it especially affects children and travelers visiting these regions. ETEC causes disease by adhering tightly to the epithelial cells in a concerted effort by adhesins, flagella, and other virulence-factors. When attached ETEC secretes toxins targeting the small intestine host-cells, which ultimately leads to osmotic diarrhea. HldE is a bifunctional protein that catalyzes the nucleotide-activated heptose precursors used in the biosynthesis of lipopolysaccharide (LPS) and in post-translational protein glycosylation. Both mechanisms have been linked to ETEC virulence: Lipopolysaccharide (LPS) is a major component of the bacterial outer membrane and is needed for transport of heat-labile toxins to the host cells, and ETEC glycoproteins have been shown to play an important role for bacterial adhesion to host epithelia. Here, we report that HldE plays an important role for ETEC virulence. Deletion of hldE resulted in markedly reduced binding to the human intestinal cells due to reduced expression of colonization factor CFA/I on the bacterial surface. Deletion of hldE also affected ETEC motility in a flagella-dependent fashion. Expression of both colonization factors and flagella was inhibited at the level of transcription. In addition, the hldE mutant displayed altered growth, increased biofilm formation and clumping in minimal growth medium. Investigation of an orthogonal LPS-deficient mutant combined with mass spectrometric analysis of protein glycosylation indicated that HldE exerts its role on ETEC virulence both through protein glycosylation and correct LPS configuration. These results place HldE as an attractive target for the development of future antimicrobial therapeutics.

Electrochemical Probes of Microbial Community Behavior

Advances in next-generation sequencing technology along with decreasing costs now allow the microbial population, or microbiome, of a location to be determined relatively quickly. This research reveals that microbial communities are more diverse and complex than ever imagined. New and specialized instrumentation is required to investigate, with high spatial and temporal resolution, the dynamic biochemical environment that is created by microbes, which allows them to exist in every corner of the Earth. This review describes how electrochemical probes and techniques are being used and optimized to learn about microbial communities. Described approaches include voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy, separation techniques coupled with electrochemical detection, and arrays of complementary metal-oxide-semiconductor circuits. Microbial communities also interact with and influence their surroundings; therefore, the review also includes a discussion of how electrochemical probes optimized for microbial analysis are utilized in healthcare diagnostics and environmental monitoring applications.

Aggregatibacter actinomycetemcomitans H-NS promotes biofilm formation and alters protein dynamics of other species within a polymicrobial oral biofilm

Aggregatibacter actinomycetemcomitans is a Gram-negative organism, strongly associated with aggressive forms of periodontitis. An important virulence property of A. actinomycetemcomitans is its ability to form tenacious biofilms that can attach to abiotic as well as biotic surfaces. The histone-like (H-NS) family of nucleoid-structuring proteins act as transcriptional silencers in many Gram-negative bacteria. To evaluate the role of H-NS in A. actinomycetemcomitans, hns mutant derivatives of serotype a strain D7S were generated. Characteristics of the hns mutant phenotype included shorter and fewer pili, and substantially lower monospecies biofilm formation relative to the wild type. Furthermore, the D7S hns mutant exhibited significantly reduced growth within a seven-species oral biofilm model. However, no apparent difference was observed regarding the numbers and proportions of the remaining six species regardless of being co-cultivated with D7S hns or its parental strain. Proteomics analysis of the strains grown in monocultures confirmed the role of H-NS as a repressor of gene expression in A. actinomycetemcomitans. Interestingly, proteomics analysis of the multispecies biofilms indicated that the A. actinomycetemcomitans wild type and hns mutant imposed different regulatory effects on the pattern of protein expression in the other species, i.e., mainly Streptococcus spp., Fusobacterium nucleatum, and Veillonella dispar. Gene ontology analysis revealed that a large portion of the differentially regulated proteins was related to translational activity. Taken together, our data suggest that, apart from being a negative regulator of protein expression in A. actinomycetemcomitans, H-NS promotes biofilm formation and may be an important factor for survival of this species within a multispecies biofilm.

