Article Cite This: Acc. Chem. Res. 2018, 51, 789−799
pubs.acs.org/accounts
Enzyme Mimicry for Combating Bacteria and Biofilms Zhaowei Chen,†,‡,¶ Zhenzhen Wang,†,‡,¶ Jinsong Ren,† and Xiaogang Qu*,† †
Laboratory of Chemical Biology and State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P.R. China ‡ Graduate School, University of Chinese Academy of Sciences, Beijing 100039, P.R. China S Supporting Information *
CONSPECTUS: Bacterial infection continues to be a growing global health problem with the most widely accepted treatment paradigms restricted to antibiotics. However, antibiotics overuse and misuse have triggered increased multidrug resistance, frustrating the therapeutic outcomes and leading to higher mortalities. Even worse, the tendency of bacteria to form biofilms on living and nonliving surfaces further increases the difficulty in confronting bacteria because the extracellular matrix can act as a robust barrier to prevent the penetration of antibiotics and resist environmental stress. As a result, the inability to completely eliminate bacteria and biofilms often leads to persistent infection, implant failure, and device damage. Therefore, it is of paramount importance to develop alternative antimicrobial agents while avoiding the generation of bacterial resistance. Taking lessons from natural enzymes for destroying cellular structural integrity or interfering with metabolisms such as proliferation, quorum sensing, and programmed death, the construction of artificial enzymes to mimic the enzyme functions will provide unprecedented opportunities for combating bacteria. Moreover, compared to natural enzymes, artificial enzymes possess much higher stability against stringent conditions, easier tunable catalytic activity, and large-scale production for practical use. In this Account, we will focus on our recent progress in the design and synthesis of artificial enzymes as a new generation of “antibiotics”, which have been demonstrated as promising applications in planktonic bacteria inactivation, wound/lung disinfection, as well as biofilm inhibition and dispersion. First, we will introduce direct utilization of the intrinsic catalytic activities of artificial enzymes without dangerous chemical auxiliaries for killing bacteria under mild conditions. Second, to avoid the toxicity caused by overdose of H2O2 in conventional disinfections, we leveraged artificial enzymes with peroxidase-mimic activities to catalyze the generation of hydroxyl radicals at low H2O2 levels while achieving efficient antibacterial outcomes. Importantly, the feasibility of these artificial enzymes was further demonstrated in vivo by mitigating mice wound and lung disinfection. Third, by combining artificial enzymes with stimuli-responsive materials, smart on-demand therapeutic modalities were constructed for thwarting bacteria in a controllable manner. For instance, a photoswitchable “Band-Aid”-like hydrogel doped with artificial enzymes was developed for efficiently killing bacteria without compromising mammal cell proliferation, which was promising for accelerating wound healing. Lastly, regarding the key roles that extracellular DNAs (eDNAs) play in maintaining biofilm integrity, we further designed a multinuclear metal complex-based DNase-mimetic artificial enzyme toward cleaving the eDNA for inhibiting biofilm formation and dispersing the established biofilms. We expect that our rational designs would boost the development of artificial enzymes with different formulations as novel antibacterial agents for clinical and industrial applications.
1. INTRODUCTION Since the discovery of antibiotics, they were believed to be the terminator of bacterial infections. Unfortunately, because of their extensive use, the continuing emergence and rapid spread of antibiotic-resistant bacterial strains have been one of the most serious threats to human health worldwide. Antibacterial resistance is currently implicated in 700,000 deaths each year, and it is predicted that, unless action is taken, the number of deaths per year will spiral to 10 million by 2050 and the cost will balloon to $100 trillion.1 Moreover, bacteria can form biofilms by shielding themselves in a self-synthesized extracellular matrix on either biological or inert surfaces, which further undermines the efficiency of conventional © 2018 American Chemical Society
antibiotics, which generally requires 100- to 1000-fold higher concentration of the antibiotics that can eradicate planktonic ones.2,3 The difficulty in fully defeating biofilms has caused significant problems in various areas including persistent infection, biomedical implants and devices, industrial settings, marine equipment, and food packaging.4 To this end, great efforts have been devoted to developing alternative antibacterial compounds and materials in recent years.5−8 Particularly, instead of modifying currently marketed antibiotics or synthesizing new antibiotics, which may sooner or Received: January 5, 2018 Published: February 28, 2018 789
DOI: 10.1021/acs.accounts.8b00011 Acc. Chem. Res. 2018, 51, 789−799
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Figure 1. Schematic illustration of leveraging enzyme mimicry for combating bacteria and biofilms. NP: nanoparticle; GQD: graphene quantum dot.
