Keywords

16.1 Introduction

Plant diseases are the major economic burden in agricultural food production, caused by bacteria, viruses, and fungi. Most plant diseases, which are caused by bacteria, can cause severe economic damage from crop losses due to spots, mosaic patterns, or pustules on leaves and fruits leading to the death of plants. Some bacteria cause hormone-based distortion of leaves and shoots; others cause crown gall, which is a proliferation of plant cells that produces swelling at the intersection of stem and soil and roots (Nivas et al. 2016).

The use of synthetic chemicals has been found to be effective in controlling plant diseases; however, the use of these chemicals induces genetic resistance in fungal and bacterial populations and creates hazardous environment for both human beings and other flora and fauna because of their nonbiodegradable nature. Therefore, the protection of plants from pathogens remains a major concern of agricultural scientists. Technologies such as bacteriophages and systemic acquired resistance have been under investigation for several years to manage plant disease; these technologies have been showing promising outcomes in managing plant diseases (Obradovic et al. 2005; Huang et al. 2012). According to Ocsoy et al. (2013), bacteriophages pose a challenge to field conditions due to limited phage viability and the specific environmental requirements for their multiplication.

Economic losses arising from crop diseases caused by phytopathogenic organisms are principally associated with yield reductions affecting crop quality and safety as well as undermining both consumer confidence and profitability to the producers (Kavitha and Satish 2011). Control of plant diseases is very critical to the reliable production of food, and it provides significant reductions in agricultural use of land, water fuel, and other inputs (Pal and Gardener 2006). Integrated Pest and Disease Management (IPDM) has recognized the importance of medicinal plants and their derivatives (extracts, essential oils, and decoctions) in crop protection (Ragsdale 2000). The potential of medicinal plants as source for new botanicals, fungicides, or bactericides is still largely unexplored. Exploitation of naturally available chemicals from plants, which retards the reproduction of undesirable microorganisms, would be a more realistic and ecologically sound method for plant protection and will play a prominent role in the development of future commercial pesticides and crop protection strategies (Gottlieb et al. 2002).

According to Aromal and Philip (2012), plant crude extract contains novel secondary metabolites such as phenolic acid, flavonoids, alkaloids, and terpenoids in which these compounds are mainly responsible for the reduction of ionic into bulk metallic nanoparticle formation. Kuppusamy et al. (2016) reported that primary and secondary metabolites are constantly involved in the redox reaction to synthesize eco-friendly nanosized particles. Biological methods of nanoparticle synthesis using microorganisms, enzymes, fungi, and plants extracts have been suggested as possible eco-friendly alternatives to chemical and physical methods (Ponneerselvam et al. 2012; Prasad et al. 2016, 2018a; Abdel-Aziz et al. 2018). Synthesis of nanoparticles using plant extracts can be beneficial over other biological processes by eliminating the elaborate processes of maintaining microbial cultures (Ponneerselvam et al. 2012; Prasad 2014; Shankar et al. 2004).

Foodborne diseases encompass a wide spectrum of illnesses and are a growing public health hazard worldwide. They are the result of ingestion of foodstuffs contaminated with microorganisms or chemicals. Foodborne diseases have been noticed as serious threats to public health all over the world. In foodborne pathogen studies, four major pathogens have emerged significantly important in terms of human health and disease. These include Escherichia coli O157:H7, Listeria monocytogenes, Salmonella typhimurium, and Vibrio parahaemolyticus. These organisms have frequently been associated with food products and linked to a number of human illness cases (Zarei et al. 2014). The worldwide statistics on foodborne diseases published for 2011–2012 by the Centers for Disease Control and Prevention reported a total of 1632 outbreaks, 29,112 affected patients, 1750 hospitalizations, and 68 deaths. Salmonella (31%), Listeria (28%), Campylobacter (5%), and Escherichia coli O157:H7 (3%) pathogens are reported to cause some of the foodborne diseases and eventual death (Inbaraj and Chen 2016).

The impact of foodborne disease is a significant economic and clinical issue, despite recent advances in food safety (Billington et al. 2014). It has been concluded by Joint Food and Agriculture Organization/World Health Organization Expert committee on food safety that illness due to contaminated food was perhaps the most widespread health problem in the contemporary world and important cause of reduced economic productivity (Käferstein et al. 1997). Reducing the occurrence of foodborne illness through the use of rapid, cost-effective detection procedures and new ways to control pathogens are the focus of much current research (Billington et al. 2014).

