Application of D-Amino Acids as Biofilm Dispersing Agent in Dental Unit Waterlines

The long-term effect of sub-boiling water on dental unit waterlines and its ability to control cross-contamination in dentistry

This study investigated the effect of water at high temperature on the physical and mechanical properties of polyurethane and on biofilm removal, aiming for its applicability in dental unit waterlines. The evaluations were carried out after simulating a 1-year period of daily immersion and measured changes in color, microhardness, surface roughness, and tensile strength before and after reproducing a disinfection protocol. For antibiofilm activity measurement, fragments of waterline were contaminated with Pseudomonas aeruginosa and submitted to the disinfection protocols. Relative to effects on the physical and mechanical properties, immersion in water at 60°C did not promote changes in color and tensile strength. However, lower values were observed for microhardness and increased values for surface roughness. Regarding antibiofilm action, water at 60°C significantly reduced the microbial load and promoted substantial changes in cells morphology. In conclusion, disinfection with water at 60°C demonstrated possible application in controlling cross-contamination in dentistry.

New insights into the inhibitory roles and mechanisms of D-amino acids in bacterial biofilms in medicine, industry, and agriculture

Biofilms are complex aggregates of microbes that are tightly protected by an extracellular matrix (ECM) and may attach to a surface or adhere together. A higher persistence of bacteria on biofilms makes them resistant not only to harsh conditions but also to various antibiotics which led to the emergence of problems in different applications. Recently, it has been discovered that many bacteria produce and release various D-amino acids (D-AAs) to inhibit biofilm formation, which made a great deal of interest in research into the control of bacterial biofilms in diverse fields, such as human health, industrial settings, and medical devices. D-AAs have various mechanisms to inhibit bacterial biofilms such as: (i) interfering with protein synthesis (ii) Inhibition of extracellular polymeric materials (EPS) productions (protein, eDNA, and polysaccharide) (iii) Inhibition of quorum sensing (autoinducers), and (iv) interfere with peptidoglycan synthesis, these various modes of action, enables these small molecules to inhibit both Gram-negative and Gram-positive bacterial biofilms. Since most biofilms are multi-species, D-AAs in combination with other antimicrobial agents are good choices to combat a variety of bacterial biofilms without displaying toxicity on human cells. This review article addressed the role of D-AAs in controlling several bacterial biofilms and described the possible or definite mechanisms involved in this process.

N-Acetyl-l-cysteine-Loaded Nanosystems as a Promising Therapeutic Approach Toward the Eradication of Pseudomonas aeruginosa Biofilms

Bacterial biofilms are a major health concern, mainly due to their contribution to increased bacterial resistance to well-known antibiotics. The conventional treatment of biofilms represents a challenge, and frequently, eradication is not achieved with long-lasting administration of antibiotics. In this context, the present work proposes an innovative therapeutic approach that is focused on the encapsulation of N-acetyl-l-cysteine (NAC) into lipid nanoparticles (LNPs) functionalized with d-amino acids to target and disrupt bacterial biofilms. The optimized formulations presented a mean hydrodynamic diameter around 200 nm, a low polydispersity index, and a high loading capacity. These formulations were stable under storage conditions up to 6 months. In vitro biocompatibility studies showed a low cytotoxicity effect in fibroblasts and a low hemolytic activity in human red blood cells. Nevertheless, unloaded LNPs showed a higher hemolytic potential than NAC-loaded LNPs, which suggests a safer profile of the latter. The in vitro antibiofilm efficacy of the developed formulations was tested against Staphylococcus epidermidis (Gram-positive) and Pseudomonas aeruginosa (Gram-negative) mature biofilms. The results showed that the NAC-loaded LNPs were ineffective against S. epidermidis biofilms, while a significant reduction of biofilm biomass and bacterial viability in P. aeruginosa biofilms were observed. In a more complex therapeutic approach, the LNPs were further combined with moxifloxacin, revealing a beneficial effect between the LNPs and the antibiotic against P. aeruginosa biofilms. Both alone and in combination with moxifloxacin, unloaded and NAC-loaded LNPs functionalized with d-amino acids showed a great potential to reduce bacterial viability, with no significant differences in the presence or absence of NAC. However, the presence of NAC in NAC-loaded functionalized LNPs shows a safer profile than the unloaded LNPs, which is beneficial for an in vivo application. Overall, the developed formulations present a potential therapeutic approach against P. aeruginosa biofilms, alone or in combination with antibiotics.