Figure 2. (A) Transmission electron microscopy (TEM) images of the MSN-AuNPs. (B) Oxidase-mimic activity of the MSN-AuNPs toward oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in different reaction solutions containing (1) TMB, (2) TMB + MSN-AuNPs, and (3) TMB + MSN in PB buffer (25 mM, pH 4.0) at 35 °C after 30 min incubation. Scanning electron microscopy (SEM) images of (C) E. coli and (D) S. aureus after treatment of MSN-AuNPs. Reprinted and modified with permission from ref 25. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.
H2O2 in the presence of oxygen.9 In the human immune system, after phagocytosis of bacteria by neutrophils, myeloperoxidase from azurophil granules can catalyze the conversion of H2O2 into highly reactive oxygen species (ROS) to attack the membranes of microbes.9 Marine algae that are often threatened by microbial fouling have evolved molecular
later raise new resistances, researchers shifted their attention to drawing inspiration from natural self-defense systems that employ enzymes to induce irreversible damage toward bacteria or disrupt biofilm integrity. For example, xanthine oxidase in breastmilk can bestow antibacterial protection to neonatal gut by generating mild bactericidal superoxide anion (O2•−) and 790
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Figure 3. (A) Schematic illustration of the GQD-assisted antibacterial system. (B) Tracking the generation of •OH catalyzed by GQDs with TA. (C) Survival rate of bacteria after treatment with different concentrations of H2O2 with or without GQDs (100 μg/mL). The error is the standard deviation from the mean (n = 3). (D) Photographs of the fabricated GQD-Band-Aid and its application for in vivo wound disinfection. (E) Photographs showing the wounds treated with H2O2 + GQD-Band-Aids (G + H), GQD-Band-Aids (G), H2O2 + blank Band-Aids (H), and saline + blank Band-Aids (blank). Reprinted with permission from ref 28. Copyright 2014, American Chemical Society.
nanoparticles, cationic polymeric compounds, graphene/ graphene oxide, and other carbon materials, which can inhibit bacterial growth by physically damaging cellular structures or inducing the intracellular accumulation of ROS.14−16 However, it should be mentioned that the unacceptable toxicity of these materials upon exposure to human cells and the environment has questioned their future utility. For developing friendly formulations, catalytically active nanomaterials have been adopted as promising alternatives for combating bacteria in a mild manner.17,18 A typical example is the application of photoactive semiconductor-based nanomaterials for inactivating a wide range of bacteria even including multidrug-resistant species via light-triggered oxidative stress.19,20 Notably, the requirement of either UV or visible light to activate the photodynamic process increases the complexity and cost of such treatment. Inspired by the fact that most reactions in biological systems are catalyzed by enzymes, bare bimetallic AuPt nanoparticles with a Pt content in the range of 10−65%, which happens to exert the best catalytic activity in various chemical reactions, were found to exhibit significant activities in inhibiting the growth of both laboratory standard and clinical multidrug-
mechanisms to secrete enzymes such as vanadium haloperoxidases (V-HPOs) to inactivate intercellular communication.10 Given that the practical utilization of natural enzymes always faces issues of high cost in preparation and purification, longterm stability, and specific reaction conditions (e.g., pH value, ionic strength, and temperature),11 the leverage of materials with enzyme-mimic functionalities (also called artificial enzymes) would provide sustainable, environmentally friendly, and cost-effective agents for antibacterial applications. Moreover, the progressive development of nanotechnology and material science has shown great success in enriching the formulations of artificial enzymes,12,13 which are promising for advanced therapeutic interventions. In this Account, we will focus on recent endeavors toward developing artificial enzymes for applications in killing bacteria, wound disinfection and healing, and eradicating biofilms (Figure 1).