Nanotechnology is an emerging field of interdisciplinary research that includes all spheres of science starting from physics, chemistry, biology, and especially biotechnology (Natarajan et al. 2010), which collectively describes the technology and science involving nanoparticles. Nanoparticles are a group of materials synthesized from a number of metals or nonmetal elements with distinct features and extensive applications in different fields of science and medicine (Matei et al. 2008). Nanoparticles also have potential biological applications, such as biosensing, catalysis, drug delivery, imaging, nanodevice fabrication, and for use as antimicrobial agents and in medicine (Ghosh et al. 1996; Geddes et al. 2003; Nair and Laurencin 2007; Jain et al. 2008; Sharma et al. 2009; Zargar et al. 2014; Prasad et al. 2016, 2018a, b; Patra and Baeke 2017).

Green synthesis of metal nanoparticles has been achieved using environmentally acceptable solvents from plant extracts has been achieved with the benefit of rapid, low-cost, eco-friendly, and a single-step method for biosynthesis process (Ponneerselvam et al. 2012; Huang et al. 2007). Plant extracts have recently been used for green synthesis of nanoparticles, since they are rich in bioactive compounds. The potential of biomolecules present in plant extracts to reduce metal ions to NPs in a single-step green synthesis process is very important (Benakashani et al. 2016). Several metal ions, such as Ag+, Au3+, Zn2+, and Cu2+, have been used to inactivate bacterial growth; among the different types of nanoparticles, Ag NPs have been used as effective biocides against a variety of pathogens, fungi, and viruses (Ocsoy et al. 2013). Ag NPs release Ag+ ions that interact with the thiol groups in bacterial proteins and affect the DNA replication, resulting in the destruction of bacteria (Marini et al. 2007). Additionally, nanoparticles have been shown to have potential antibacterial activity and significantly higher synergistic effects when applied with many antibiotics (Devi and Joshi 2018; Aziz et al. 2015, 2016).

This chapter reviews the latest research development and mechanism of action during green synthesis of silver nanoparticles (Ag NPs) utilizing of plants extract, the antimicrobial activity, and mechanism of action of Ag NPs against agriculture (phytopathogens) and foodborne pathogens. Second, the benefits of using green synthesis of Ag NPs as new antimicrobial agent are explored. Finally, we highlight the potential impacts of plant-mediated synthesized Ag NPs on agriculture and food sector and economic benefits in sub-Saharan Africa (SSA) countries.

16.2 Green Synthesis of Silver Nanoparticles Using Plant Extract

16.2.1 Plant Broth Preparation from Plant Extract

Different plant materials have been used for the preparation of plant broth for green synthesis of Ag NPs such as seeds, roots, stems, leaves, flowers, and fruits (Phanjom et al. 2012; Prasad 2014; Ahmed et al. 2016; Dhayalan et al. 2017; Khan et al. 2018).

In order to prepare plant broth for green syntheses of Ag NPs, the following simple steps have to be pursued:

  • Step 1: Collection of plant materials is the first step for the preparation of plant broth from plant materials.

  • Step 2: Washing step, part 1; in this step, plant materials are washed with tap water to remove epiphytes and necrotic plants. This process can be done twice or more.

  • Step3: Washing step, part 2; this is another washing step; however, in this step, plant materials are washed with sterile distilled water in order to remove any debris or potential contaminants.

  • Step 4: Drying stage; in this stage, materials are kept in the shade for 10–15 days until they dried; this is done in order to protect degradation of plant materials.

  • Step 5: Preparation of plant extract/powder; in this step, plant materials are blended with domestic blender in order to obtain dried powder.

  • Step 6: Preparation of plant broth; in order to prepare plant broth from dried powder, 10 g of powder or plant extract was boiled in 100 ml of deionized water or using sunlight as the primary source of energy.

  • Step 7: Filtration; in this step all insoluble materials are filtered out using microfilter until all insoluble materials are no longer visible in the plant broth (Fig. 16.1).

Fig. 16.1
figure 1

Schematic illustration of seven (7) steps for preparation of plant broth from plant extract

Full size image

16.2.2 Single-Step Method for Green Synthesis of Silver Nanoparticles Using Plant Extract

Green synthesis is a simple synthesis technique that utilizes microorganisms and plant extract to form metallic nanoparticles with different sizes and shapes. This technique does not require any chemical reducing agent for the formulation of metal nanoparticles, because they contain highly toxic substances (Roy and Das 2015).

