Abstract
Pathogenic microorganisms and their chronic pathogenicity are significant concerns in biomedical research. Biofilm-linked persistent infections are not easy to treat due to resident multidrug-resistant microbes. Low efficiency of various treatments and in vivo toxicity of available antibiotics drive the researchers toward the discovery of many effective natural anti-biofilm agents. Natural extracts and natural product-based anti-biofilm agents are more efficient than the chemically synthesized counterparts with lesser side effects. The present review primarily focuses on various natural anti-biofilm agents, i.e., phytochemicals, biosurfactants, antimicrobial peptides, and microbial enzymes along with their sources, mechanism of action via interfering in the quorum-sensing pathways, disruption of extracellular polymeric substance, adhesion mechanism, and their inhibitory concentrations existing in literature so far. This study provides a better understanding that a particular natural anti-biofilm molecule exhibits a different mode of actions and biofilm inhibitory activity against more than one pathogenic species. This information can be exploited further to improve the therapeutic strategy by a combination of more than one natural anti-biofilm compounds from diverse sources.
Background
The antimicrobial tolerance of biofilms has emerged as a significant challenge to medical scientists across diverse healthcare sectors. Synthetic drugs, combinational therapy, and antibiotic hybrids could not achieve and deliver the desired results during the treatment. The hunt for novel antimicrobials in drug resistance emergency insists on the scientific society to search novel natural anti-biofilm agents. The focus of the present review is to revisit various natural products to overcome the biofilm-forming microorganisms and provide concise information on existing confines and recent developments in the modification of different natural anti-biofilm agents to make them effective drug candidates for clinical exploitation.
The Biofilms
The concept of biofilm was first developed by and further described by Fletcher, Characklis, and Costerton, “Biofilm is the unique pattern of growth in the life cycle of microbes that provides specific properties, advantages and higher level of organization to the free living bacterial cells during colonization” (; ; ; ). The description of biofilm is much more clarified by that biofilms are aggregates of microorganisms with distinct sessile cells followed by cell division to form small clusters, microcolonies, and larger sums. The film underneath the biofilm is only in direct contact with the substratum in a multilayered heterogeneous microbial mat. Biofilms are extensively used in various biotechnological applications, for example, biofuel production, degradation of wastewater, and filtration of drinking water (). The negative impact of biofilm includes bio-fouling (), corrosion (), and deterioration of the drinking water quality (). All higher eukaryotes, including humans, are populated by microorganisms that form biofilms (). Human dental plaque, skin, and gut represent one of the dominant biofilms in eukaryotic habitats. The widespread uses of medical devices create several new niches for bacterial biofilm formations ().
Cells in biofilm survive harsh growth conditions as biofilms are surrounded by high molecular weight extracellular polymeric substances (EPS) that attach cells (; ). The EPS are composed of proteins, lipids, polysaccharides, and extracellular DNA and play an essential function in the pathogenesis of the numerous microbial infections (). It has also been reported that microbial cells inside the biofilms are found to be resistant against UV, metal toxicity, acid exposure, desiccation, pH gradients, etc. (; ). In accretion to various physical and chemical tolerances, EPS confers immune resistance to many resident pathogenic microbes within biofilms by inhibiting neutrophil-mediated phagocytosis (). reported that the eDNA and intercellular adhesins of EPS act as a barrier for the penetration of a variety of antimicrobials. The eDNA present within the EPS chelate human antimicrobial peptides (AMPs) and lessen the antimicrobial activity of these peptides (). So far, many studies have been carried out to identify the method of biofilm formation and subsequent preventive strategies to strike the challenges, especially the drug resistance due to biofilm formation ().
The presence of glycocalyx, outer membrane structure, and efflux pumps; and heterogeneity in growth rate, genetic adaptation, metabolic state, and metabolism of cells within a biofilm are the leading causes of biofilm that acquire resistance against antimicrobials (). The mode of biofilm establishment in several human pathogens, as well as its drug resistance mechanism, is well documented and reviewed by different researchers and plotted (Figure 1). This figure explains the shared mechanism of biofilm tolerance under three sections. (1) Physical tolerance: the excess production of EPS restricts the penetration and diffusion of antimicrobials; as a result, cells in the biofilm get more time to become tolerant. Similar observations found that EPS production augments antimicrobial tolerance; an isogenic ΔcsgD mutant of Salmonella Typhimurium (EPS-deficient mutant) is much susceptible to hydrogen peroxide () and ciprofloxacin (). Therefore, therapeutic strategies that destabilize and inhibit EPS are the best anti-biofilm approaches to inhibit biofilms and reduce significant problems of antimicrobial resistance. A similar observation has been recorded by and that EPS inhibition reduces cell adhesion as well as drug tolerance in biofilms. (2) Passive tolerance: enzymes present in the biofilm matrix inactivate the antimicrobial molecules. The mechanism for the neutralization of antimicrobials through the biofilm matrix components have also been reported (), and there are reports that catalase enzymes present in the biofilm matrix are responsible for tolerance of Staphylococcus epidermidis biofilm against various physicochemical agents (). (3) Physiological tolerance: metabolically inactive cells in the deeper layers of biofilm exhibit adaptive stress responses that regulate the tolerance of biofilms to various antimicrobials. Persister cells become more tolerant of a variety of antibiotics after phenotypic and reversible changes induced by starvation, ecological factors, and several other adaptive responses such as SOS and stringent response (; ; ).