Molecular and Mechanistic Characterization of PddB, the First PLP-Independent 2,4-Diaminobutyric Acid Racemase Discovered in an Actinobacterial D-Amino Acid Homopolymer Biosynthesis

We recently disclosed that the biosynthesis of antiviral γ-poly-D-2,4-diaminobutyric acid (poly-D-Dab) in Streptoalloteichus hindustanus involves an unprecedented cofactor independent stereoinversion of Dab catalyzed by PddB, which shows weak homology to diaminopimelate epimerase (DapF). Enzymological properties and mechanistic details of this enzyme, however, had remained to be elucidated. Here, through a series of biochemical characterizations, structural modeling, and site-directed mutageneses, we fully illustrate the first Dab-specific PLP-independent racemase PddB and further provide an insight into its evolution. The activity of the recombinant PddB was shown to be optimal around pH 8.5, and its other fundamental properties resembled those of typical PLP-independent racemases/epimerases. The enzyme catalyzed Dab specific stereoinversion with a calculated equilibrium constant of nearly unity, demonstrating that the reaction catalyzed by PddB is indeed racemization. Its activity was inhibited upon incubation with sulfhydryl reagents, and the site-directed substitution of two putative catalytic Cys residues led to the abolishment of the activity. These observations provided critical evidence that PddB employs the thiolate-thiol pair to catalyze interconversion of Dab isomers. Despite the low levels of sequence similarity, a phylogenetic analysis of PddB indicated its particular relevance to DapF among PLP-independent racemases/epimerases. Secondary structure prediction and 3D structural modeling of PddB revealed its remarkable conformational analogy to DapF, which in turn allowed us to predict amino acid residues potentially responsible for the discrimination of structural difference between diaminopimelate and its specific substrate, Dab. Further, PddB homologs which seemed to be narrowly distributed only in actinobacterial kingdom were constantly encoded adjacent to the putative poly-D-Dab synthetase gene. These observations strongly suggested that PddB could have evolved from the primary metabolic DapF in order to organize the biosynthesis pathway for the particular secondary metabolite, poly-D-Dab. The present study is on the first molecular characterization of PLP-independent Dab racemase and provides insights that could contribute to further discovery of unprecedented PLP-independent racemases.

A Mixture of D-Amino Acids Enhances the Biocidal Efficacy of CMIT/MIT Against Corrosive Vibrio harveyi Biofilm

Biocides are widely used for the mitigation of microbial contamination, especially in the field of the aviation fuel industry. However, the long-term use of biocide has raised the concerns regarding the environmental contamination and microbial drug resistance. In this study, the effect of a mixture of D-amino acids (D-tyrosine and D-methionine) on the enhancement of the bactericidal effect of 5-Chloro-2-Methyl-4-isothiazolin-3-one/2-Methyl-2H-isothiazole-3-one (CMIT/MIT) against corrosive Vibrio harveyi biofilm was evaluated. The results revealed that D-Tyr and D-Met alone can enhance the biocidal efficacy of CMIT/MIT, while the treatment of 5 ppm CMIT/MIT, 1 ppm D-Tyr and 100 ppm D-Met showed the best efficacy comparable to that of 25 ppm CMIT/MIT alone. The triple combination treatment successfully prevented the establishment of the corrosive V. harveyi biofilm and effectively removed the mature V. harveyi biofilm. These conclusions were confirmed by the results of sessile cell counts, images obtained by scanning electron microscope and confocal laser scanning microscope, and the ATP test kit.

Multimodal Role of Amino Acids in Microbial Control and Drug Development

Amino acids are ubiquitous vital biomolecules found in all kinds of living organisms including those in the microbial world. They are utilised as nutrients and control many biological functions in microorganisms such as cell division, cell wall formation, cell growth and metabolism, intermicrobial communication (quorum sensing), and microbial-host interactions. Amino acids in the form of enzymes also play a key role in enabling microbes to resist antimicrobial drugs. Antimicrobial resistance (AMR) and microbial biofilms are posing a great threat to the world's human and animal population and are of prime concern to scientists and medical professionals. Although amino acids play an important role in the development of microbial resistance, they also offer a solution to the very same problem i.e., amino acids have been used to develop antimicrobial peptides as they are highly effective and less prone to microbial resistance. Other important applications of amino acids include their role as anti-biofilm agents, drug excipients, drug solubility enhancers, and drug adjuvants. This review aims to explore the emerging paradigm of amino acids as potential therapeutic moieties.