Single-step technique for the synthesis process of Ag NPs involved the mixing 1 mM final concentration of silver nitrate (AgNO3) solution and plant broth (plant extract) through magnetic stirrer at room temperature or using sunlight (Bindhani and Panigrahi 2015). The formation of Ag NPs is confirmed by color change in the mixture due to the reduction of pure Ag+ ions to zero valent state Ag 0 just after 2–5 minutes (Fig. 16.2). UV-visible spectra analysis showed a strong surface plasma resonance and absorption property that confirmed the formation of nanoparticles. The shape, size, surface area analysis, morphology, oxidation state, and polydispersity of Ag NPs were characterized using various techniques such as dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), auger electron spectroscopy (AES), X-ray diffraction (XRD), low energy ion scattering (LEIS), energy dispersive spectroscopy (EDS), and Fourier-transform infrared spectroscopy (FTIR) (Kumar et al. 2007; Aguilar-Mendez et al. 2010; Banerjee et al. 2014; Park 2014; Bindhani and Panigrahi 2015; Lateefa et al. 2016; Joshi et al. 2007; Joshi et al. 2018).

Fig. 16.2
figure 2

Schematic illustration of green synthesis of silver nanoparticles from plant extracts (Gholami-Shabani et al. 2017)

Full size image

16.2.3 The Mechanism of Silver Nanoparticle Synthesis Using Plant Extracts

The mechanism of synthesis of Ag NPs using plant extracts is influenced by biomolecules such as flavonoids, protein, vitamins, citric acid, ascorbic acids, carbohydrates, carboxylic acids, aldehydes, hydrocarbons, amides, and ketones (Fig. 16.2). The reduction of pure Ag (+) ions into Ag(0) nanoparticles is could be achieved by blending of silver ions together with plant extracts, which can act as reducing and stabilizing agents (Prabhu and Poulose 2012; Kulkarni and Muddapur 2014). A study conducted by Tran et al. (2013) showed that the presence of carbonyl group of amino acid and proteins facilitated the reduction and capping of metal ions to form Ag NPs that are very stable. Another study conducted by Makarov et al. (2014) showed that the presence of free aldehyde group of glucose used to facilitate ion reduction during the formation of Ag NPs. Roopan et al. (2013) indicated that the presence of hydrocarbons as a stabilizing agent influenced the formation of stable Ag NPs. The concentration of AgNO3 and different experimental environments (e.g., temperature and pH) also shown to govern ion reduction process during nanoparticle synthesis. The temperature improves the reaction rate and efficiency of nanoparticle synthesis. Different pH values of the biomolecules from various plant extracts showed to influence capping process during nanoparticle formation (Makarov et al. 2014; Mohammadlou et al. 2016).