FIGURE 1
Persister cells are slow-dividing bacteria, less susceptible to antibiotics, and they have an essential role for biofilm re-establishment (
However, with the advance of sequencing technology, a ton of genomic data have been generated, which allows further illustration of the unknown molecular mechanism in the association of biofilm formation and drug resistance. The RNA-seq transcriptome analysis identified arsenic resistance operon genes (arsR and arsD), sporulation regulatory gene (paiA), ABC drug transporter classes, and penicillin-binding proteins associated with the Enterococcus faecalis biofilm formation and drug resistance (
The present tendency of antifungal tolerance is also a significant area of concern; therefore, direct research in a direction of novel antifungal compounds with targeted mechanisms of action are required (
Anti-biofilm Agents Based on Natural Products
The formation and development of biofilms is a complicated procedure involving different stages which can be the target of natural anti-biofilm agents for the prevention of biofilm development. Some of the well-studied stages of biofilm development include (1) attachment of bacterial cells to a suitable biotic/abiotic surface, (2) development of biofilm structure, (3) maturation of biofilm, and (4) dispersion (
FIGURE 2

Workflow of the portrayed natural anti-biofilm agents based on their mode of actions.
Phytochemicals
There are broadly five classes of natural compounds that have high anti-biofilm properties. Those are phenolics, essential oils, terpenoids, lectins, alkaloids, polypeptides, and polyacetylenes (
Several solvents, i.e., water, methanol, ethanol, chloroform, ether, dichloromethanol, and acetone, were used for the extraction of natural compounds from various sources for anti-biofilm activity. Various experiments carried out by researchers found that water extracts anthocyanins, sugars like tannins, saponins, terpenoids, polypeptides, and lectins. Ethanol extracts compounds, i.e., tannins, polyphenols, polyacetylenes, flavonol, terpenoids, sterols, alkaloids, and propolis whereas methanol extracts anthocyanins, terpenoids, saponins, tannins, xanthoxyllines, quassinoids, totarol, flavones, lactones, phenones, and polyphenols (
Phytochemicals inhibit the quorum sensing mechanism mainly by blocking the quorum sensing inducers like AHL, autoinducers, and autoinducers type 2 (
There are reports on the anti-adhesive properties of ethanolic and acetone extract of Psidium guajava L. (
Members of Enterobacteriaceae express curli, an amyloid fiber on the cell surface that helps in the attachment to characters and cell aggregation and enhances biofilm formation as well as cellular invasion (
TABLE 1
| Compound | Source | Experimental details | Pathogenic species | Molecular mechanism | Inhibitory concentration | References |
| Ajoene | Allium sativum L. | In vitro (PMNs killing assays) and in vivo (pulmonary infection mice model) | Pseudomonas aeruginosa P. aeruginosa Staphylococcus aureus | Downregulates rhamnolipid production Inhibits small regulatory RNA molecules (rsmY, rsmZ, and rnaIII) that operate in the later phase of QS signaling | 20 μg/ml ajoene reduces rhamnolipid production by 1/3 IC50 for rsmY = 2.5 μg/ml rsmZ = 2.3 μg/ml | |
| Allicin | Allium sativum L. | In vitro (ΔpqsABCD knockout strain) | Pseudomonas aeruginosa | Decreases the bacterial adhesion in the initial stages of biofilm formation as it reduces EPS formation It controls the expression of virulence factors hence interfere with the QS system | 250 μM inhibit production of virulence factors such as pyocyanin, elastase, and pyoverdine and rhamnolipids | |
| Carvacrol (monoterpenoid) | Origanum vulgare L. | In vitro (qPCR for relative expression of lasI/lasR genes) and docking modeling of proteins LasI and LasR | Pseudomonas aeruginosa | Post-translational inhibition against lasI, which effects AHL production. It mainly acts on QS machinery | C6-AHL production reduced up to 80% with 1.9 mM of carvacrol | |
| Emodin (anthraquinone) | Polygonum cuspidatum Siebold & Zucc. and Rheum palmatum L. | In vitro (crystal violet biofilm assay and SEM analysis) | Staphylococcus aureus | Decreases the release of eDNA and downregulates the expression of biofilm-forming related genes like cidA, icaA, dltB, agrA, sortaseA, and sarA | MIC = 8 μg/ml | |
