Natural products as antivibrio agents: insight into the chemistry and biological activity

Vibriosis causes serious problems and economic loss in aquaculture and human health. Investigating natural products as antivibrio agents has gained more attention to combat vibriosis. The present review highlights the chemical diversity of antivibrio isolated from bacteria, fungi, plants, and marine organisms. Based on the study covering the literature from 1985–2021, the chemical diversity ranges from alkaloids, terpenoids, polyketides, sterols, and peptides. The mechanisms of action are included inhibiting growth, interfering with biofilm formation, and disrupting of quorum sensing. Relevant summaries focusing on the source organisms and the associated bioactivity of different chemical classes are also provided. Further research on in vivo studies, toxicity, and clinical is required for the application in aquaculture and human health.


Introduction
The genus Vibrio is Gram-negative, curved-rod shape bacteria, halophilic, fermentative, motile by polar agella, catalase, and oxidase-positive. The genus inhabits aquatic environment, freshwater, water column, sediment, and is associated with marine organisms. 1,2 Vibrio spp. play roles as nutrient cyclers in aquatic ecosystems, take up organic material, produce polyunsaturated fatty acids to the aquatic food web, and degrade chitin. 3 These groups of bacteria are responsible for several serious infections and opportunistic pathogens to aquatic animals and humans. 1,4,5 Studies about the effects of increasing sea surface temperature on the biology and ecology of Vibrio showed that there are correlations between the escalation of the emergence of Vibrio infections and global warming. Climate change induces global warming and as a result, the rising sea surface temperature corresponds to the number and distribution of Vibrio as reported in many places worldwide. Salinity less than 25 ppt contributes to Vibrio prevalence and infection in the marine system. [5][6][7][8] The term vibriosis is used to refer to infections by the member family of Vibrionaceae both in aquatic animals and humans. 9 Vibriosis is one of the primary problems in aquaculture that causes severe economic losses and large-scale mortality of shrimp, sh, and shellsh. 10 Comprehensive reviews are available focusing on vibriosis in sh, [11][12][13] shrimps, 14 crustaceans, [15][16][17] and mollusks. 18 More than a hundred Vibrio species have been identied and caused infections in humans. About 14 species of Vibrio reported as causative agents of human vibriosis cause foodborne and nonfoodborne Vibrio infections such as V. cholerae, V. parahaemolyticus, V. alginolyticus, and V. vulnicus. 9,19 Vibrio spp. infect humans worldwide and is responsible for gastroenteritis, septicemia, and invasive skin and so tissue infection (SSTI). 9,20 Non-foodborne Vibrio infections, caused by V. vulnicus, V. alginolyticus, and V. parahaemolyticus are fatal and oen leads to the amputation or death of immunocompromised patients suffering from liver disease, alcohol abuse, or diabetes. [20][21][22] A single or combinational antibiotic is the treatment for curative against vibriosis both in aquatic animals and humans. 20 Most Vibrio spp. are susceptible to most antibiotics for animals or humans. Overuse and unregulated antibiotic used in aquaculture are contributing to growing problems and concerns in antimicrobial resistance that impacts human health. Antimicrobial resistance may reduce the effectiveness of treatment options for sh and human health management. 23,24 Multiple-antibiotic resistance of V. vulnicus and V. parahaemolyticus were reported in countries such as the United States, Italy, Brazil, Philippines, Malaysia, Indonesia, Thailand, China, India, Iran, South Africa, and Australia. [24][25][26][27][28][29][30] Antibiotic resistance and the restricted choices of available antivibrio agents are the reasons for searching natural products as new antivibrio agents. The increase in the emergence of antibiotic-resistant bacterial pathogens, including Vibrio spp. is a major public health concern. Therefore, it has intensied the interest in research on the search for effective alternatives to cope with the issue of antibiotic-resistant bacteria. Attempts have been done on screening, isolation, and structure determination of antivibrio compounds from natural products. This review intends to deliver the exploration of natural products for new antivibrio compounds.