A member of a specific group of gene-regulating proteins promotes biofilm formation by a bacterium associated with aggressive forms of gum disease. Forming biofilms helps the bacterium to cause persistent infections. Researchers at Karolinska Institutet and Umeå University (Sweden), and University of Zürich (Switzerland), led by Jan Oscarsson at Umeå University, investigated the role of the “histone-like” protein H-NS in Aggregatibacter actinomycetemcomitans infections. These proteins are known to suppress the activity of specific genes in many bacteria, a property confirmed in this research. By studying mutant bacterial strains deficient in H-NS protein, the researchers demonstrated that this protein promotes the formation of biofilms by the bacteria. The results suggest that H-NS plays a significant role in allowing Aggregatibacter actinomycetemcomitans to thrive in biofilms containing mixed populations of bacteria. This effect appears to involve activating production of hair-like appendages called pili on the bacterial surface.

A single-step transconjugation system for gene deletion in Aggregatibacter actinomycetemcomitans

Aggregatibacter (A.) actinomycetemcomitans is a periodontopathogenic bacterium causing aggressive periodontitis. Here we describe a single-step transconjugation system as novel and easily applicable protocol for site-specific genetic manipulation of A. actinomycetemcomitans. Deletion of PgaC, which is involved in the synthesis of biofilm matrix, led to a reduced biofilm formation.

Identification and functional characterization of type II toxin/antitoxin systems in Aggregatibacter actinomycetemcomitans

Type II toxin/antitoxin (TA) systems contribute to the formation of persister cells and biofilm formation for many organisms. Aggregatibacter actinomycetemcomitans thrives in the complex oral microbial community subjected to continual environmental flux. Little is known regarding the presence and function of type II TA systems in this organism or their contribution to adaptation and persistence in the biofilm. We identified 11 TA systems that are conserved across all seven serotypes of A. actinomycetemcomitans and represent the RelBE, MazEF and HipAB families of type II TA systems. The systems selectively responded to various environmental conditions that exist in the oral cavity. Two putative RelBE‐like TA systems, D11S_1194‐1195 and D11S_1718‐1719 were induced in response to low pH and deletion of D11S_1718‐1719 significantly reduced metabolic activity of stationary phase A. actinomycetemcomitans cells upon prolonged exposure to acidic conditions. The deletion mutant also exhibited reduced biofilm biomass when cultured under acidic conditions. The D11S_1194 and D11S_1718 toxin proteins inhibited in vitro translation of dihydrofolate reductase (DHFR) and degraded ribosome‐associated, but not free, MS2 virus RNA. In contrast, the corresponding antitoxins (D11S_1195 and D11S_1719), or equimolar mixtures of toxin and antitoxin, had no effect on DHFR production or RNA degradation. Together, these results suggest that D11S_1194‐1195 and D11S_1718‐1719 are RelBE‐like type II TA systems that are activated under acidic conditions and may function to cleave ribosome‐associated mRNA to inhibit translation in A. actinomycetemcomitans. In vivo, these systems may facilitate A. actinomycetemcomitans adaptation and persistence in acidic local environments in the dental biofilm.

Therapeutic Potential of Biofilm-Dispersing Enzymes

Surface-attached colonies of bacteria known as biofilms play a major role in the pathogenesis of medical device infections. Biofilm colonies are notorious for their resistance to antibiotics and host defenses, which makes most device infections difficult or impossible to eradicate. Bacterial cells in a biofilm are held together by an extracellular polymeric matrix that is synthesized by the bacteria themselves. Enzymes that degrade biofilm matrix polymers have been shown to inhibit bio film formation, detach established bio film colonies, and render biofilm cells sensitive to killing by antimicrobial agents. This review discusses the potential use of biofilm matrix-degrading enzymes as anti-biofilm agents for the treatment and prevention of device infections. Two enzymes, deoxyribonuclease I and the glycoside hydrolase dispersin B, will be reviewed in detail. In vitro and in vivo studies demonstrating the anti-biofilm activities of these two enzymes will be summarized, and the therapeutic potential and possible drawbacks of using these enzymes as clinical agents will be discussed.