| Emodin (anthroquinone) | Rheum palmatum L. | In vitro (microdilution assay, kinase assay) and molecular docking for emodin in CK2 (Autodock Vina) | Candida albicans Candida krusei Candida parapsilosis Candida tropicalis | Biofilm formation is inhibited by targeting cellular kinase signaling It acts on planktonic cells by reducing hyphal formation. It acts as a competitive inhibitor of CK2 | MIC = 12.5 μg/ml MFC = 25 μg/ml MIC and MFC = 25 μg/ml MIC and MFC = 50 μg/ml | |
| Aloe-emodin | Rheum officinale Baill. | In vitro (CLSM assays and Congo red assay) | Staphylococcus aureus | Reduce the production of extracellular proteins and polysaccharide intercellular adhesin | Inhibited biofilm formation on polyvinyl chloride surfaces at 32 μg/ml | |
| Hordenine | Hordeum vulgare L. (sprouting) | In vitro (SEM and CLSM assays, qPCR for QS-related genes) | Pseudomonas aeruginosa | Decreases AHL production Virulence factors (proteases, elastase, pyocyanin, rhamnolipid, alginate, and pyroviridine) production decreased significantly. Inhibit swimming and swarming activity Down-regulates the expression of lasI, lasR, rhlI and rhlR genes. | 1 mg/ml of hordenine along with 0.4 μg/ml of netilmicin reduced biofilms by 88% C4-HSL production decreased up to 69% at 0.5 mg/ml | |
| Pulverulentone A | Callistemon citrinus (Curtis) skeels leaves | In vitro (broth microdilution assay, CLSM, TEM analysis) | Methicillin-resistant Staphylococcus aureus | Reduces styphyloxanthin production, thus inhibiting biofilm formation Disrupts the cell membrane | MIC = 125 μg/ml Production of the virulence factor decreased by 65.9% | |
| Vitexin (flavon) | Vitex species | In vitro (safranin staining, microscopy methods, EPS quantification) In vivo murine model (catheter-associated infection), molecular docking | Pseudomonas aeruginosa | Attenuates formation of EPS, QS-associated factors (swarming motility, production of protease, pyoverdin and pyocyanin) Molecular docking studies confirmed it attnuates Las A, Las B, and Lux R | MIC = 260 μg/ml 39.04% decrease in Las A protease and 37.54% Las B elastase | |
| 5-Hydroxymethylfurfural | Musa acuminata Colla. | In vitro (biofilm, Las B elastase, protease, and rhamnolipid quantification assays) | Pseudomonas aeruginosa | Inhibits the production of biofilm proteins, EPS, and cell surface hydrophobicity productions Downregulates the expression of QS-regulated virulence genes | MBIC = 400 μg/ml Reduces production of biofilm proteins, biofilm adherence, EPS and CSH to the level of 79, 82, and 77%, respectively Inhibits the production of LasA protease, LasB elastase, pyocyanin, alginate, and rhamnolipid 77, 75, 68, 80, 78, and 69%, respectively | |
| Phytol | Piper betle L. | In vitro (microscopic analysis, transcriptional analysis of QS-regulated genes) | Serratia marcescens | Inhibits the swarming motility and hydrophobicity Downregulates QS genes | Significantly inhibits the production of biofilm and EPS to the level of 65 and 43% | |
| Isolimonic acid and ichangin | Citrus species | In vitro (Caco-2 cell adhesion and survival assay, AI-3 reporter assay) | Enterohaemorrhagic Escherichia coli Vibrio harveyi | Decreases the adherence Downregulates flagellar genes, ler (transcriptional regulator of LEE) Represses the expression of the flagellar master regulator (flhC and flhD) Regulates luxO expression, thus acting as potent modulators of bacterial cell–cell signaling | IC25 (isolimonic acid) = 19.7 μM IC25 (ichangin) = 28.3 μM ler repressed by 5-fold, flhC and flhD repressed by 4.6 and 6.9, respectively IC50 (isolimonic acid) = 94.18 μM | |
| (R)-Bgugaine | Arisarum vulgare O. Targ. Tozz. | In vitro (static biofilm inhibition assay) | Pseudomonas aeruginosa | Affects flagella related functions, inhibits pyocyanin pigmentation, LasA protease, rhamnolipid production. | Reduces biofilm density by 83% at 1.8 mM | |
| Zingerone | Zingiber officinale Roscoe | In vitro (microtiter plate assay, motility assay, quorum sensing signal molecules quantitative assay) and molecular docking of TraR, LasR, and PqsR proteins | Pseudomonas aeruginosa PAO1 | Reduces swimming, swarming, and twitching motility. Suppresses pyocyanin, hemolysin, rhamnolipid, protease and elastase Molecular docking analysis proved that it could bind with all the quorum sensing receptors and stops receptor–ligand interaction, suppresses QS-dependent gene expression | Sub MIC = 10 mg/ml Reduces the BFC of P. aeruginosa PAO1 as A570 from 1.1 to 0.5 | |