Targets for antivibrio
Antibiotic resistance is becoming an important issue in the world of medicine. Newly developed antibiotics also starting to lose their effectiveness against some bacterial strains. As a result, it is critical to look for novel antimicrobial agents that are both effective against resistant microbes and long-lasting.
Quorum sensing (QS) is a small diffusible signaling molecule that trigger the expression of multiple genes that govern a wide range of activities including bioluminescence, virulence control, sporulation, host colonization, biolm development, defense against competition, and environmental adaptability. Vibrio scheri, V. harveyi, V. cholerae, V. anguillarum, and V. vulnicus use QS to regulate their pathogenicity. 31 Biolm is a complex structure of microbiome having different bacterial colonies or single type of cells in a group; adhere to the surface that are embedded in a membrane structure of the extracellular polymeric substances (EPS) composed of eDNA, proteins and polysaccharides. The matrix complex are attached to the biotic or abiotic surface, showed high resistance to antibiotics. 32,33 Biolms formation is the key factor for accelerating Vibrio spp. to grow and survive by providing access to nutrients, protecting from the host immune system, defending from the predator, and antimicrobial compounds. Studies showed that biolm is important for survival, virulence and stress resistance of Vibrio sp. 34,35 The formation and maintenance of biolms, as well as their resistance to antimicrobials and the host's innate immune system, are controlled by QS-regulated gene expression. [35][36][37] In the aquaculture system, QS regulates virulence factors and the formation of biolm. Thus, disruption of QS is a potential strategy for preventing disease in aquaculture systems. Quorum sensing inhibitors (QSIs) or quorum quenchers inhibit both the expression of virulence-associated genes and attenuate the virulence of aquaculture pathogens. 32 Quorum sensing plays a role in the formation of biolms. Thus, ghting Vibrio spp. by interrupting quorum sensing and biolms formation are the right strategies to combat vibriosis. 38,39 Inhibiting growth, interrupting quorum sensing, and interfering biolms formation are targets for antagonistic effects in searching for antivibrio.

Actinobacteria
Actinobacteria are important assets for microbial natural products with therapeutic properties for medicinal, agricultural, veterinary, and aquaculture applications including chloramphenicol, tetracycline, erythromycin, rifamycin, rapamycin, vancomycin, bleomycin, and avermectin. Actinobacteria are known to produce 70% of the antibiotics used today. 40,41 Screening have been carried out to obtain isolates that produce antivibrio compounds. Actinobacteria mainly Streptomyces spp. from different sources were tested for the antagonistic effect against Vibrio spp. 42-47 A comprehensive review showed a list of 128 strains of Streptomyces isolated from terrestrial and marine environments that are active against Vibrio spp. 48 Most of the studies have focused on the preliminary screening bioactivity of crude extract fermentation. To date, only a limited number of structure elucidations and identied the bioactive compounds that displayed potent antibacterial activity against Vibrio spp. Herein, we collected data on antivibrio compounds isolated from Actinobacteria presented in Fig. 1 and Table 1.

Pseudoalteromonas
The genus Pseudoalteromonas is Gram-negative bacteria, heterotrophic, aerobic, and belongs to the g-Proteobacteria class. This genus attracts attention due to its wide array of metabolites and ecological role in the ocean. The metabolites of Pseudoalteromonas have bioactivity including antimicrobial agents. 58,59 Antivibrio compounds isolated from Pseudoalteromonas spp. are presented in Fig. 2 and Table 2.

Antivibrio from marine fungi
Since the discovery of penicillin from Penicillium chrysogenum in the twentieth century, the fungus is an important source of natural products for drug discovery. Marine fungi have been explored for their bioactive secondary metabolites and potential for anti-microbial agents. [70][71][72] To date, 38% of 22.000 bioactive microbial metabolites are from fungi. 73 Among those metabolites, there are only a few antivibrio agents derived from marine fungi as presented in Fig. 5 and Table 5.
In vitro bacteriostatic assay conrms the MIC value at 31.25 mg mL −1 . 79 The MIC of each compound is presented in Table 5.