16.2.4 Green Synthesis Approaches to Synthesis of Silver Nanoparticles

Many plant extracts have been used for green synthesis of Ag NPs (Table 16.1), such as Moringa oleifera (Moodley et al. 2018), Buchu plant extracts (Chiguvare et al. 2016), stem bark hydrosol of Acacia mearnsii (Avoseh et al. 2017), root bark aqueous extract of Annona muricata Linn (Ezealisiji et al. 2017), plant extract from Ziziphus spinachristi and Garcinia kola (Abalaka et al. 2017), aqueous leaf extract of Costus afer (Elemike et al. 2017), and pod extract of Cola nitida (Lateef et al. 2016). Moodley et al. (2018) reported green synthesis of Ag NPs from the leaf extracts of Moringa oleifera. The study demonstrated the use of sunlight as the primary source of energy during the reaction mixture of 1 mM aqueous AgNO3 and aqueous plant extract. The FTIR study confirmed the presence of biomolecules from the extract to be flavones, polysaccharides, and terpenoids. The biomolecules have been shown to be responsible for both reduction and capping of silver ions during the formation of Ag NPs. The formation of Ag NPs was confirmed by color change from initial reagent yellow solutions to dark brown mixture solution and validated by surface plasmon resonance peak between 450 nm and 440 nm. The nanoparticle sizes were confirmed by X-ray diffraction analysis and DLS and were found to have particle size ranging from 3 to 50 nm. Chiguvare et al. (2016) have reported a green synthesis of Ag NPs from Buchu plant extracts from South Africa. Phytochemical screening of the crude using FTIR revealed the presence of proteins, flavonoids, alkaloids, and saponins. The presence of phytochemicals facilitated the reduction of silver ions during the formation of Ag NPs. The TEM analysis confirmed the morphology of Ag NPs as spherical in shape and particles size in a range between 19.95 and 7.76 nm, respectively. Avoseh et al. (2017) demonstrated a rapid green synthesis of Ag NPs using stem bark hydrosol of Acacia mearnsii within 15 minutes at 60 °C. The presence of plant extract facilitated ion reduction and capping process during the formation of Ag NPs. TEM images confirmed that the presence of spherical Ag NPs with the diameter in the range of 19.95 ± 7.76 nm. Ezealisiji et al. (2017) also reported green synthesis of Ag NPs using the root bark aqueous extract of Annona muricata with an average particle size of 22 ± 2 nm. In this study, Malvern Nano ZS was used to reveal zeta potential of – 27.90 ± 0.01 mV, a negative surface charge, and the polydispersity index (PDI) of 0.44 ± 0.02. Abalaka et al. (2017) reported synthesis of Ag NPs using plant extract from Ziziphus spinachristi and Garcinia kola. The plant extract acted as reducing and capping agent; however, in this study, hyaluronic acid was added to enhance particle stability and to prevent aggregation during synthesis. The presence of nanoparticles, particle size, and shape was determined by TEM. Elemike et al. (2017) reported synthesis of Ag NPs employing aqueous leaf extracts of Costus afer with a mean diameter of 20 nm. Loo et al. (2012) reported the synthesis of Ag NPs using tea leaf extract of Chinese tea from Camellia sinensis, with a particle size of 4 nm. Lateef et al. (2016) demonstrated the synthesis of Ag NPs employing pod extracts of Cola nitida. The formation of Ag NPs was observed by the color change in the solution from yellow brown to dark brown. The presence of nanoparticles was confirmed by UV-visible spectroscopy and TEM images with a spherical shape and the diameter in a range between 12 and 80 nm. Yadav et al. (2018) reported the synthesis of spherical Ag NPs with a size ranging from 40 to 95 nm using leaf extract of Ocimum sanctum and Ocimum americanum. Sowmya et al. (2018) reported synthesis of Ag NPs from leaf extract of Phyllanthus acidus. The physicochemical characterization revealed the formation of Ag NPs with a spherical shape, and the average size ranged from 65 to 250 nm. Very recently, Singh et al. (2018) reported synthesis of Ag NPs using dried tulsi leaves. The catalytic activity during the formation of Ag NPs was evaluated through the reduction of 4-nitrophenol to 4-aminophenol in alkaline medium. The formation Ag NPs was revealed by UV-visible spectrum with peak at 430 nm and TEM image with the average diameter of 5–10 nm. The study conducted by Balashanmugam et al. (2016) showed synthesis of Ag NPs to be influenced by different physicochemical conditions. Highly stable Ag NPs were synthesized with 1.0 mL of C. roxburghii leaf extract and 1.0 mM AgNO3 (pH 7.0) at 37 °C. The synthesized Ag NPs were characterized by XPS, DLS, and ZETA potential. In DLS and ZETA potential analysis, the average size of Ag NPs was 35 nm, and the zeta potential was −18.3 mV. The studies by Chahardooli et al. (2014) showed green synthesis of Ag NPs characterized by UV-visible spectroscopy gave surface plasmon resonance for synthesized Ag NPs peak at 415–445 nm. Further, the Ag NPs showed an effective antibacterial activity toward plant pathogenic bacteria (Pectobacterium carotovorum, Ralstonia solanacearum, Erwinia amylovora, and Xanthomonas citri). The synthesized Ag NPs were characterized using UV-visible spectroscopy, dynamic light scattering spectroscopy (DLS), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy. The DLS study revealed the surface charge of the resulting nanoparticles that was highly negative, i.e., −25.0 ± 7.84 mV, and the size was 74.56 ± 0.46 nm. The phytochemical and FTIR analysis confirmed the role of water-soluble phyto-compounds for the reduction of silver ions to silver nanoparticles. The biosynthesized Ag NPs were characterized by UV-visible spectrophotometry with surface plasmon resonance at 450 nm followed by the analysis using scanning electron microscope, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis.