| Baicalin | Scutellaria baicalensis Georgi | In vivo (mouse peritoneal implant infection model) | Pseudomonas aeruginosa | Inhibits LasA protease, LasB elastase, pyocyanin, rhamnolipid, motilities and exotoxin A virulence factors Decreases the expression of lasI, lasR, rhlI, rhlR, pqsR, and pqsA genes and reduces the QS signaling molecule 3-oxo-C12-HSL and C4-HSL | MIC > 1024 μg/ml C4-HSL levels decreased 77.2% at 64 μg/ml baicalin | |
| Curcumin | Curcuma longa L. | In vitro (crystal violet biofilm assay, pellicle formation assay, surface motility assay, mixed culture biofilm assay) In vivo (Caenorhabditis elegans model organism, C. elegans killing assay) | Acinetobacter baumannii, C. albicans | Inhibits pellicle formation, Pilli motility, and ring biofilm formation Molecular docking analysis proved that curcumin interacts with the biofilm response regulator BfmR | MIC > 500 μg/ml for A. baumannii ATCC 17978 planktonic cell Reduces A. baumannii ATCC 17978 biofilm production by 93% at 100 μg/ml | |
| Epigallocatechin-3-gallate (EGCG) | Camellia sinesis (L.) Kuntze (green tea) | In vitro (growth assay, CR-binding assay, TEM analysis) | Escherichia coli BW25113 | Suppresses curli production and expression of curli-related proteins csgA, csgB, and csgD Enhances the degradation of sigma factor (RpoS) by ClpXP protease | IC50 = 5.9 ± 0.8 μM | |
| Ginkgolic acid (GA) and hydroginkgolic acid | Pistacia lentiscus L. (fruit) | In vitro, in vivo (human lung A549 infection model, C. elegans infection model) | Pseudomonas aeruginosa H103 | Decreases virulence factor production Modifies the membrane fluidity Regulates virulence through the ECFσSigX | IC50 of pyocyanin inhibition = 6.3 μg/ml 100 μg/ml GA reduces pyocyanin production by 82% | |
| 7-Epiclusianone | Rheedia brasiliensis (Mart.) Planch. & Triana | In vivo A rodent model of dental caries | Streptococcus mutans | Increases cariostatic activity by disrupting insoluble exopolysaccharides and intracellular polysaccharides | 70–80% less severe smooth-surface lesions and 50–70% less severe sulcal-surface lesions than the vehicle control treatment 50–70% reduction of exopolysaccharides | |
| Tannic acid | Not specified | In vitro (crystal violet microplate biofilm assay, Congo red binding assay) | E. coli BW25113 | Efficiently killed bacteria in pgaA mutant biofilms by inhibiting the formation of polysaccharide in the matrix Affects intracellular SOS response and decreases the expression of genes involved in this pathway | MIC = 1 mg/ml | |
| Diterpene derivative (C31H50O3) | Myrmecodia pendens Merr. & L.M. Perry | In vitro (broth microdilution assay, MBIC analysis by Perumal method) | Streptococcus mutans ATCC 25175 | Not specified | MBIC = 50 ppm and MIC = 40 ppm | |
| Chelerythrine | Bocconia cordata Willd. | In vitro (broth microdilution assay, crystal violet assay) and in vivo (mono- and dual species culture models) | Candida albicans and Staphylococcus aureus | Inhibits hyphae formation Reduces biofilm formation by decreasing eDNA, polysaccharide, and protein levels | The MICs (monospecies) = 4 μg/ml and MBIC90S (monospecies) = 2 μg/ml MICs (dual species) = 6 μg/ml and MBIC90S (dual species) = 3 μg/ml | |
| Hyperforin | Hypericum perforatum L. | In vitro (quorum sensing inhibition assay, human plasma protein-coated assay, static microtiter plate crystal violet assay) | Staphylococcus aureus AH1872 | Exhibits anti-biofilm activity and a moderate amount of quroum quenching activity, but a detailed mechanism is not specified | MIC50 (flowering aerial part) = 0.512% v/v Exhibit moderate inhibition of quorum sensing (QSIC50 = 0.064–0.512% v/v) | |
| Warburganal, polygodial, alpha-linolenic acid (ALA) | Warburgia ugandensis Sprague subsp. ugandensis | In vitro (tetrazolium reduction assay, checkerboard assay) | Candida albicans Candida glabrata S. epidermidis S. aureus | α,β-unsaturated 1,4-dialdehyde in polygodial and warburganal is responsible for the potent antifungal activity on developing biofilms Polygodial affects mitochondrial ATPase and leads to reduced ergosterol levels | BIC50 (warburganal) = 4.5 ± 1 μg/ml and BIC50 (polygodial) 10.8 ± 5 μg/ml BIC50 (warburganal) = 37.9 ± 8 μg/ml BIC50 (ALA) = 25 μg/ml |
Anti-biofilm activity of phytocompounds with their mechanism of action.
