Keywords

5.1 Introduction

From the past decade, we have been receiving many alerts on emerging and re-emerging diseases. Though the traditional medicine obtained from plants and microbes was neglected for various reasons, now people have started to pay attention to the conventional herbal medicine . There are very many incidents where plant-derived metabolites have served as a preventive and curative drug. For instance, Nilavembu concoction was advised to consume during dengue outbreak in India. These are instant remedies to patients and have been found to be effective in preventing the disease. The side effects are not much harmful and provide remedy in the long run.

The secondary metabolites obtained from plants do not play a vital part in the plant’s reproduction, growth, etc. (Fraenkel 1959). However, there are evidences that these products serve in the defence mechanism of plants (Stamp 2003). With this cue, researchers are inspecting the derivatives of herbal plants. Plants have a specialized pathway to synthesize the compounds. The study of these compounds has been elucidated with the help of various techniques, namely solvent extraction; microwave-assisted extraction; supercritical fluid extraction (SFE); solid phase micro extraction (SPME); chromatography methods such as HP-TLC, TLC, UPLC, GC-MS, LC-MS; phytochemical screening assays, nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), etc.

Plant-derived compounds are comparatively safer than their chemically synthesized counterparts (Upadhyay et al. 2014; Rajeh et al. 2010). These compounds are primarily responsible for their antimicrobial activity (Savoia 2012). Phenolics and polyphenols such as flavonoids, quinones, coumarins and other types like alkaloids and terpenoids do possess the antimicrobial properties from the secondary metabolite group (Cowan 1999; Savoia 2012). The mechanisms by which plants control the microbial population are: (a) inhibition of biofilm formation, (b) inhibition of capsule production of bacteria, (c) microbial membrane disruption, (d) weakening of cell wall and metabolism and (e) by toxins production (Upadhyay et al. 2014).

Microbial-derived metabolites on the other side perpetuate their properties in the form of antibacterial, antifungal, antitumour and anticancer compounds. The major group of microbes that had a remarkable place in the pharmaceutical industry is Actinomycetes (Bérdy 1989). It was followed by fungi, bacteria and cyanobacteria, which had a notable role in metabolite production (Prasad et al. 2016, 2018).

This chapter enlightens the readers about the novel plant metabolites and microbial metabolites in the recent years. The merits and demerits of the recent extraction and analytical techniques with respect to the metabolite production are discussed in detail in Sect. 5.2.

5.2 Traditional Medicine

Traditional medicine has been a magical potion since prehistoric times for human population. It has been followed in many countries all over the world such as traditional medicine in China (Fabricant and Farnsworth 2001; Qi et al. 2013), Ayurveda and Unani in India (Parasuraman et al. 2014), Kampo in Japan (Yakubo et al. 2014), Sasang constitutional medicine in Korea (Kim and Noble 2014), traditional Aboriginal medicine in Australia (Oliver 2013), traditional medicine in Africa (Boakye et al. 2015) and Russian herbal medicine (Shikov et al. 2014). Traditional medicine has been a platform for formulating the modern pharmaceutical products. Plants which are used in traditional medicine can be derived from a single or mixed plant material and might contain a range of phytoconstituents. These components might act solely or in integration with other drugs to treat a particular disease (Parasuraman et al. 2014). The World Health Organization (WHO) recognized that traditional medicine plays a vital part in meeting the essential health maintenance requirement of the global population and emphasize definite types of the medicine (WHO 2014).

The medicinal components from both flora and fauna, despite being used in traditional medicine, have also been given much value as a primary source during the development of herbal preparations and modern medicines (Kang and Phipps 2003). Animal parts like hooves, skins, bones, feathers, tusks and their by-products such as honey, venom, wax and ambergris serve as the key constituents in the preparation of curative, protective and preventive medicine (Adeola 1992). Further, a major portion of bioactive compounds are derived from plant sources like herbs, shrubs, trees and higher plants followed by microorganisms and others (Farnsworth and Morris 1976). Figure 5.1 represents the group of antimicrobial compounds that are produced from plants.