Extracellular DNA Contributes to Dental Biofilm Stability

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

Role of de-N-acetylase PgaB from Aggregatibacter actinomycetemcomitans in exopolysaccharide export in biofilm mode of growth

Aggregatibacter actinomycetemcomitans, a Gram-negative bacterium, is the causative agent of localized aggressive periodontitis. Attachment to a biotic surface is a critical first step in the A. actinomycetemcomitans infection process for which exopolysaccharides have been shown to be essential. In addition, the pga operon, containing genes encoding for biosynthetic proteins for poly-N-acetyl glucosamine (PNAG), plays a key role in A. actinomycetemcomitans virulence, as a mutant strain lacking the pga operon induces significantly less bone resorption. Among the genes in the pga operon, pgaB codes for a de-N-acetylase that is responsible for the deacetylation of the PNAG exopolysaccharide. Here we report the role of PgaB in regulation of virulence genes using a markerless, scarless deletion mutant targeting the coding region of the N-terminal catalytic domain of PgaB. The results demonstrate that the N-terminal, catalytic domain of PgaB is crucial for exopolysaccharide export.

Bacteriophage Lysin CF-301, a Potent Antistaphylococcal Biofilm Agent

Biofilms pose a unique therapeutic challenge because of the antibiotic tolerance of constituent bacteria. Treatments for biofilm-based infections represent a major unmet medical need, requiring novel agents to eradicate mature biofilms. Our objective was to evaluate bacteriophage lysin CF-301 as a new agent to target Staphylococcus aureus biofilms. We used minimum biofilm-eradicating concentration (MBEC) assays on 95 S. aureus strains to obtain a 90% MBEC (MBEC₉₀) value of ≤0.25 μg/ml for CF-301. Mature biofilms of coagulase-negative staphylococci, Streptococcus pyogenes, and Streptococcus agalactiae were also sensitive to disruption, with MBEC₉₀ values ranging from 0.25 to 8 μg/ml. The potency of CF-301 was demonstrated against S. aureus biofilms formed on polystyrene, glass, surgical mesh, and catheters. In catheters, CF-301 removed all biofilm within 1 h and killed all released bacteria by 6 h. Mixed-species biofilms, formed by S. aureus and Staphylococcus epidermidis on several surfaces, were removed by CF-301, as were S. aureus biofilms either enriched for small-colony variants (SCVs) or grown in human synovial fluid. The antibacterial activity of CF-301 was further demonstrated against S. aureus persister cells in exponential-phase and stationary-phase populations. Finally, the antibiofilm activity of CF-301 was greatly improved in combinations with the cell wall hydrolase lysostaphin when tested against a range of S. aureus strains. In all, the data show that CF-301 is highly effective at disrupting biofilms and killing biofilm bacteria, and, as such, it may be an efficient new agent for treating staphylococcal infections with a biofilm component.

Plasticity of Candida albicans Biofilms

Candida albicans, the most pervasive fungal pathogen that colonizes humans, forms biofilms that are architecturally complex. They consist of a basal yeast cell polylayer and an upper region of hyphae encapsulated in extracellular matrix. However, biofilms formed in vitro vary as a result of the different conditions employed in models, the methods used to assess biofilm formation, strain differences, and, in a most dramatic fashion, the configuration of the mating type locus (MTL). Therefore, integrating data from different studies can lead to problems of interpretation if such variability is not taken into account. Here we review the conditions and factors that cause biofilm variation, with the goal of engendering awareness that more attention must be paid to the strains employed, the methods used to assess biofilm development, every aspect of the model employed, and the configuration of the MTL locus. We end by posing a set of questions that may be asked in comparing the results of different studies and developing protocols for new ones. This review should engender the notion that not all biofilms are created equal.

A novel intrinsically disordered outer membrane lipoprotein of Aggregatibacter actinomycetemcomitans binds various cytokines and plays a role in biofilm response to interleukin-1β and interleukin-8

Intrinsically disordered proteins (IDPs) do not have a well-defined and stable 3-dimensional fold. Some IDPs can function as either transient or permanent binders of other proteins and may interact with an array of ligands by adopting different conformations. A novel outer membrane lipoprotein, bacterial interleukin receptor I (BilRI) of the opportunistic oral pathogen Aggregatibacter actinomycetemcomitans binds a key gatekeeper proinflammatory cytokine interleukin (IL)-1β. Because the amino acid sequence of the novel lipoprotein resembles that of fibrinogen binder A of Haemophilus ducreyi, BilRI could have the potential to bind other proteins, such as host matrix proteins. However, from the tested host matrix proteins, BilRI interacted with neither collagen nor fibrinogen. Instead, the recombinant non-lipidated BilRI, which was intrinsically disordered, bound various pro/anti-inflammatory cytokines, such as IL-8, tumor necrosis factor (TNF)-α, interferon (IFN)-γ and IL-10. Moreover, BilRI played a role in the in vitro sensing of IL-1β and IL-8 because low concentrations of cytokines did not decrease the amount of extracellular DNA in the matrix of bilRI⁻ mutant biofilm as they did in the matrix of wild-type biofilm when the biofilms were exposed to recombinant cytokines for 22 hours. BilRI played a role in the internalization of IL-1β in the gingival model system but did not affect either IL-8 or IL-6 uptake. However, bilRI deletion did not entirely prevent IL-1β internalization, and the binding of cytokines to BilRI was relatively weak. Thus, BilRI might sequester cytokines on the surface of A. actinomycetemcomitans to facilitate the internalization process in low local cytokine concentrations.