NS, not specified.
Biosurfactants
Biosurfactants (BS) hinder biofilm formation by varying the cell adhesion ability through less cell surface hydrophobicity, membrane disruption, and inhibited electron transport chain, thus restricting cellular energy demand (
TABLE 2
| Class | Source microorganism | Pathogen strains | Effect on biofilm | Dose | References |
| Lipopeptide biosurfactants (LPBs) | Acinetobacter junii | Biofilm of Staphylococcus aureus, Proteus mirabilis, and Pseudomonas aeruginosa | Biofilm disruption 35, 10, and 32%, respectively Biofilm disruption 52, 31, and 70%, respectively | 1250 μg/ml 2500 μg/ml | |
| Lipopeptide | Beauveria bassiana | Microsporum canis | 25.76% biofilm eradication | 1.95 μg/ml | |
| Lipopeptide surfactin-C15 | B. subtilis #309 | Candida albicans | 85% inhibition to biofilm formation | 960 μg/ml | |
| Lipopeptide surfactin | Bacillus safensis F4 | Staphylococcus epidermidis | 80% anti-adhesive activity | 6.25 mg/ml | |
| Lipopeptide pontifactin | Pontibacter korlensis strain SBK-47 | Bacillus subtilis, Staphylococcus aureus, Salmonella typhi, and Vibrio cholerae | 99% anti-adhesive activity | 2 mg/ml | |
| Lipopeptide | Bacillus subtilis AC7 | Candida albicans | Reduced adhesion up to 67–69% and biofilm formation up to 56–57% | 2 mg/ml | |
| Glycolipoprotein | Acinetobacter indicus M6 | Methicillin-resistant Staphylococcus aureus | 82.5% removal of biofilm | 500 μg/ml | |
| Glycolipid | Burkholderia sp. WYAT7 | Staphylococcus aureus | 41% inhibition to biofilm formation 79% inhibition to biofilm formation | 1 mg/ml 2 mg/ml | |
| Rhamnolipids | Pseudomonas aeruginosa MN1 | Streptococcus mutans | Dissociation of 67% of the preformed biofilm | 12.5 mg/ml | |
| Rhamnolipids | Burkholderia thailandensis E264 | Streptococcus oralis, Actinomyces naeslundii, Neisseria mucosa, and Streptococcus sanguinis | 90% inhibition of S. sanguinis biofilm 70% inhibition of S. oralis biofilm 70% inhibition of N. mucosa biofilm 50% inhibition of A. naeslundii biofilm | 0.39 mg/ml 0.78 mg/ml 6.25 mg/ml 12.5 mg/ml | |
| Exopolysaccharides | Pandorea pnomenusa MS5 | Burkholderia cepacia | Inhibit Burkholderia cepacia biofilm formation | 0.25 mg/ml |
Biosurfactants reported recently with anti-biofilm activities.
Rhamnolipids produced from Pseudomonas aeruginosa MN1 have higher anti-adhesive and anti-biofilm activity than that of surfactin (
Biosurfactants are appropriate coating agents for medical implants such as urinal catheters, bone implants, etc. to inhibit biofilms originated from pathogenic organisms without using synthetic drugs. Rhamnolipids and sorphorolipids are reported to be potential agents for the inhibition of biofilms formed by Gram-negative and Gram-positive microbes (
Antimicrobial Peptides
AMPs are broad-acting antimicrobial agents widely used in the treatment of both fungal and bacterial biofilms (
Amphibian skin is a source for many AMPs effective against various biofilm-causing microorganisms.
An AMP from frog skin named esculentin-1a, i.e., Esc (1-21), and its D-amino acid–containing diastereomer Esc (1-21)-1c inhibited P. aeruginosa biofilm formation by its membrane-perturbing activity. Previous studies reported that Esc (1-21)-1c showed potential activity against chronic lung Pseudomonas infections of cystic fibrosis patients (
In summary, Esc (1-21)-1c lowers the expression of virulence genes and bacterial motility genes, and ultimately prevents biofilm formation. These two anti-pseudomonal peptides esculentin-1a (1-21) and its diastereomer Esc (1-21)-1c have shown promising results in bronchial epithelium repair of cystic fibrosis patients.