Fig. 5.1
figure 1

Plant metabolite classification

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India has been known to possess the rich source of medicinal plants. Around 8000 medical preparations from plants have been classified in Ayurveda. The famous Vedas have acknowledged more medicinal plants, accounting for 67 medicinal plants in Rig Veda around 5000 BC, 290 species in Atharva Veda during the 4500–2500 BC, 81 species in Yajur Veda, 1270 species in Sushrut Samhita and 1100 plant varieties in Charak Samhita for the drug preparation and its use. To our surprise, these plant preparations are still in use as a standard material in Ayurvedic medicine (Joy et al. 2001).

5.2.1 Technologies/Methodologies for the Compounds from Plants and Microbes

The assays and assessment techniques used to validate the antimicrobial potential of the secondary metabolites against pathogens need to be addressed in an elaborate manner. In this way, the choice of method for plants/microbes can be made, and accordingly, the precise results are obtained (Rios et al. 1988). Though the plant/microbial derived drugs are in a successful rate, their screening procedures in the past were strenuous, expensive and sluggish. This has eventually led to the increase of high-end technologies to screen and identify the metabolites. Table 5.1 discusses the recent methods in the identification of bioactive components and phytochemical analysis.

Table 5.1 Merits and demerits of phytochemical analysis
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The standard antimicrobial screening methods availed currently comprise diffusion, dilution and bioautographic methods. Both diffusion and bioautographic method accomplish a qualitative analysis of the samples by revealing the presence or absence of inhibitory compounds. In contrast, dilution method defines the quantity of the compound (Valgas et al. 2007). Table 5.2 reviews the common and recent technologies in antimicrobial activity screening and evaluation methods.

Table 5.2 Common and recent methods to evaluate the antimicrobial activity
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5.2.2 Novel Metabolites from Plants

Gymnema sylvestre from family Apocynaceae has been reported to have larvicidal, anticancer, hypolipidemic, antimicrobial, antiviral and antioxidant activities. Ten percent aqueous extract of the leaves inhibited 9 out of 13 microbial strains, with a wide-spectrum activity showing inhibition zone of 14–23 mm. Among the gram-negative bacteria, Klebsiella pneumoniae was the most sensitive with 22.1 mm of inhibition zone, followed by Salmonella typhimurium with 21.2 mm and Escherichia coli with 19.6 mm. Pseudomonas aeruginosa was the most vulnerable bacterium for the plant metabolites with 23.3 mm of inhibition zone followed by Candida albicans with 22.6 mm of inhibition zone. The zones of inhibition of Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) were 20 mm and 19 mm, respectively. Staphylococcus epidermidis and Enterococcus faecalis were unresponsive to the plant metabolites (Arora and Sood 2017).

Ficus species from family Moraceae have been shown to have antifungal (Hassan et al. 2006), anti-diarrhoeal (Ahmadu et al. 2007), anti-inflammatory and anti-nociceptive activities (Amos et al. 2002). In addition to possessing the common metabolites, the ethanolic extracts of leaf and stem bark of Ficus sycomorus and Ficus platyphylla showed prominent antimicrobial activity against Trichophyton mentagrophytes, Staphylococcus aureus and ciprofloxacin-resistant Salmonella typhi (Adeshina et al. 2009). Table 5.3 lists the antibacterial activity of the medicinal plants.

Table 5.3 Antibacterial secondary metabolites from medicinal plants
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Recently, an interesting study on green tea and Korean mixed tea subjected to antibacterial and antifungal activities was done, and the results indicated that green tea possessed substantial antibacterial and antifungal activities, while the mixed tea presented less activity. It was evident that epigallocatechin gallate (EGCG) was the responsible compound for the property (Muthu et al. 2016).

Moringa oleifera is known to possess immense pharmacological properties such as antioxidant, antidiabetic, antihypertensive, antitumor, antiulcer, cholesterol-lowering agent and is used in the traditional medicine. Though most of the plant parts are potent in producing metabolites, seed coat has been analysed for antimicrobial activity. Candida albicans was the most sensitive microbe for the ethanolic extract. Staphylococcus epidermidis and Staphylococcus aureus were the most sensitive Gram-positive bacteria. Among Gram-negative bacteria, Klebsiella pneumoniae was the most sensitive (Arora and Onsare 2014). A potent bioactive compound from turmeric was found to be effective against Helicobacter pylori. The metabolite curcumin (diferuloylmethane) was anticipated as a promising antimicrobial compound to solve gastroduodenal diseases like peptic ulcer, gastritis, etc. The minimum inhibitory concentration (MIC) of curcumin against different strains of H. pylori was in the range of 5–50 μg/mL (De et al. 2009). Tables 5.4 and 5.5 list the antifungal and antiviral activities of the medicinal plants, respectively.