Bacillus cereus Biofilms-Same, Only Different

Bacillus cereus displays a high diversity of lifestyles and ecological niches and include beneficial as well as pathogenic strains. These strains are widespread in the environment, are found on inert as well as on living surfaces and contaminate persistently the production lines of the food industry. Biofilms are suspected to play a key role in this ubiquitous distribution and in this persistency. Indeed, B. cereus produces a variety of biofilms which differ in their architecture and mechanism of formation, possibly reflecting an adaptation to various environments. Depending on the strain, B. cereus has the ability to grow as immersed or floating biofilms, and to secrete within the biofilm a vast array of metabolites, surfactants, bacteriocins, enzymes, and toxins, all compounds susceptible to act on the biofilm itself and/or on its environment. Within the biofilm, B. cereus exists in different physiological states and is able to generate highly resistant and adhesive spores, which themselves will increase the resistance of the bacterium to antimicrobials or to cleaning procedures. Current researches show that, despite similarities with the regulation processes and effector molecules involved in the initiation and maturation of the extensively studied Bacillus subtilis biofilm, important differences exists between the two species. The present review summarizes the up to date knowledge on biofilms produced by B. cereus and by two closely related pathogens, Bacillus thuringiensis and Bacillus anthracis. Economic issues caused by B. cereus biofilms and management strategies implemented to control these biofilms are included in this review, which also discuss the ecological and functional roles of biofilms in the lifecycle of these bacterial species and explore future developments in this important research area.

Profound Effects of Aggregatibacter actinomycetemcomitans Leukotoxin Mutation on Adherence Properties Are Clarified in in vitro Experiments

Leukotoxin (Ltx) is a prominent virulence factor produced by Aggregatibacter actinomycetemcomitans, an oral microorganism highly associated with aggressive periodontitis. Ltx compromises host responsiveness by altering the viability of neutrophils, lymphocytes, and macrophages. Previously, we developed a Rhesus (Rh) monkey colonization model designed to determine the effect of virulence gene mutations on colonization of A. actinomycetemcomitans. Unexpectedly, an A. actinomycetemcomitans leukotoxin (ltxA) mutant (RhAa-VS2) failed to colonize in the Rh model. No previous literature suggested that Ltx was associated with A. actinomycetemcomitans binding to tooth surfaces. These results led us to explore the broad effects of the ltxA mutation in vitro. Results indicated that LtxA activity was completely abolished in RhAa-VS2 strain, while complementation significantly (P<0.0001) restored leukotoxicity compared to RhAa-VS2 strain. RT-PCR analysis of ltx gene expression ruled out polar effects. Furthermore, binding of RhAa-VS2 to salivary-coated hydroxyapatite (SHA) was significantly decreased (P<0.0001) compared to wild type RhAa3 strain. Real time RT-PCR analysis of the genes related to SHA binding in RhAa-VS2 showed that genes related to binding were downregulated [rcpA (P = 0.018), rcpB (P = 0.02), tadA (P = 0.002)] as compared to wild type RhAa3. RhAa-VS2 also exhibited decreased biofilm depth (P = 0.008) and exo-polysaccharide production (P<0.0001). Buccal epithelial cell (BEC) binding of RhAa-VS2 was unaffected. Complementation with ltxA restored binding to SHA (P<0.002) but had no effect on biofilm formation when compared to RhAa3. In conclusion, mutation of ltxA diminished hard tissue binding in vitro, which helps explain the previous in vivo failure of a ltxA knockout to colonize the Rh oral cavity. These results suggest that; 1) one specific gene knockout (in this case ltxA) could affect other seemingly unrelated genes (such as rcpA, rcpB tadA etc), and 2) some caution should be used when interpreting the effect attributed to targeted gene mutations when seen in a competitive in vivo environment.