The melittin peptide of bee venom exhibit antibacterial activity, prevents MRSA systemic infections and initiates the wound healing process in MRSA-infected mice model (
TABLE 3
| Name of AMPs | Amino acid sequence | Net charge | 3D structure | Source | Effects on biofilm | Disadvantages | References |
| Japonicin-2LF | FIVPSIFLLK KAFCIALKKC | 4 | Helix | Frog skin secretion | Eradicates the methicillin-resistant S. aureus biofilm matrix as well as kills all the sessile bacteria | Futile against P. aeruginosa biofilms. The anti-biofilm activity concealed by the changes in LPS contents and cell wall structure of microorganisms | |
| Dermaseptin-PT9 | GLWSKIKDAAKT AGKAALGFVNEMV | 2 | Helix | Frog skin secretion | Inhibits the biofilm formation of S. aureus, MRSA, and E. coli | More potent activity against Gram-negative bacteria | |
| Phylloseptin-PTa | FLSLIPAA ISAVSALANHF | 2 | Helix | Frog skin secretion | More potent against S. aureus biofilm | Anti-biofilm activity changed by the hydrophobicity, charges and α-helicity of the peptides | |
| Moronecidin-like | FFRNLWKGAK AAFRAGHAAWRA | 6 | Unknown | Seahorse | Inhibits surface attachment of S. aureus biofilm | More effectual against Gram-positive bacteria than Gram-negative bacteria The outer membrane proteins of Gram-negative bacteria may hinder translocation of AMPs through the outer membrane | |
| Mastoporan | LNLKALL AVAKKIL | 4 | Helix | European hornet venom | Suppresses biofilm formation by S. aureus and P. aeruginosa | Release histamine from mammalian mast cells may lead to an immune response | |
| Melittin | GIGAVLKVLTTG LPALISWIKRKRQQ | 6 | Helix | Honeybee venom | Induce disintegration of the MDR P. aeruginosa and degrades the biofilm | The toxicity of melittin on normal cells is a disadvantage for clinical applications (in case of third-degree burn patients, all three layers of skin are destroyed, so cytotoxicity of melittin hardly limits its applications) | |
| NA-CATH | KRFKKFFKKLKNSV KKRAKKFFKKPKVIGVTFPF | 15 | Helix | Chinese cobra (Naja atra) | Prevent biofilm formation of Burkholderia thailandensis | The small size of the peptide restricts its large-scale synthesis | |
| Defensin ZmD32 | RTCQSQSHRFRGPCLRRS NCANVCRTEGFPGG RCRGFRRRCFCTTHC | 12 | Combine Helix and Beta structure | Corn, Zea mays | Active against Candida albicans biofilms | Anti-biofilm activity of many defensins lost in the presence of salt | |
| Capsicumicine | RSCQQQIQQ AQQLSSCQQYLKQ | – | Unknown | Red pepper, Capsicum bacattum | Prevents the establishment of S. epidermidis biofilm by matrix anti-assembly (MAA) mechanism | NS | |
| Rhesus theta defensin-1 | GFCRCLCRRGVCRCICTR | 5 | Beta | Monkey leukocytes | Active against established C. albicans biofilms | Most of the host defense peptides exhibit undesirable pro-inflammatory properties and low bioavailability |
Sources and effects of AMPs.
NS, not specified.
Pathogens which form the biofilms on the implanted medical devices, human skin, gut, and oral cavities generally communicate through quorum sensing (QS) signals. The quorum sensing inhibiting potential of AMPs from natural sources offers an alternative antibiotic-free approach to overcome biofilm-associated infections. To date, more than 3000 AMPs have been discovered, but only seven of them have been approved by the FDA (
Efforts have been made to design novel specifically targeted multi-domain AMPs composed of a species-targeting peptide linked to a broad-spectrum antimicrobial killing peptide domain (
Similarly, attempts have been made to target only the pathogenic organisms of the biofilm without influencing the normal microflora (
Therapeutic Strategies Using Natural Products
The failure of conventional antibiotic therapies indicates that biofilm treatments need auxiliary upgradation (
FIGURE 3

The stages of biofilm formation and potential targets for anti-biofilm agents. The bacterial cells in humans attach to the matrix-forming proteins by forming a covalent linkage with peptidoglycan structure or by non-covalent attachment. With attachment and aggregation of a sufficient number of cells, the formation of EPS matrix takes place, and the attachment now becomes resistant to external repulsive forces. With the maturation of biofilm, the cells within the bulk structure start further communication with each other and start secreting specialized proteins and DNA, and some of them are involved in the formation of the efflux pump. At last, the dispersion of free planktonic cells from the formed biofilm further promotes the formation of new biofilms in the periphery. The natural anti-biofilm compounds can attack at one or different stages of biofilm formation and development, thus inhibiting it.