Table 5.4 Antifungal secondary metabolite from medicinal plants
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Table 5.5 Antiviral secondary metabolites from medicinal plants
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5.3 Novel Metabolites from Microbes

5.3.1 Antibacterial Activity

Penicillium sp. is a well-renowned fungus that produces an array of secondary metabolites like antibacterial substances (Lucas Esther et al. 2007), immunosuppressants, cholesterol-lowering agents (Kwon et al. 2002), antiviral substances (Nishihara et al. 2000), antifungal substances (Nicoletti et al. 2007) and also strong mycotoxins (Frisvad Jens and Samson Robert 2004). A group of scientists from Brazil has reported three metabolites from Penicillium sp. Among the three compounds, methyl 6-acetyl-4-methoxy-5,7,8-trihydroxynaphthalene-2-carboxylate worked as a wonder substance having a broad spectrum of antimicrobial activity against Candida albicans with a minimum inhibitory concentration of 32 μg/mL and Bacillus cereus and Listeria monocytogenes with MIC of 64 μg/mL (Petit et al. 2009). Another novel compound extracted from Penicillium sp. is 7-methoxy2,2-dimethyl-4-octa-4′,6′-dienyl-2H-napthalene-1-one which exhibited antimicrobial activity against K. pneumoniae, gram-negative bacteria and MRSA, S. aureus, S. epidermidis, Gram-positive bacteria. This study has also proven results in cytotoxicity and mutagenicity assay, revealing the suitability of the compound for further use (Kaur et al. 2015).

Two metabolites elucidated as 4-Chromanone, 6-hydroxy-2-methyl-(5CI) and Modiolide A were isolated from a unique endophytic fungus Periconia siamensis extracted from leaves of Thysanoleana latifolia. These compounds were found to be effective against Listeria monocytogenes, MRSA, Bacillus cereus and Pseudomonas aeruginosa (Bhilabutra et al. 2007). Table 5.6 provides the list of bacterial metabolites from various microbes.

Table 5.6 Bacterial secondary metabolites with antimicrobial activity
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The lactic acid bacterial cultures were treated against Mycobacterium avium subsp. Paratuberculosis to test for the production of bacteriocin (Kralik et al. 2018). Recently, volatile organic compounds with antimicrobial activity were extracted from the bacterium Bacillus amyloliquefaciens strain S13. These compounds were found to be effective against Saccharomyces cerevisiae, Agrobacterium tumefaciens and Bacillus subtilis (Sonia Hamiche et al. 2019).

Bacillus subtilis isolated from soil sample served as a potent biological agent by controlling the fatal phytopathogens, namely Botrytis cinerea, Monilia linhartiana, Alternaria solani, Rhizoctonia sp. and Phytophthora cryptogea. This paves the way to control several plant diseases. The same bacterium also showed antibacterial activity against two pathogens Pseudomonas syringae and Xanthomonas campestris (Todorova and Kozhuharova 2010). There is more impending work from the researchers in this field, and the microbial-derived drugs should be used wisely by the human population.

5.3.2 Antifungal Activity

Very few antifungal drugs are successful in the market, owing to their poor efficacy and other reasons like azole resistance (Carledge et al. 1997). However, there are recent enhancements involving polyene lipid formulation with less toxicity and novel triazoles possessing a wide range of action against many fungal isolates (Granier 2000). The drug target is presumably on the fungal cell wall, which comprises three major constituents: chitin, glucan and mannoproteins. Hence, the drug’s action is determined by the nature of the fungal cell wall composition. There are also natural fungal inhibitors isolated from microbes which are covered in detail in the subsequent sections.