2. CATALYTICALLY ACTIVE MATERIALS WITHOUT ADDING CHEMICAL AUXILIARIES The rise of antibiotic-resistant pathogens has sparked great interest in the development of materials with intrinsic antibacterial potencies; these include metal or metal oxide 791
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Figure 4. (A) Schematic illustration of g-C3N4@AuNP peroxidase mimicry to activate biologically safe H2O2 for bacteria killing. (B) Assessment of the peroxidase activity of (1) control, (2) AuNPs, (3) g-C3N4, and (4) g-C3N4@AuNPs by using TMB as the substrate. Inset shows the corresponding photograph of 1−4. (C) Schematic illustration of the construction and treatment of the acute lung infection model. (D) Histological changes at 96 h after the different treatments: (1) PBS, (2) vancomycin, (3) g-C3N4@AuNPs, and (4) positive control (no lung infection). (E) Remaining number of bacteria in the lungs of mice with different treatments. The error is the standard deviation from the mean (n = 3). Reprinted and modified with permission from ref 36. Copyright 2016, Elsevier Ltd.
resistant strains.21 The antibacterial activity originated from the collapse of microbial membrane potential and elevation in intracellular ATP levels, which differed from the traditional metal agent-based antibacterial mechanism that involves ROS. The increase in ATP concentration was possibly due to the fact that the AuPt either acted as an alternative “enzyme” that catalyzed the generation of ATP or inhibited the synthesis of proteins that depleted ATP. As surface properties greatly affect the catalytic properties of nanomaterials, Au nanoparticles (AuNPs) with different surface modifications have been found to exhibit glucose oxidase-, peroxidase-, superoxide dismutase-, and catalase-mimic activities, respectively.22−24 Along this line, our group discovered that AuNPs immobilized on biofunctionalized mesoporous silica (amino groups on inner channels and carboxyl groups on the external surface, designated MSN-AuNPs) exhibited intrinsic oxidase-mimic activity (Figure 2A, B), which could be directly utilized as antibacterial agents under mild conditions.25 Intriguingly, the MSN-AuNPs exhibited high oxidase-like activity and stability over broad pH, temperature, and salt concentration ranges, which were important for antibacterial application under harsh conditions. Further experiments revealed that single oxygen (1O2), O2•−, and hydroxyl radical (•OH) were the main active intermediates generated by MSN-AuNPs. With this unique property, significant germicidal activity was observed against both
Gram-negative E. coli and Gram-positive S. aureus. A morphological study indicated that the bacterial surface became wrinkled after treatment with MSN-AuNPs (Figure 2C, D), which was a result of lipid membrane oxidation induced by the generated ROS. Very recently, several other materials with rationally designed structures such as octahedral dealloyed Pt/Ag nanoparticles and GQD/silver nanoparticle hybrids with strong oxygen reduction activity were also found to inhibit bacterial growth in a similar biomimetic way (Figure 1).26,27 Development of catalytic materials with enzyme-like activities that could directly inhibit bacterial proliferation in a noninvasive manner would provide a new avenue for preventing bacterial infections.
3. ENZYME MIMICRY-ASSISTED ANTIBACTERIAL THERAPEUTICS As a common and commercially available ROS, H2O2 has long been used as a medical cleaning and wound disinfection agent. Generally, H2O2 acts by inducing oxidation of biomolecules and hence affecting the function of bacteria, but the process is rather slow.9 As a result, high concentration of H2O2 (0.5−3%, approximately 166 mM to 1 M) is typically required to achieve desired antibacterial effects, which, however, also causes damage to normal tissues and even delays wound healing.28 Recently, the discovery that many bactericidal antibiotics share 792
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Figure 5. (A) Schematic illustration of the preparation of the “on demand” antibacterial platform. (B) Bioresponsive process of the platform for in situ generation of •OH on the bacteria surface. (C) Fluorescence spectra of TA in different solutions to demonstrate the generation •OH from AA catalyzed by MNPs. Survival percentage of (D) S. aureus and (E) E. coli treated with AA@GS@HA-MNPs and near-infrared light irradiation with GS@HA-MNPs and AA@GS@HA-MNPs. Data are expressed as mean ± SD (n = 3). Reprinted and modified with permission from ref 42. Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.