Antivibrio from sponges
Sponges are the oldest metazoan and have been investigated extensively for bioactive metabolites. Three new alkaloids isonaamide D, di-isonaamide A, and leucettamine D, and two known compounds isonaamine A and isonaanidine from a sponge Leucetta chagosensis Dendy, 1863 from French Polynesia. The compounds were screened for quorum sensing (QS) inhibitor of V. harveyi. The result showed that Isonaamidine A (61) inhibited the QS pathway at IC 50 1 mg mL −1 . None of the compounds affected bacterial growth at 50 mg mL −1 . 82 In the searching for antimicrobial agents against V. vulnicus twelve pure marine compounds from a variety of sponges were screened for inhibition effect. Psammaplin A (62), a bromotyrosine derivative from the sponge Poecillastra sp., Jaspis sp., and Psammaplin aplysilla inhibited V. vulnicus in vitro and in vivo assay at 5-50 mg (Table 6). 83 Alkaloid aaptamin and derivates from sponge Aaptos aaptos were tested against Vibrio spp. and V. harveyi. Aaptamine (63), 9demethylaaptamine (64), 4-N-methylaaptamine (65), 9-methoxyaaptamine (66) were active at concentration 1 mg mL −1 (Fig. 6). 84

Antivibrio from seaweeds
Seaweeds are well known as rich sources of primary and secondary metabolites with diverse applications for food, feed, agriculture, pharmaceutical, and cosmetics. 86,87 Numerous substances were isolated from seaweed such as halogenated compounds, 88,89 polyether, 90 phenolic compounds, 91 and polyunsaturated fatty acid. 92 Antimicrobial activity testing of seaweed extracts support the possibility of using seaweeds as a source of antimicrobial agents or as a health-promoting feed for aquaculture. 93 Bioactive compounds from seaweed can be applied in aquaculture health and disease management to control bacterial infection. [94][95][96] Seaweeds are rich in fatty acid and the mechanism of action of fatty acid as an antibacterial agent through inhibition of the electron transport chain and normal oxidative phosphorylation in bacterial cell membranes. 97 Polysaccharides from seaweed have been examined for the purpose as prebiotic or immunostimulant in Table 5 (Contd. ) No.  (Table 8). 99 Water-soluble fractions of red algae Palmaria paltata and Grateloupia turuturu were examined for the activity against V. harveyi. The NMR data suggested that the active water fraction of Palmaria paltata contains oridoside (73) (Fig. 7). 16 Further structure elucidation should be done to identify principal compounds responsible for an antivibrio agent. 16 Red algae Delisea pulchra produced halogenated furanones called mbrolide (Fig. 8). 101 Brominated furanones from marine algae inhibited biolm formation and quorum sensing (QS) Gram-negative without affecting their growth. The structure is similar to bacterial acyl homoserine lactones (AHL). 100 Some marine algae produced halogenated furanones as AHL antagonists as a response to the negative impact of bacterial colonization. Fimbrolide 1 (74) and Fimbrolide 2 (75) were tested for inhibiting bioluminescence in V. harveyi and V. campbellii with the target on LuxS, PhaB, and uncharacterized IMPD protein. 101 Extracts of Indonesian red seaweeds have been screened for bioactivity against sh pathogens including Vibrio spp. Extract of Gracilaria arcuata was active against Vibrio sp. at a concentration of 2.5 mg mL −1 . The active fraction contained hexadecanoic acid and sterol compounds such as Ergost-5-en-3-ol    fraction showed inhibition against V. uvialis at MIC 2.5 mg mL −1 . 103 Ethanolic extract of Gracilaria scheri exhibited anti-quorum sensing activity in V. harveyi and V. parahaemolyticus at concentrations of 5, 10, and 100 mg mL −1 . The extract also reduced the luminescence of V. harveyi. 104 Further investigation showed G. sheri contains N-benzyl cinnamamide (84) and aresorcylic acid (85) and which are responsible for antivibrio activity. 105