Characterization of Biofilm Formation in [Pasteurella] pneumotropica and [Actinobacillus] muris Isolates of Mouse Origin

[Pasteurella] pneumotropica biotypes Jawetz and Heyl and [Actinobacillus] muris are the most prevalent Pasteurellaceae species isolated from laboratory mouse. However, mechanisms contributing to their high prevalence such as the ability to form biofilms have not been studied yet. In the present investigation we analyze if these bacterial species can produce biofilms in vitro and investigate whether proteins, extracellular DNA and polysaccharides are involved in the biofilm formation and structure by inhibition and dispersal assays using proteinase K, DNase I and sodium periodate. Finally, the capacity of the biofilms to confer resistance to antibiotics is examined. We demonstrate that both [P.] pneumotropica biotypes but not [A.] muris are able to form robust biofilms in vitro, a phenotype which is widely spread among the field isolates. The biofilm inhibition and dispersal assays by proteinase and DNase lead to a strong inhibition in biofilm formation when added at the initiation of the biofilm formation and dispersed pre-formed [P.] pneumotropica biofilms, revealing thus that proteins and extracellular DNA are essential in biofilm formation and structure. Sodium periodate inhibited the bacterial growth when added at the beginning of the biofilm formation assay, making difficult the assessment of the role of β-1,6-linked polysaccharides in the biofilm formation, and had a biofilm stimulating effect when added on pre-established mature biofilms of [P.] pneumotropica biotype Heyl and a majority of [P.] pneumotropica biotype Jawetz strains, suggesting that the presence of β-1,6-linked polysaccharides on the bacterial surface might attenuate the biofilm production. Conversely, no effect or a decrease in the biofilm quantity was observed by biofilm dispersal using sodium periodate on further biotype Jawetz isolates, suggesting that polysaccharides might be incorporated in the biofilm structure. We additionally show that [P.] pneumotropica cells enclosed in biofilms were less sensitive to treatment with amoxicillin and enrofloxacin than planktonic bacteria. Taken together, these findings provide a first step in understanding of the biofilm mechanisms in [P.] pneumotropica, which might contribute to elucidation of colonization and pathogenesis mechanisms for these obligate inhabitants of the mouse mucosa.

Aggregatibacter actinomycetemcomitans, a potent immunoregulator of the periodontal host defense system and alveolar bone homeostasis

Aggregatibacter actinomycetemcomitans is a perio-pathogenic bacteria that has long been associated with localized aggressive periodontitis. The mechanisms of its pathogenicity have been studied in humans and preclinical experimental models. Although different serotypes of A. actinomycetemcomitans have differential virulence factor expression, A. actinomycetemcomitans cytolethal distending toxin (CDT), leukotoxin, and lipopolysaccharide (LPS) have been most extensively studied in the context of modulating the host immune response. Following colonization and attachment in the oral cavity, A. actinomycetemcomitans employs CDT, leukotoxin, and LPS to evade host innate defense mechanisms and drive a pathophysiologic inflammatory response. This supra-physiologic immune response state perturbs normal periodontal tissue remodeling/turnover and ultimately has catabolic effects on periodontal tissue homeostasis. In this review, we have divided the host response into two systems: non-hematopoietic and hematopoietic. Non-hematopoietic barriers include epithelium and fibroblasts that initiate the innate immune host response. The hematopoietic system contains lymphoid and myeloid-derived cell lineages that are responsible for expanding the immune response and driving the pathophysiologic inflammatory state in the local periodontal microenvironment. Effector systems and signaling transduction pathways activated and utilized in response to A. actinomycetemcomitans will be discussed to further delineate immune cell mechanisms during A. actinomycetemcomitans infection. Finally, we will discuss the osteo-immunomodulatory effects induced by A. actinomycetemcomitans and dissect the catabolic disruption of balanced osteoclast-osteoblast-mediated bone remodeling, which subsequently leads to net alveolar bone loss.