μM) formed scabs, and no erythema or edema was observed during the whole processes (Figure 3E). Encouraged by these results, we then embarked on the development of artificial enzymes that could utilize H2O2 at biologically relevant concentrations (50−100 μM). A hybrid nanozyme composed of AuNPs and ultrathin graphitic carbon nitride (designated g-C3N4@AuNPs) was synthesized, and its ability in facilitating the treatment of bacterial infections was tested (Figure 4A).36 Systematic assessment of the peroxidaselike activity indicated that g-C3N4@AuNPs showed much higher activity than those of g-C3N4, AuNPs, and their physical mixtures (Figure 4B). This could be attributed to the positive synergistic coupling effect at the interface between AuNPs and g-C3N4, which had also been discovered with other AuNPbased hybrid artificial enzymes.37 On the basis of the high activity of g-C3N4@AuNPs, ∼80% decrease in growth of both drug-resistant Gram-positive and Gram-negative bacteria was achieved with 20 μg/mL of g-C3N4@AuNPs and 100 μM H2O2. Compared to GQDs, 10-times lower H2O2 (10 μM) was required for g-C3N4@AuNPs to prevent wound infection. Moreover, by using acute lung infection caused by multidrugresistant S. aureus as a model, g-C3N4@AuNPs could effectively alleviate the inflammation reaction with the endogenous H2O2, which was generally very high at inflammation sites (Figure 4C, D). The remaining number of bacteria was comparable with that in mice treated with the current clinical option vancomycin (Figure 4E). More importantly, the g-C3N4@AuNPs showed no significant systemic toxicity, as proven by the negligible changes in body weight and lack of obvious histological abnormalities or inflammation lesions. Different from oxidase mimicry in section 2, which leverages oxygen to generate bactericidal ROS, the peroxidase-mimic materials in this part catalyze the conversion of externally added or endogenous H2O2.
a common bacterial killing mechanism involving •OH formation via Fenton reaction and generation of secondary ROS by the natural defense system has spurred efforts to leverage materials such as Fe3O4 and V2O5 with peroxidase-/ haloperoxidase-mimic activities to potentiate the antibacterial efficiency of H2O2.9,29,30 However, concerns regarding the cytotoxicity induced by aberrantly accumulated iron and the carcinogenic effect of vanadium compounds prompt research for developing more biocompatible enzyme mimicries.31,32 Having discovered the peroxidase-mimic activity of GO,33 we moved on to study its low dimensional counterpart-graphene quantum dots (GQDs).34 Owing to the excellent electron transport property, GQDs display much higher enzymatic activity than that of GO.34 Moreover, GQDs have been demonstrated to show no apparent toxicity either in vitro or in vivo.35 In light of these advantages, we proposed improving the antibacterial efficiency of H2O2 with the assistance of GQDs (Figure 3A).28 By using terephthalic acid (TA) as a tracking probe, we found that the peroxidase-like activity of GQDs stemmed from its ability to convert H2O2 into •OH radicals (Figure 3B). This is quite suitable for antibacterial application because •OH is more acute and cannot be deactivated by natural enzymes.9 As a result, the marriage of GQDs and H2O2 rendered remarkably improved antibacterial outcomes against both E. coli and S. aureus (Figure 3C) and reduced the survival of bacteria below 10%, whereas the concentration of H2O2 needed was 100-times lower than that in cases where H2O2 was used alone, for example. Moreover, the adherence of GQDs to the cellular membrane allowed the generation of •OH in situ, which also contributed to the system’s strong bactericidal activity. To further demonstrate the versatility of the system, we designed a new kind of Band-Aid with GQDs for preventing wound infection (Figure 3D). After 72 h of treatment, the wounds in mice treated with GQD-Band-Aids plus H2O2 (100 793
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Figure 6. (A) Schematic illustration of the construction of a light-controlled “Band-Aid”-like hydrogel containing multiple enzyme-mimic CeO2 nanoparticles for programmable infected wound healing. (B) Photoresponsive processes inside the hydrogels. (C) Optical density at 600 nm (OD600 nm) of the bacterial suspension after incubating with different hydrogels under different conditions for 4 h. Data are expressed as mean ± SD (n = 3). (D) Fluorescence microscopy images of 2H11 cells (SV40 transferred murine endothelial cells) that are control (a) and incubated with the suspension of bacteria (b) and MGCB-GO-CeO2 (c) and MGCB-GO (d) containing hydrogel treated bacteria; the mixture was treated with ultraviolet light irradiation before transfer. Reprinted and modified with permission from ref 45. Copyright 2017, American Chemical Society.