Antivibrio from plants
Plants are well known as a source of bioactive compounds and are used in traditional medicine. Various plant extracts containing phenolic, alkaloid, avonoid, and polysaccharide have been tested and used in aquaculture as an immunostimulant, antioxidant, prebiotic, antibacterial, and antifungal. 106,107 Plant extracts have been screened as sources for antivibrio agents. 108,109 Phytochemicals can be used to interfere with bacterial quorum sensing to counteract the biolm resistance. Medicinal plants are rich resources for screening bioactive QS. 110 Antivibrio compounds identied from plants are shown in Table 9.
The essential oil from aromatic plants Mentha longifolia, M. pulegium, Eugenia caryophyllata, Thymus vulgaris, and Rosmarinus officinalis were tested against V. alginolyticus, V. parahaemolyticus, V. vulnicus, and V. uvialis strains. Results showed variable activity and essential oils of T. vulgaris yielded the highest zone of growth inhibition against V. parahaemolyticus. 111 One of the approaches in the screening of natural products as antivibrio is targeting the production of virulence factors such as capsaicin and curcumin. Extract methanol of Capsicum annum containing capsaicin was reported to inhibit CT (cholera toxin) production in V. cholerae. The transcriptions of ctxA, tcpA, and toxT genes were repressed by capsaicin (86). On the contrary, capsaicin signicantly enhanced the transcription of the hns gene, the product of which is known to regulate negatively the transcription of ctxAB, tcpA, and toxT genes. These results suggest that capsaicin might act as a potent repressor for CT production possibly by enhancing the transcription of hns. 112 Curcumin (87) from Curcuma longa reduced 88% of bioluminescence of V. harveyi and inhibited components of biolms and virulence factor in V. parahaemolyticus, V. vulnicus, V. harveyi. 113 Three compounds piperidine (88), chlorogenic acid (89), and eugenyl acetate (90) isolated from Piper bettle were reported as bactericidal against several pathogenic Vibrio spp. The MIC range 0.6 to 16 mg mL −1 . Piperidine has the strongest inhibition effect on Vibrio spp. compare to chlorogenic acid and eugenyl acetate (Fig. 9). 114 Punicalagin (91) from pomegranate (Punica granatum Linn.) was reported against V. anguillarum at MIC 25 mg mL −1 . 115 Punicalagin (91) Punica granatum Linn V. anguillarum MIC 25 mg mL −1 Inhibition of the growth 115 Fig. 9 Antivibrio compounds isolated from plants.

Conclusions and perspective
Climate change and global warming will impact increasing cases of vibriosis in the future. Vibrio spp. cause serious problems in aquaculture with consequent huge economic losses. Moreover, vibriosis threatens human health through seafood contamination and contact with seawater during wound events.
To date, an effective vaccine to prevent vibriosis has not been available yet. Efforts have been done to prevent vibriosis in aquaculture with probiotics, prebiotics, and immunostimulants. The rising incidence of Vibrio resistance to antimicrobial agents and the limited option of antibiotics have driven the search for new antivibrio agents. Different stages of work have been performed ranging from the preliminary screening to an in-depth characterization of antivibrio compounds. This review provides proof that natural products are promising as a source of antivibrio agents. Screening of natural products from different sources has been carried out to discover antivibrio agents. Fig. 10 summarizes the exploration of natural resources to discover antivibrio agents. Natural product compounds exhibit bioactivity against Vibrio spp. through mechanism of action inhibiting the growth, disrupting quorum sensing, and interfering with biolm formation.
This review shows that natural products as antivibrio are produced by prokaryotes and eukaryotes living in terrestrial and marine environments (Fig. 11). Based on data on this review, marine fungi demonstrated prolic sources of antivibrio and contribute 36% of bioactive antivibrio. Actinobacteria and sponges are well-known as sources of bioactive compounds for decades, but their compounds account for only 16% and 7%, respectively for antivibrio. The type classes of natural antivibrio derived from natural product compounds are alkaloid, polyketide, peptide, sterol, terpene, organic acid, and fatty acid. Fig. 10 Summary of the chemistry of natural products as antivibrio and their mechanism of actions. Fig. 11 The structure type of antivibrio compounds derived from natural resources. Polyketide and alkaloid are the major class of antivibrio compounds and count about 28% and 23%, respectively presented in this review (Fig. 12). The alkaloids are produced by fungi and sponges, while polyketides were produced by mostly all organisms except coral. Antivibrio from coral and seaweed are mostly sterol.
This review summarizes that nature has provided a plethora of natural products with extraordinary chemistry and bioactivity against Vibrio spp. Further research and development of promising compounds are necessary for application in aquaculture and human health. Future efforts are necessary to evaluate the biological activities in vivo, toxicity, and mechanisms of action. Biolms is the leading cause of multidrug resistance among microorganisms including Vibrio spp. Thus, study and examination of antivibrio compounds as inhibitor of biolm formation is needed. The clinical study of antivibrio compounds has not been reported yet.

Author contribution
NK and MU wrote the manuscript, DCR helped supervise the project and reviewed the manuscript, MU contributed to collect data and references. NK received the funding and DCR provided lab access to do research on antivibrio.

Conflicts of interest
The authors report there are no competing interests to declare.