A recent study reported that elasnin (an anti-biofilm compound from an actinobacteria Streptomyces mobaraensis DSM 40847) destroyed the matrix in a multispecies biofilm and making them more vulnerable to antibiotics (
FIGURE 4

Graphical representation of anti-biofilm strategies covered in this review. EPS targeting: EPS matrix is targeted by matrix-degrading enzymes (DNaseI, dispersin B, lysostaphin) that inhibit microbial adhesion to a surface. Quorum sensing targeting: This strategy focused on the use of natural agents that block cell–cell communication in preformed biofilms and regulate virulence factor production (
Extracellular Polymeric Substance (EPS)–Targeting Strategies
Microbial EPSs secreted by a large variety of microorganisms mainly composed of polysaccharides, structural proteins, and extracellular DNA. The EPS matrix supports microbial adhesion to a surface, aggregation in multilayered biofilms, and functions as a three-dimensional scaffold that provides hydration, digestive capacity, and protection against antimicrobial compounds, antibiotics, and host effecter molecules (
TABLE 4
| Enzymes | Source | Pathogenic bacteria | Molecular mechanism of biofilm inhibition | References |
| Serine protease, Esp | Staphylococcus epidermidis | Staphylococcus aureus | Esp degrades S. aureus surface proteins and host receptors | |
| Lysostaphin | Staphylococcus simulans | Methicillin-resistant Staphylococcus aureus (MRSA) | Cleaves the pentaglycine cross-bridges of peptidoglycan and destroyed EPS matrix | |
| α-Amylase | Bacillus subtilis S8-18 | Methicillin-resistant Staphylococcus aureus (MRSA) | Degrades the preform mature biofilm by disrupting EPS matrix | |
| Cellulase | Penicillium funiculosum Trichoderma reesei | Pseudomonas aeruginosa | Decreases the adhesion of cells to the surface and polysaccharide matrix | |
| Cellulase | Aspergillus niger Bacillus sp. DGV19 | Burkholderia cepacia | Degrades the exopolysaccharide | |
| Alginate lyase | Bacillus circulans ATCC 15518 | Pseudomonas aeruginosa | Degrades the exopolysaccharide | |
| Hyaluronan | Streptococcus equi | Staphylococcus aureus | NS | |
| Cysteine, histidine dependent amidohydrolase/peptidase CHAPK | Myoviridae staphylococcal Phage K | Staphylococcus aureus | Cleaves the peptide bond involving D-alanine and the first glycine in the pentaglycine cross-bridge of Staphylococcal cell wall peptidoglycan | |
| Endolysin LysH5 | Phage vB_SauS-phiIPLA88 | Staphylococcus aureus, Staphylococcus epidermidis | Anti-persister agents | |
| DNase I | Human stratum corneum | Pseudomonas aeruginosa, Staphylococcus aureus | Degradation of extracellular DNA prevents the formation of biofilm | |
| DNase I and Proteinase K | NS | Actinomyces oris, Fusobacterium nucleatum, Streptococcus mutans, Streptococcus oralis, and Candida albicans | Affected the structural integrity of the biofilms by removal of eDNA and extracellular proteins | |
| Trypsin | Pancreatic serine endoprotease | Pseudomonas aeruginosa | Destroy the protein contents of the biofilm matrix |
Biofilm-degrading enzymes against various human pathogens.
NS, not specified.
The existing enzymes which have less catalytic activity can enhance their catalytic properties against the biofilms by modeling and engineering approach. The site-directed mutational analysis is considered as another approach to modulate the biofilm-inhibiting properties of the enzymes. Thus, broad-spectrum enzymes/peptides, as well as secondary compounds, must be isolated from bacteria for bioprospection, which can target a broad range of QS signaling molecules and structural part of the biofilms. The complete elimination of heterogeneous biofilms needs amalgamation of hydrolytic enzymes that can degrade proteins, polysaccharides, eDNA, and QS molecules (
Quorum Sensing Targeting Strategies
Prevention of cell-to-cell communication (quorum sensing) is an efficient strategy to restrain biofilm formation (
On the other hand, quorum quenchers are usually species specific; therefore, a combination of quenchers is required to eliminate mixed-species biofilms. Ajoene, a sulfur-rich molecule from garlic, decreases the expression of small regulatory RNAs (sRNAs) in both Gram-negative (P. aeruginosa) and Gram-positive (S. aureus) bacteria. Ajoene is the first compound to be identified to target broad-spectrum range quorum sensing inhibitors, i.e., lowers the RNAIII expressions in S. aureus (
The anti-biofilm peptide Human Cathelicidin LL-37 affects the bacterial cell signaling system and inhibits P. aeruginosa biofilm formation at 0.5 μg/ml by downregulating genes of the QS system (
Phage Therapy
Lytic bacteriophages have been used as an effective therapeutic strategy to remove biofilm cells. A recently published study proved that two lytic phages vB_SauM_ME18 and vB_SauM_ME126 are potential natural antimicrobials for inhibiting biofilm of MDR S. aureus (
Combination Therapy
Natural anti-biofilm agents sensitize antibiotics and established to be more effective when used in amalgamation (
Anti-biofilm Biomaterial Therapy
The adhesion of biofilm-associated pathogenic organisms on implant surfaces restricts their clinical applications, so many attempts have been made by various researchers to coat biomaterial as a preventive strategy. Natural polymer-based surface coatings, such as anti-adhesive coatings of algal polysaccharide ulvan, dextran, and dermatan sulfate, and antimicrobial-releasing polysaccharide coatings etc. have been popularized during the last decade (
Conclusion and Future Directions
The occurrence of many biofilm-based human infections and their multiple antimicrobial resistance is a major concern in medicine and human health. The elevated rate of resistance to antibiotics in biofilm leads to the discovery and characterization of novel natural anti-biofilm agents. This review describes different types of phytocompounds, antimicrobial peptides, and biosurfactants that exhibit promising biofilm-inhibiting ability. Natural anti-biofilm agents could be effectively used to deal with certain surgeries and diseases where there is a possibility of untraceable infection sites like bone, dental, eye lenses, and breast implants. These agents of natural origin are structurally and functionally more diverse in comparison with conventional antibiotics. The structure and function of natural anti-biofilm agents from various sources have been exploited to develop numerous advanced therapeutic strategies showing increased activity, stability, and reliability. Here, we continue to analyze the efficacy of specially targeted AMPs against drug-tolerant pathogenic biofilms without disturbing the natural microflora.