4. SMART MULTIFUNCTIONAL ANTIBACTERIAL THERAPEUTIC MODALITIES
oxide-iron oxide hybrid nanocomposites for combating multidrug-resistant S. aureus in subcutaneous abscesses.41 Rather than just simply integrating several functions together, our group put forward different designs from the perspective of controlling artificial enzyme reactions to “on demand” antibacterial aims. Considering the difficulty in direct delivery of •OH to infection sites, we designed a bioresponsive platform to control the contact between peroxidase-mimic artificial enzyme and its substrate and thereby stimulated the generation of •OH in situ on the surface of bacteria (Figure 5).42 By sequentially loading ascorbic acid (AA) inside sandwich-like graphene-mesoporous silica nanosheets, coating hyaluronic acid (HA) on the mesopore surface, and conjugating vancomycinmodified ferromagnetic nanoparticles (MNPs), a smart antibacterial system was built (designated AA@GS@HAMNPs, Figure 5A). Upon accumulation at the infection site with the assistance of vancomycin, which recognized the terminal dipeptide D-alanyl-D-alanine moieties on the bacterial cell wall,39 the capping HA was degraded by the bacteriasecreted hyaluronidase, leading to the release of the packaged
Incorporation of multiple functionalities into one single system has presented considerable appeal in overcoming the low efficiency of the individual treatment, minimizing adverse risks and especially achieving a synergistic effect in cancer therapy.38 Recently, such strategies have been extended to the field of fighting bacteria and offered additional possibilities for improving the activity and selectivity of bactericidal agents.39−41 For instance, on the basis of the inherent photothermal capacity and peroxidase-like activity of molybdenum disulfide nanoflowers, a synergistic antibacterial system capable of heat and •OH generation was built.40 The generated •OH could destroy the microbial membrane and enhance the sensitivity and permeability of bacteria to heat, which would help to avoid the toxicity of a high concentration of H2O2 and overcome the requirement of high temperature that could destroy bacteria but cause damage to surrounding healthy tissues. A similar synergistic effect was also observed with reduced graphene 794
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Figure 7. (A) Schematic illustration of synthesis DMAE and application for combating S. aureus biofilms. (B) TEM elemental mappings of the DMAE. 3D confocal microscopy images: (C) biofilms formed on bare and DNase I- and DMAE-modified surfaces after different incubation times, and (D) biofilms of different ages after treatment with DNase I and DMAE. (E) Orthogonal views of 120 h old biofilms treated with rhodamine (red)-labeled DNase and DMAE for 3 h. Bacteria were stained by Calcein-AM (green). The size of each confocal microcopy image is 639.5 × 639.5 μm. Reprinted and modified with permission from ref 55. Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.
mimicry and produce antibacterial ROS. Meanwhile, nearinfrared light irradiation had a positive effect on the antibacterial performance because increasing temperature could improve the catalytic activity (Figure 6C). After further treating the mixture containing the hydrogel and the few surviving bacteria with ultraviolet light and transferring it to incubate with mammalian cells, the cells surprisingly showed high density and normal morphology (Figure 6D). This was because the pH increasing inside the hydrogel induced by the release of OH− from MGCB could enable CeO2 to act as an antioxidant, which helped to promote cell growth and proliferation.51 With such unique advantages, this new kind of “Band-Aid” could help infected wounds to proceed via a timely and orderly healing pathway.
AA. AA has been widely utilized as a moderate antibacterial and anticancer agent as it has been demonstrated to act as a prodrug of H2O2.43,44 In this way, the attached peroxidase-like MNPs on the cell wall enabled the on-site conversion of AA into strong bactericidal •OH (Figure 5B, C). Moreover, the photothermal property of graphene further allowed for synergistic chemohyperthermia antibacterial outcomes, from which just 2.6 and 1.7 mg mL−1 AA could inhibit the survival percentage of E. coli and S. aureus, respectively, below 10% (Figure 5D, E). This bioresponsive design showed negligible cytotoxicity toward human cells due to the lack of factors inducing the premature release of AA. As a further evolution, we constructed a programmable system that could not only fight bacteria in the first phase but also promote cell proliferation in the second phase (Figure 6A),45 which agreed well with the ideal process for healing infected wounds.46 The key factor in this design is the leverage of multifaceted CeO2 nanoparticles that can exhibit oxidase-like activity at acidic pH while acting as superoxide dismutase and catalase mimicry under neutral/basic conditions.47−49 By introducing CeO2 nanoparticles into a “Band-Aid”-like hydrogel that was doped with a photothermal agent (graphene oxide) and a photobase reagent (malachite green carbinol base, MGCB),50 the oxidant and antioxidant functions of CeO2 could be readily switched and tuned in response to external light irradiation (Figure 6B). Before irradiation with ultraviolet light, the MGCB-GO-CeO2-containing hydrogel exhibited excellent antibacterial efficiency because the initial acidic microenvironment inside the hydrogel enabled CeO2 to behave as oxidase
5. COMBATING BIOFILMS WITH ARTIFICIAL ENZYMES Bacteria can form biofilms, and the formed biofilms have their own defense and communication systems, which in turn makes the flocculated bacteria harder to remove than solitary ones.4 Recent advances in understanding of the molecular basis of biofilm formation provide clues for developing therapeutic targets that can inhibit the formation or induce the detachment of biofilms. In this sense, artificial enzymes have been proposed to combat biofilms mainly from the following three perspectives. 795
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Figure 8. (A) Examples of autoinducers. (B) Bromination of 3-oxo-acyl homoserine lactone by HOBr and the subsequent hydrolysis.