A potent strain named Lactococcus lactis LI4 strain isolated from dairy foods was found to prevent the growth of Candida albicans DMST 5239. Surprisingly, the activity of the culture was persistent in pH 2.0–4.0 (Lertcanawanichakul 2011). It was reported that by combining both natural and engineered lactococci strains, a group of compounds termed alkyl ketones were produced, and they were responsible for the antifungal activity against yeast genera Candida and Rhodotorula and Aspergillus and Fusarium (Stoyanova et al. 2006, 2010). Few reports have suggested that these strains can be efficiently used in the preservation of vegetables and fruits from fungal contamination (Trias et al. 2008).

In 2007, Gai et al. studied antifungal compounds in the ethanolic extracts of Fusarium sp. Based on the identification studies of the bioactive compound, it was interpreted as Fusarielin E (Gai et al. 2007). In a recent study, two glycolipids isolated from Bacillus licheniformis exhibited antifungal activities against Rhizoctonia solani, Colletotrichum acutatum, Aspergillus niger, Candida albicans and Botrytis cinerea. They were identified as leodoglucomide and leodoglycolipid (Tareq et al. 2015).

Haliangicin, a beta-methoxyacrylate antibiotic, exhibited effective inhibition of growth against Pythium ultimum, B. cinerea and Saprolegnia parasitica (Fudou et al. 2001a, b). Bacillus laterosporus isolated from Papua produced Basiliskamides A and B and showed potent antifungal action against Aspergillus fumigatus and Candida albicans (Barsby et al. 2002).

A novel compound named Saadamycin isolated from Streptomyces sp. exhibited antifungal action against A. fumigatus, T. mentagrophytes, A. niger, F. oxysporum, Epidermophyton floccosum, Microsporum gypseum and C. albicans (El-Gendy and El-Bondkly 2010). A novel hexapeptide, Sclerotide B, derived from marine-based Aspergillus sclerotiorum PT06-1 possessed antifungal action against Candida albicans (Zheng et al. 2009). The same organism from an anonymous source exhibited antimycotic activity against A. flavus (Bao et al. 2010). Two sesquiterpene type compounds isolated from Penicillium bilaiae MA-267 showed antifungal activity against phytopathogenic fungi such as Colletotrichum gloeosporioides, Gaeumannomyces graminis, F. graminearum and Alternaria brassicae (Meng et al. 2014). Peniciadametizine A isolated from Penicillium adametzioides AS-53, presented selective antimycotic activity against the phytopathogenic fungus A. brassicae (Liu et al. 2015). Table 5.7 shows the list of fungal secondary metabolites.

Table 5.7 Fungal secondary metabolites with antimicrobial activity
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5.3.3 Antiviral Activity

At present, there are many well reputable microbial-driven secondary metabolites used for antiviral activity. Among the Actinomycetes, Streptomyces roseus played a promising role in producing leupeptin, a serine and cysteine protease (Aoyagi et al. 1969). This enzyme has proven to impede the glycoprotein-mediated admittance of Marburg virus (Gnirss et al. 2012). Similarly, protease inhibitors like Antipain and Elastatinal were utilized to hinder poliovirus protease (George A. Belov 2004). An interesting analogue of sialic acid, siastatin B, extracted from Streptomyces verticillus showed anti-influenza activity and in vitro propagation (Yoshio Nishimura et al. 1993).

There is an increasing need to focus on drugs for Hepatitis B virus (HBV) since the number of individuals who are infected with HBV is on steep escalation. The existing approved antiviral drugs comprise immunomodulators and nucleotide analogues that help to interfere in the disease advancement. Nevertheless, these drugs have their limitations such as side effects, limited efficacy and development of drug-resistant varieties. In contrast to the phytochemicals, there are very few microbial metabolites to act against HBV (Zhang and Wang 2014; Wu 2016).

To treat influenza virus, the following synthetic antiviral drugs have been approved: (a) amantadine (b) ribavirin (c) favipiravir (d) rimantadine (e) peramivir (f) and laninamivir octanoate (g) oseltamivir and (h) zanamivir. The drugs from natural products are currently under research, and the targets of these drugs are neuraminidase and haemagglutinin. Stachyflin, a novel compound isolated from Stachybotrys sp. RF-7260 provides conformational changes in haemagglutinin, thus preventing the fusion of host cells with virus (Nakatani et al. 2002; Minagawa et al. 2002).