Transcriptome Profiling of Wild-Type and pga-Knockout Mutant Strains Reveal the Role of Exopolysaccharide in Aggregatibacter actinomycetemcomitans

Exopolysaccharides have a diverse set of functions in most bacteria including a mechanistic role in protecting bacteria against environmental stresses. Among the many functions attributed to the exopolysaccharides, biofilm formation, antibiotic resistance, immune evasion and colonization have been studied most extensively. The exopolysaccharide produced by many Gram positive as well as Gram negative bacteria including the oral pathogen Aggregatibacter actinomycetemcomitans is the homopolymer of β(1,6)-linked N-acetylglucosamine. Recently, we reported that the PGA-deficient mutant of A. actinomycetemcomitans failed to colonize or induce bone resorption in a rat model of periodontal disease, and the colonization genes, apiA and aae, were significantly down regulated in the mutant strain. To understand the role of exopolysaccharide and the pga locus in the global expression of A. actinomycetemcomitans, we have used comparative transcriptome profiling to identify differentially expressed genes in the wild-type strain in relation to the PGA-deficient strain. Transcriptome analysis revealed that about 50% of the genes are differently expressed (P < 0.05 and fold change >1.5). Our study demonstrated that the absence of the pga locus affects the genes involved in peptidoglycan recycling, glycogen storage, and virulence. Further, using confocal microscopy and plating assays, we show that the viability of pga mutant strain is significantly reduced during biofilm growth. Thus, this study highlights the importance of pga genes and the exopolysaccharide in the virulence of A. actinomycetemcomitans.

Cyclic-di-GMP signalling and biofilm-related properties of the Shiga toxin-producing 2011 German outbreak Escherichia coli O104:H4

In 2011, nearly 4,000 people in Germany were infected by Shiga toxin (Stx)-producing Escherichia coli O104:H4 with > 22% of patients developing haemolytic uraemic syndrome (HUS). Genome sequencing showed the outbreak strain to be related to enteroaggregative E. coli (EAEC), suggesting its high virulence results from EAEC-typical strong adherence and biofilm formation combined to Stx production. Here, we report that the outbreak strain contains a novel diguanylate cyclase (DgcX)--producing the biofilm-promoting second messenger c-di-GMP--that shows higher expression than any other known E. coli diguanylate cyclase. Unlike closely related E. coli, the outbreak strain expresses the c-di-GMP-controlled biofilm regulator CsgD and amyloid curli fibres at 37°C, but is cellulose-negative. Moreover, it constantly generates derivatives with further increased and deregulated production of CsgD and curli. Since curli fibres are strongly proinflammatory, with cellulose counteracting this effect, high c-di-GMP and curli production by the outbreak O104:H4 strain may enhance not only adherence but may also contribute to inflammation, thereby facilitating entry of Stx into the bloodstream and to the kidneys where Stx causes HUS.

Enzymatic modifications of exopolysaccharides enhance bacterial persistence

Biofilms are surface-attached communities of bacterial cells embedded in a self-produced matrix that are found ubiquitously in nature. The biofilm matrix is composed of various extracellular polymeric substances, which confer advantages to the encapsulated bacteria by protecting them from eradication. The matrix composition varies between species and is dependent on the environmental niche that the bacteria inhabit. Exopolysaccharides (EPS) play a variety of important roles in biofilm formation in numerous bacterial species. The ability of bacteria to thrive in a broad range of environmental settings is reflected in part by the structural diversity of the EPS produced both within individual bacterial strains as well as by different species. This variability is achieved through polymerization of distinct sugar moieties into homo- or hetero-polymers, as well as post-polymerization modification of the polysaccharide. Specific enzymes that are unique to the production of each polymer can transfer or remove non-carbohydrate moieties, or in other cases, epimerize the sugar units. These modifications alter the physicochemical properties of the polymer, which in turn can affect bacterial pathogenicity, virulence, and environmental adaptability. Herein, we review the diversity of modifications that the EPS alginate, the Pel polysaccharide, Vibrio polysaccharide, cepacian, glycosaminoglycans, and poly-N-acetyl-glucosamine undergo during biosynthesis. These are EPS produced by human pathogenic bacteria for which studies have begun to unravel the effect modifications have on their physicochemical and biological properties. The biological advantages these polymer modifications confer to the bacteria that produce them will be discussed. The expanding list of identified modifications will allow future efforts to focus on linking these modifications to specific biosynthetic genes and biofilm phenotypes.