5.1. Cleavage of eDNA
inducers, such as N-acyl homoserine lactones (AHL) and furanosyl compounds (Figure 8A).10 Disruption of QS by either quenching autoinducers or using antagonists has been suggested as an attractive strategy for manipulating biofilms. Inspired by the antifouling mechanism of certain seaweeds that utilize V-HPOs to generate hypohalous acids to halogenate AHL (Figure 8B), fluorite-type CeO2−x nanorods with haloperoxidase-mimic activity have recently been shown to reproduce such a scenario when used as paint additives in both laboratory-simulated and natural marine environment conditions.31 Although the V2O5 nanowire was also reported to exert similar enzymatic functions, its antibiofouling activity was mainly attributed to the oxidative effect of the produced HOBr and 1O2.30 Maybe both mechanisms exist simultaneously in these two cases.
Extracellular matrixes (ECM) are primarily composed of exopolysaccharides, proteins, and eDNA, all of which complementarily provide a strong but flexible three-dimensional architecture to keep the encased bacteria in close proximity and protect them from external assaults.2 Being the longest polymer in ECM, eDNA functions as both a cell-to-cell interconnector and a bridge associating bacteria with substrate surfaces or other ECM components.52 Cleavage of eDNA by DNase has shown certain potency in combating biofilms.52,53 However, its broad application faces two critical issues: (1) lack of long-term activity for prohibiting the formation of new generated biofilms, and (2) limited penetration depth and stability in dispersing established biofilms. On the basis of the progress in synthetic metallonucleases,54 our group designed a DNase-mimetic artificial enzyme (DMAE) by confining cerium(IV) complex-passivated AuNPs on the surface of Fe3O4/SiO2 core/shell particles to overcome these two challenges (Figure 7A, B).55 First, by conjugating DMAE and DNase onto substrate surfaces, less than 10% of bacterial attachment was observed in both cases relative to that of bare surfaces after 1 h incubation. During the following incubation, the biofilm formed on DMAE-modified surfaces remained rather low over 120 h, whereas the biofilm thickness on DNase-conjugated surfaces remained low only up to 24 h (Figure 7C). The longer biofilm inhibition capacity of DMAE could be attributed to its intrinsic stability, which would help avoid the continuous addition of fresh DNase. Second, to disintegrate preformed biofilms, DMAE showed high efficiency toward biofilms with ages from 12 to 120 h, whereas DNase only worked with young biofilms (Figure 7D). Orthogonal views from a confocal microcopy study indicated that DMAE penetrated much deeper than DNase (Figure 7E). Moreover, the DMAE could be easily recovered with a magnet and reused for at least 5 runs, which was almost impossible for natural DNase. Considering the complexity of ECM, other artificial enzymes trained to degrade different components are also possible.
5.3. Destroy Whole Biofilm
Although the two approaches presented above hold promising potential in biofilm inhibition or dispersion, the detached bacteria can still produce virulence or develop new biofilms on unprotected surfaces. Thus, ideally, we should thoroughly eliminate the whole biofilm. By virtue of the strong oxidative activity and nonselectivity of •OH toward any biomolecules (e.g., DNA, lipids, proteins), enzyme mimicry with oxidase/ peroxidase-like activity has been shown to be competent for this task. For instance, the GQDs and g-C3N4@AuNPs-H2O2 systems in section 3 were able to destroy biofilms and kill the embedded bacteria simultaneously. Similar results were also achieved with MNPs in treating experimental and dental biofilms.57,58 In addition, the prodrug strategy with AA@GS@ HA-MNPs in section 4 could effectively disperse preformed biofilms, inactivate the shielded bacteria, and prevent the development of new biofilms both in vitro and in vivo. Besides peroxidase mimicry, other artificial enzymes that can generate highly ROS such as hypohalous acid, 1O2, and nitric oxide may also completely destroy biofilms.