Controlling biofilms using synthetic biology approaches

Bacterial biofilms are formed by the complex but ordered regulation of intra- or inter-cellular communication, environmentally responsive gene expression, and secretion of extracellular polymeric substances. Given the robust nature of biofilms due to the non-growing nature of biofilm bacteria and the physical barrier provided by the extracellular matrix, eradicating biofilms is a very difficult task to accomplish with conventional antibiotic or disinfectant treatments. Synthetic biology holds substantial promise for controlling biofilms by improving and expanding existing biological tools, introducing novel functions to the system, and re-conceptualizing gene regulation. This review summarizes synthetic biology approaches used to eradicate biofilms via protein engineering of biofilm-related enzymes, utilization of synthetic genetic circuits, and the development of functional living agents. Synthetic biology also enables beneficial applications of biofilms through the production of biomaterials and patterning biofilms with specific temporal and spatial structures. Advances in synthetic biology will add novel biofilm functionalities for future therapeutic, biomanufacturing, and environmental applications.

d-Amino Acids and Lactic Acid Bacteria

Proteins are composed of l-amino acids except for glycine, which bears no asymmetric carbon atom. Accordingly, researchers have studied the function and metabolism of l-amino acids in living organisms but have paid less attention to the presence and roles of their d-enantiomers. However, with the recent developments in analytical techniques, the presence of various d-amino acids in the cells of various organisms and the importance of their roles have been revealed. For example, d-serine (d-Ser) and d-aspartate (d-Asp) act as neurotransmitters and hormone-like substances, respectively, in humans, whereas some kinds of d-amino acids act as a biofilm disassembly factor in bacteria. Interestingly, lactic acid bacteria produce various kinds of d-amino acids during fermentation, and many d-amino acids taste sweet, compared with the corresponding l-enantiomers. The influence of d-amino acids on human health and beauty has been reported in recent years. These facts suggest that the d-amino acids produced by lactic acid bacteria are important in terms of the taste and function of lactic-acid-fermented foods. Against this background, unique d-amino-acid-metabolizing enzymes have been searched for and observed in lactic acid bacteria. This review summarizes and introduces the importance of various d-amino acids in this regard.

Plasma Sterilization Effectively Reduces Bacterial Contamination in Dental Unit Waterlines

Objective

To investigate the effectiveness of plasma sterilization in reducing bacterial contamination and controlling biofilms in dental unit waterlines.

Materials and Methods

Ten identical dental chair units (DCUs) were used. Five DCUs were installed with an automated plasma sterilization system (PSS) and the other five were kept as nontreated controls (CTL). Water flushed from the airotor line served as the output water of the dental unit waterlines (DUWLs). Water samples were collected at the beginning and on a weekly basis for 4 months. Water was analyzed for bacterial contamination (CFU/mL). Scanning electron microscopy (SEM) was used to investigate the amount of biofilm in the waterlines. Biofilm viability was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assays. All statistical analyses were performed using the Mann-Whitney U test. A value of p < 0.05 was considered significant.

Results

The DCU output water was found to be heavily contaminated with bacteria. Plasma sterilization effectively reduced bacterial contamination from an average of 212 CFU/mL to 8 CFU/mL. During the entire period of 4 months, the level remained below 500 CFU/mL, the standard level recommended by the Centers for Disease Control and Prevention (CDC) of the USA. The reduction in the bacterial count was significant compared with the CTL group (p < 0.05). Plasma sterilization could not eradicate the existing biofilms in the waterlines, and it did reduce biofilm mass and viability. Moreover, treatment with plasma sterilization did not induce a change in the composition of microorganisms, as analyzed by Gram staining.

Conclusion

Plasma sterilization, which is part of electrochemically activated water, effectively reduces bacterial contamination and reduces biofilms in dental unit waterlines.

Comparison between Two Types of Dental Unit Waterlines: How Evaluation of Microbiological Contamination Can Support Risk Containment

Infection risk management in a dental unit waterline (DUWL) involves healthcare personnel and patients and is related to routine exposure to water and aerosols that may contain bacterial species. To improve water safety plans, maintenance, and sanitation procedures, analyses of heterotrophic plate counts (HPCs) at 36 °C, and two other microorganisms frequently associated with biofilms, Pseudomonas aeruginosa and Legionella spp., were performed in order to evaluate differences in microbiological contamination between two types of DUWLs: Type A, provided by a water tank, and Type B, directly connected to municipal water. The data showed that the water supply and water safety plan differentially influenced microbiological contamination: Type A DUWLs were more contaminated than Type B DUWLs for all microbiological parameters tested, with significant changes in the percentage of positive samples and contamination levels that were beyond the limits of standard guidelines. The results obtained show how the storage tank, the absence of anti-retraction valves, and the disinfection procedures performed are the main critical points of Type A DUWLs, which confirms that dental unit management (maintenance/sanitization) is often missed or not correctly applied by stakeholders, with an underestimation of the real risk of infection for patients and operators.