Quantitation of biofilm and planktonic life forms of coexisting periodontal species

BACKGROUND

Complexity of oral polymicrobial communities has prompted a need for developing in vitro models to study behavior of coexisting bacteria. Little knowledge is available of in vitro co-growth of several periodontitis-associated species without early colonizers of dental plaque.

THE AIM

was to determine temporal changes in the quantities of six periodontal species in an in vitro biofilm model in comparison with parallel planktonic cultures.

MATERIAL AND METHODS

Porphyromonas gingivalis, Aggregatibacter actinomycetemcomitans, Prevotella intermedia, Parvimonas micra, Campylobacter rectus and Fusobacterium nucleatum were anaerobically grown as multispecies and monospecies biofilms and parallel planktonic cultures using cell culture plates and microfuge tubes, respectively. After incubating 2, 4, 6, 8 days, biofilms and planktonic cultures were harvested, DNA extracted and the target species quantified using qPCR with species-specific 16S rDNA primers. Biofilm growth as monocultures was visualized at day 2 and 8 with confocal microscopy and crystal violet staining.

RESULTS

The six species were found throughout the test period in all culture conditions, except that P. gingivalis and F. nucleatum were not detected in multispecies planktonic cultures at day 8. In multispecies biofilm, P. gingivalis qPCR counts (cells/ml) increased (P<0.05) from day 2-8 and were then higher (P<0.05) than those of A. actinomycetemcomitans and C. rectus, whereas in monospecies biofilm, P. gingivalis counts were lower (P<0.05) than those of the other species, except A. actinomycetemcomitans. When multi- and monospecies biofilm cultures were compared, P. gingivalis counts were higher (P<0.05) but those of the other species, except P. intermedia, lower (P<0.05) in multispecies biofilm. Comparison between planktonic and biofilm cultures showed that A. actinomycetemcomitans, P. micra and C. rectus had higher (P<0.05) counts in planktonic cultures no matter whether grown in mono- or multispecies environment.

CONCLUSIONS

Six periodontal species were able to form multispecies biofilm up to 8 days in vitro without pioneer plaque bacteria. P. gingivalis seemed to prefer multispecies biofilm environment whereas P. micra and A. actinomycetemcomitans planktonic culture.

Role of exopolysaccharide in Aggregatibacter actinomycetemcomitans-induced bone resorption in a rat model for periodontal disease

Aggregatibacter actinomycetemcomitans a causative agent of periodontal disease in humans, forms biofilm on biotic and abiotic surfaces. A. actinomycetemcomitans biofilm is heterogeneous in nature and is composed of proteins, extracellular DNA and exopolysaccharide. To explore the role played by the exopolysaccharide in the colonization and disease progression, we employed genetic reduction approach using our rat model of A. actinomycetemcomitans-induced periodontitis. To this end, a genetically modified strain of A. actinomycetemcomitans lacking the pga operon was compared with the wild-type strain in the rat infection model. The parent and mutant strains were primarily evaluated for bone resorption and disease. Our study showed that colonization, bone resorption/disease and antibody response were all elevated in the wild-type fed rats. The bone resorption/disease caused by the pga mutant strain, lacking the exopolysaccharide, was significantly less (P < 0.05) than the bone resorption/disease caused by the wild-type strain. Further analysis of the expression levels of selected virulence genes through RT-PCR showed that the decrease in colonization, bone resorption and antibody titer in the absence of the exopolysaccharide might be due to attenuated levels of colonization genes, flp-1, apiA and aae in the mutant strain. This study demonstrates that the effect exerted by the exopolysaccharide in A. actinomycetemcomitans-induced bone resorption has hitherto not been recognized and underscores the role played by the exopolysaccharide in A. actinomycetemcomitans-induced disease.

Aggregatibacter actinomycetemcomitans biofilm can be inactivated by methylene blue-mediated photodynamic therapy

BACKGROUND

The purpose of this study was to evaluate the antibacterial effects of photodynamic action of methylene blue (MB) against Aggregatibacter actinomycetemcomitans organized on biofilm.