6. SUMMARY AND OUTLOOK In this Account, we summarized the recent achievements of utilizing enzyme mimicry for antibacterial/antibiofilm applications (Table S1). Increasing understanding of the mechanisms of the natural self-defense system as well as the molecular basis of bacterial group behaviors bestow valuable information for
5.2. Disrupting Quorum Sensing (QS)
QS is a cell-to-cell communication process that enables bacteria to orchestrate group behaviors at all stages of biofilm development.56 QS relies on chemical signals called auto796
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Accounts of Chemical Research developing enzyme mimicry to deactivate bacteria, disable their communication, and disrupt the entire aggregates. This is different from conventional studies relying on synthesizing new or modifying existing antibiotics. Importantly, opposed to the vulnerability of natural enzymes, the higher stability of artificial enzymes enables combating bacteria/biofilms under harsh and complicated environmental conditions. Moreover, the rapid development and versatile features of artificial enzymes provides us a rich toolbox to design tailor-made multifunctional therapeutic modalities. Although notable progress has been made in developing enzyme mimicry for combating bacteria/biofilms, this field is young and faces many issues and challenges. (1) The exact antibacterial/antibiofilm mechanism of each artificial enzyme is still not fully understood. Gathering clear information would inspire more efficient and even bacteria-specific therapeutics. (2) The final fate of enzyme mimicry covering potential toxicity toward human beings, other biological systems, and the environment should be systematically examined before translation for biomedical or industrial applications. (3) The potent acquisition of resistance against artificial enzyme-based “antibiotics” deserves serious investigation. (4) Proper standards and certain general guidelines should be considered for performing antibacterial/antibiofilm tests with artificial enzymes, and reliable analytical techniques are also required for evaluating the therapeutic outcomes. Because this field is still expanding, we hope that these concerns will be addressed in the near future. We believe that combating bacteria/biofilm with enzyme mimicry will bring this field to a new era.
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Zhenzhen Wang received her B.Sc. degree at Henan Normal University in 2012. She then joined to the Changchun Institute of Applied Chemistry as a Ph.D. candidate, majoring in Chemical Biology. Her current scientific research is focused on constructing functional nanomaterials for on-demand medical therapeutics. Jinsong Ren received her B.Sc. degree at Nanjing University in 1990 and Ph.D. from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 1995. From 1996 to 2002, she worked at the School of Medicine, UMMC and Department of Chemistry and Chemical Engineering, California Institute of Technology. In 2002, she took a position as a principal investigator at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Her current research is mainly focused on drug screening and DNA-based nanofunctional materials. Xiaogang Qu received his Ph.D. from the Chinese Academy of Sciences (CAS) in 1995 with the President’s Award of CAS. He moved to the USA afterwards and worked with Professor J. B. Chaires at the Mississippi Medical Center and Nobel Laureate Professor Ahmed H. Zewail at the California Institute of Technology. Since late 2002, he has been a professor at the Changchun Institute of Applied Chemistry, CAS. From 12/2006 to 05/2007, he visited the group of Nobel Laureate Professor Alan J. Heeger at UCSB. His current research is focused on ligand−nucleic acids or related protein interactions and biofunctional materials for advanced medical technology.
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ACKNOWLEDGMENTS This work was supported by NSFC (21210002, 21431007, and 21533008) and the Key Program of Frontier of Sciences, CAS QYZDJ-SSW-SLH052.
ASSOCIATED CONTENT
S Supporting Information *
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.8b00011. Summarization of the enzyme mimicry for combating bacterial and biofilms discussed in this Account (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jinsong Ren: 0000-0002-7506-627X Xiaogang Qu: 0000-0003-2868-3205 Author Contributions ¶
Z.C. and Z.W. contributed equally to this work.
Notes
The authors declare no competing financial interest. Biographies Zhaowei Chen received his B.Sc. degree at Northwestern Polytechnical University in 2010 and Ph.D. in Inorganic Chemistry and Chemical Biology at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2016. He then started his postdoctoral research in the Joint Department of Biomedical Engineering at University of North Carolina at Chapel Hill and North Carolina State University. His research interests are focused on the design and synthesis of biomimetic materials for theranostic, biocatalytic, and energy conversion applications. 797
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