Table 16.1 Plant-mediated synthesis of silver nanoparticles
Full size table

16.2.5 The Advantages of Using Green Synthesis of Silver Nanoparticles

Metallic nanoparticle synthesis using green chemistry route has recently received a lot of attention as nanofactories over the conventional methods. The conventional methods such as chemical and physical synthesis involving toxic chemicals are harmful to humans and the environment (Patra and Baek 2014). The use of plant extracts and microorganisms toward biosynthesis of Ag NPs promises to overcome setbacks associated with conventional synthesis. Green synthesis for the formation of Ag NPs employs aqueous plant extracts as reducing and stabilizing agents during the synthesis of Ag NPs (Suna et al. 2014; Sadeghi and Gholamhoseinpoor 2015; Mohammadlou et al. 2016). This approach offers simplicity, rapid synthesis, and inexpensive biological procedure for nanoparticle fabrication and is environment friendly and nontoxic (Ahmed et al. 2016). Additionally, green synthesis approaches are easy to scale up for large-scale production of nanoparticles and are economically viable. Although plant and microbiological approach promises to overcome the setbacks associated with conventional methods during metallic nanoparticle synthesis, the use of plant extract during green synthesis is considered to be more advantageous and safer over microorganism approach. During metallic nanoparticle synthesis, plant extracts are able to reduce and cap metal ions faster than bacteria, fungi, and viruses (Iravani 2011). Additionally, microorganisms are associated with biohazards and have setbacks toward nanoparticle isolation and identification. The approach is not eco-friendly particularly toward maintaining cell culture processes for microorganisms (Kalishwaralal et al. 2010; Roy and Das 2015). The toxicity of Ag NPs is shown to influence the type of plant extract used and also by nanoparticle size, concentration, dosage, pH of the medium, and exposure time to pathogens (Banerjee et al. 2014; Das et al. 2018; Kumar et al. 2018). In addition, green synthesis of Ag NPs is biocompatible to the human cell line and nontoxic to mammalian cells (Ahmed et al. 2016). The study conducted by Balashanmugam et al. (2016) showed synthesis of Ag NPs to be influenced by different physicochemical conditions. Highly stable Ag NPs were synthesized with 1.0 mL of C. roxburghii leaf extract and 1.0 mM AgNO3 (pH 7.0) at 37 °C. The synthesized AgNPs were characterized by XPS, DLS, and ZETA potential. In DLS and ZETA potential analysis, the average AgNPs size was 35 nm, and the zeta potential was −18.3 mV. Chahardooli et al. (2014) reported green synthesis of Ag NPs using Protium serratum leaf extract. UV-visible spectroscopy confirmed the formation of Ag NPs with surface plasmon resonance peak at 415–445 nm. The synthesized Ag NPs were characterized using UV-visible spectroscopy, dynamic light scattering spectroscopy (DLS), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy. The DLS study revealed the surface charge of the resulting nanoparticles that was highly negative, i.e., −25.0 ± 7.84 mV, and the size was 74.56 ± 0.46 nm. The phytochemical and FTIR analysis confirmed the role of water-soluble phyto-compounds for the reduction of silver ions to silver nanoparticles. The biosynthesized AgNPs were characterized by UV-visible spectrophotometry with surface plasmon resonance at 450 nm followed by the analysis using scanning electron microscope, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. The AgNPs displayed moderate antibacterial activity (9.26–11.57 mm inhibition zone) against all five foodborne pathogenic bacteria. Patra and Baeke (2017) reported biosynthesis of Ag NPs using the aqueous extract of corn leaf waste of Zea mays. Ag NPs were characterized by UV-visible spectrophotometry with surface plasmon resonance at 450 nm followed by the analysis using scanning electron microscope, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis.