Natural products, mainly phytochemicals, as anti-biofilm agents have been studied more in in vitro and in vivo conditions, but not a single FDA-approved drug was developed despite huge efforts. Most of them failed in phase II and phase III clinical trials (
The review also explains the quorum quenching molecules and EPS-degrading enzymes of natural origin along with their mode of action on various biofilms. The mechanism of action of various natural agents against biofilm remains unknown. More studies on the mode of action may help to identify novel anti-biofilm agents. Anti-adhesin strategy can be a novel strategy for biofilm treatments on a broad range of bacteria as it targets and prevents attachment of bacteria to the cell surface. Very few studies have been made in this area, so future research in targeting biofilm in the direction of adhesin proteins may lead to the discovery of unique natural anti-biofilm agents. Pili and curli gene expression regulating phytocompounds can control biofilm formation. More work in these directions or a combination of phytocompound which has anti-adhesin properties may be a better therapeutic strategy for biofilm-related ailments.
The failure of natural medicines in clinical trials can be checked by rigorous quality control. The discovery of accurate markers that are sensitive and stable can resolve the problem and help in better quality control of natural anti-biofilm agents. It is a significant challenge faced by natural product research for the discovery of useful QC markers as natural compounds have a very complex structural lattice (
Statements
Author contributions
AP, RM, SD, MS, and SS drafted the manuscript. AP, RM, and JK were responsible for preparing the tables and figures in the manuscript. AP and RM equally contributed to the development of this manuscript. SS assisted to revise the manuscript. All the authors read and approved the final manuscript.
Acknowledgments
The authors gratefully acknowledge Prof. Hans-Curt Flemming, Biofilm Centre, Faculty of Chemistry, University of Duisburg-Essen, Essen, Germany, and Visiting Professor, Singapore Centre for Environmental Life Sciences Engineering (SCELSE), Singapore for his invaluable conceptual and technical advice to this work.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Abbreviations
- AHL
acyl-homoserine lactone
- AMP
anti-microbial peptide
- BFC
biofilm-forming capacity
- CLSM
confocal laser scanning microscopy
- CSH
cell surface hydrophobicity
- eDNA
extracellular DNA
- EPS
extracellular polymeric substances
- GA
ginkgolic acid
- HSL
homoserine lactone
- MBIC
minimum biofilm inhibitory concentration
- MFC
minimal fungicidal concentration
- MIC
minimum inhibitory concentration
- PMNs
polymorphonuclear leukocytes
- QS
quorum sensing
- SEM
scanning electron microscopy
- TEM
transmission electron microscopy.
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Summary
Keywords
microbial biofilm, therapeutic strategies, phytocompounds, multidrug resistance, antimicrobial peptides, biosurfactant
Citation
Mishra R, Panda AK, De Mandal S, Shakeel M, Bisht SS and Khan J (2020) Natural Anti-biofilm Agents: Strategies to Control Biofilm-Forming Pathogens. Front. Microbiol. 11:566325. doi: 10.3389/fmicb.2020.566325
Received
27 May 2020
Accepted
30 September 2020
Published
29 October 2020
Volume
11 - 2020
Edited by
Sujogya Kumar Panda, KU Leuven, Belgium
Reviewed by
Murugan Kasi, Manonmaniam Sundaranar University, India; Fazlurrahman Khan, Sharda University, India
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Copyright
© 2020 Mishra, Panda, De Mandal, Shakeel, Bisht and Khan.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Rojita Mishra, rojitamishra@gmail.com
†These authors have contributed equally to this work and share first authorship
This article was submitted to Antimicrobials, Resistance and Chemotherapy, a section of the journal Frontiers in Microbiology
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