METHODS

After the biofilm growth in 96 flat-bottom well plate, the following groups were used: control group, untreated by either laser or photosensitizer (PS); MB group or dark toxicity group, which was exposed to MB alone (100μM) for 1min (pre-irradiation time); laser group, irradiated with laser for 5min in the absence of PS and three antimicrobial photodynamic inactivation (APDI) groups, with three exposure times of 1, 3 and 5min of irradiation, corresponding to fluences of 15, 45, and 75J/cm(2) respectively. The results were compared to the control group for statistical proposes. Scanning electronic microscope analysis was used to access structural changes in biofilm.

RESULTS

Red laser alone and MB alone were not able to inactivate bacterial biofilm. APDI groups showed differences when compared to the control group and they were dependent on the exposure time. No statistically significant differences were observed among the APDI groups at 1 and 3min of irradiation. On the other hand, 5min of APDI showed 99.85% of bacterial reduction (p=0.0004). In addition, the biofilm loose its structure following 5min APDI.

CONCLUSIONS

The results of this study suggest that A. actinomycetemcomitans biofilm can be inactivated by MB mediated APDI.

Surface adhesins and exopolymers of selected foodborne pathogens

The ability of bacteria to bind different compounds and to adhere to biotic and abiotic surfaces provides them with a range of advantages, such as colonization of various tissues, internalization, avoidance of an immune response, and survival and persistence in the environment. A variety of bacterial surface structures are involved in this process and these promote bacterial adhesion in a more or less specific manner. In this review, we will focus on those surface adhesins and exopolymers in selected foodborne pathogens that are involved mainly in primary adhesion. Their role in biofilm development will also be considered when appropriate. Both the clinical impact and the implications for food safety of such adhesion will be discussed.

Mini-review: Antimicrobial peptides and enzymes as promising candidates to functionalize biomaterial surfaces

Biomaterial-associated infections remain a serious concern in modern healthcare. The development of materials that can resist or prevent bacterial attachment constitutes a promising approach to dealing with this problem. Antimicrobial peptides (AMPs) and enzymes have been recognized as promising candidates for the new generation of antimicrobial surfaces. AMPs have been the focus of great interest in recent years owing to a low propensity for developing bacterial resistance, broad-spectrum activity, high efficacy at very low concentrations, target specificity, and synergistic action with classical antibiotics. Biofilm-dispersing enzymes have been shown to inhibit biofilm formation, detach established biofilm, and increase biofilm susceptibility to other antimicrobials. This review critically examines the potential of these protein-like compounds for developing antibacterial coatings by reporting their immobilization into different substrata using different immobilization strategies.

The pga gene cluster in Aggregatibacter actinomycetemcomitans is necessary for the development of natural competence in Ca(2+) -promoted biofilms

Natural competence is the ability of bacteria to incorporate extracellular DNA into their genomes. This competence is affected by a number of factors, including Ca(2+) utilization and biofilm formation. As bacteria can form thick biofilms in the presence of extracellular Ca(2+) , the additive effects of Ca(2+) -promoted biofilm formation on natural competence should be examined. We evaluated natural competence in Aggregatibacter actinomycetemcomitans, an important periodontal pathogen, in the context of Ca(2+) -promoted biofilms, and examined whether the pga gene cluster, required for bacterial cell aggregation, is necessary for competence development. The A. actinomycetemcomitans cells grown in the presence of 1 mm CaCl2 exhibited enhanced cell aggregation and increased levels of cell-associated Ca(2+) . Biofilm-derived cells grown in the presence of Ca(2+) exhibited the highest levels of natural transformation frequency and enhanced expression of the competence regulator gene, tfoX. Natural competence was enhanced by the additive effects of Ca(2+) -promoted biofilms, in which high levels of pga gene expression were also detected. Mutation of the pga gene cluster disrupted biofilm formation and competence development, suggesting that these genes play a critical role in the ability of A. actinomycetemcomitans to adapt to its natural environment. The Ca(2+) -promoted biofilms may enhance the ability of bacteria to acquire extracellular DNA.

Biofilm matrix-degrading enzymes

Polymeric substances such as proteins, polysaccharides, and DNA constitute a major component of the biofilm matrix. Enzymes that depolymerize and degrade these components are useful tools for investigating the composition and function of the biofilm matrix. This chapter provides a brief overview of the most commonly used biofilm matrix-degrading enzymes and presents examples of their applications in biofilm research.