The antiviral drugs that are derived from natural products exhibited a mild anti-HIV activity: disorazol, tubulysin and stigmatellin. Two myxobacterial strains named Polyangium sp. and Myxococcus stipitatus produced compounds such as thiangazole, phenalamide A1 and phenoxan with anti-HIV activity (Martinez et al. 2013). Myriocin and NA255, potent serine palmitoyl transferase inhibitors, were isolated from Myriococcum albomyces and Fusarium incarnatum and known to prevent the propagation of HBV, HCV and influenza virus (Kluepfel et al. 1972; Sakamoto et al. 2005; Tafesse et al. 2013).

5.4 Challenge to Antimicrobial Resistance

Antimicrobial resistance has turned up as a growing threat to the human health. It is affecting both the developing and developed nations, and its needs to be addressed immediately to save several lives. In the past 10 years, increasing awareness over this issue has been observed, and this is the right time that we use this occasion to obviate the disaster projected on health.

As per the meeting held on 21 September 2016 to combat the antimicrobial resistance, the UN General Assembly has decided to organize a global effort with certain plans. Peter Thomson, president of the UN General Assembly, informed the representatives that the entire world should join hands to fight against this deadly factor with the integrative approaches. Besides endorsing WHO’s current antimicrobial resistance plan and 2030 Agenda for Sustainable Development, the chapter represents a pledge by each member country (Muoio and Adalja 2016):

  1. (a)

    To organize and manage investment into novel therapeutic technologies, research and investigation.

  2. (b)

    To develop multisectoral programmes and strategies concentrated on antimicrobial resistance.

  3. (c)

    To raise alertness of antimicrobial resistance to inculcate positive activities from the general public.

  4. (d)

    To demand the launch of an ad hoc interagency coordination group in consultation with WHO, the Food and Agricultural Organization.

Though antibiotics are treated as a lifesaver globally, the CDC has reported that around 50% of the antibiotics that are prescribed for the patients are actually not desirable (CDC report 2013). The overuse of antibiotics commonly comes from recommending these medications to cure nonbacterial ailments for which they are inadequate. For instance, in USA, more than 60 million cases/year of viral flu are suggested antibiotics (CDC-About Antimicrobial resistance n.d.). In USA, every year, the number of people acquiring antibiotic-resistant bacterial infection is around two million, and 23,000 people die because of the chronic infection. On the other hand, over usage of these drugs might pose yeast infection in intestine, certain allergic reactions and diarrhoea (Bartlett 2002). On the contrary, no or late antibiotic treatment in case of bacterial infections is quite normal (Houck et al. 2004). This may decrease the risk of antibiotic-resistant bacteria but might pose a serious threat to life, leading to mortality (Little 2005; Spiro et al. 2006).

5.5 Future of Natural Medicine

Both microbes and medicinal herbs will endure to play a better part in pharmaceutical industry since only 0.001% of microbes and half a million plants are explored in world till date. Scientists will come up with astonishing metabolites to treat the life-threatening diseases in the near future (Singh 2015). Herbal medicine is extensively used across the globe in a constant way. This has led to the trend in replacing the synthetic drugs with the conventional medicine. However, plants/microbes are prone to contamination or infection, making their recovery and usage a critical issue. Hence, they should be subjected to a set of tests before the extraction process to ensure the safety and efficacy of the drug (Clark 1996; Firenzuoli and Gori, 2007).

One of the challenges that is imposed on herbal therapy is the extensive use of the whole plants leading to the threat of extinction. Habitat destruction, increasing population and wildlife resource exploitation are the serious problems that are associated with this therapy (Bentley 2010). However, they can be solved by wise usage, proper plantation and harvest of the medicinal plants.

5.6 Conclusion

There is no doubt that both plants and microbes had made the entire globe to accept the fact that secondary metabolites derived from them turn to be a lifesaver. This is well correlated with the recent improvements in the research field. The development of new medicines and advanced approaches towards the disease diagnosis will surely combat the threatening antibiotic resistance. On the other hand, personalized medicine is also gaining more attention. Henceforth, both microbial and plant products will serve the pharmaceutical needs of the human population in a safe way.