16.3 Plant and Foodborne Pathogens

Pathogenic microorganisms affecting plant health are a major and chronic threat to food production and ecosystem stability worldwide. As agricultural production intensified over the past few decades, producers became more and more dependent on agrochemicals as a relatively reliable method of crop protection helping with economic stability of their operations. However, increasing use of chemical inputs causes several negative effects, i.e., development of pathogen resistance to the applied agents and their nontarget environmental impacts (Compant et al. 2005). Foodborne pathogens are also posing a big threat to the public health and food security. Bacteria, fungi, viruses, and parasites are the main cause of foodborne disease worldwide. Salmonella typhimurium, Escherichia coli, Listeria monocytogenes, Vibrio parahaemolyticus, Vibrio cholera, Pseudomonas aeruginosa, and Streptococcus pyogenes are the major bacterial pathogens that cause foodborne illness. Current traditional antibacterial and disinfection agents are facing the challenge of bacterial resistance, which result in food spoilage and outbreaks with high mortality. Escherichia coli O157:H7 has been reported to be resistant to ampicillin and Streptococcus pyogenes resistant to erythromycin (Armstrong et al. 1996; Jay 2000; Lara et al. 2010). In the agriculture and food industry, microbial pathogens are threatening food production, food quality, and food security by failing to protect plant crops, maintaining food quality, and shortening food shelf life (Tareq et al. 2017; Prasad et al. 2017). Therefore, there is an urgent need to develop a new generation of antimicrobial agents that are effective against both plant and foodborne pathogens. In sub-Saharan Africa countries, where the economy of the majority of the countries is heavily dependent on agriculture and food production, and where there are no appropriate systems to deal with foodborne disease outbreaks or advanced analytical tools to analyze food samples, developing a new generation of antimicrobial agents is an issue of high priority. Ag NPs promise to overcome challenges associated with plant and foodborne pathogens by offering an effective antimicrobial agent that could protect crops, extend food shelf life, and maintain food quality for a longer period (Lara et al. 2010; Zandi et al. 2013; Rajeshkumar and Malarkodi 2014; Zarei et al. 2014; Tareq et al. 2017).

16.4 Antimicrobial Activity of Silver Nanoparticles on Plant and Foodborne Pathogens

Green synthesis of Ag NPs promises to offer an effective antimicrobial agent against plant and foodborne pathogens (Jo et al. 2009). According to Jo et al. (2009) and Conrad et al. (1999), silver particles display multiple modes of inhibitory action to microorganisms, it may be used for controlling various plant pathogens in a relatively safer way compared to synthetic fungicides (Park et al. 2006). Ag NPs exhibit new or improved properties depending upon their size, morphology, and distribution, which can be achieved through different (physical and chemical) methods that are employed for the synthesis of metal nanoparticles (Shankar et al. 2004; Panacek et al. 2006). Krishnaraj et al. (2012) tested the inhibitory effect of fungal plant pathogens, namely, Alternaria alternata, Sclerotinia sclerotiorum, Macrophomina phaseolina, Rhizoctonia solani, Botrytis cinerea, and Curvularia lunata using different concentrations of Ag NPs. Interestingly, 15 mg concentration of Ag NPs showed excellent inhibitory activity against all the tested pathogens. Thus, the obtained results clearly suggest that Ag NPs may have important applications in controlling various plant diseases caused by fungi. Balashanmugam et al. (2016) exhibit no antifungal activity using Ag NPs comparing with the conventional antifungal drug amphotericin B against all the tested plant fungal pathogens R. solani, Fusarium oxysporum, and Curvularia sp. Chahardooli et al. (2014) showed good antibacterial activity against the foodborne pathogens Pseudomonas aeruginosa (74.26 ± 0.14 μg/ml), Escherichia coli (84.28 ± 0.36 μg/ml), and Bacillus subtilis (94.43 ± 0.4236 μg/ml). This finding displayed the potential use of Protium serratum leaf extract as a good bioresource for the biosynthesis of Ag NPs and their implementation in diverse applications, specifically as antibacterial agent in food packaging and preservation to combat against various foodborne pathogenic bacteria (Mohanta et al. 2017). Menon et al. (2017) synthesized silver nanoparticles by using the medicinal plant Acalypha indica, which was characterized using various advanced tools with the help of UV-visible spectrophotometer. The food pathogen strain Aspergillus fumigates showed ZOI (Zone of inhibition) of 133% at 75 μl of concentration proving that Ag NPs can act effectively against this strain when compared to other strains even at low concentrations. The study concluded that Ag nanoparticle can be used for therapeutic purposes and for large-scale synthesis in food industries for food preservation or packaging. Patra and Baeke (2017) evaluated synthesized Ag NPs for their antibacterial activity against foodborne pathogenic bacteria (Bacillus cereus ATCC 13061, Listeria monocytogenes ATCC 19115, Staphylococcus aureus ATCC 49444, Escherichia coli ATCC 43890, and Salmonella Typhimurium ATCC 43174). The anticandidal activity of Ag NPs was evaluated against Candida species (C. albicans KACC 30003 and KACC 30062, C. glabrata KBNO6P00368, C. geochares KACC 30061, and C. saitoana KACC 41238). The AgNPs displayed moderate antibacterial activity (9.26–11.57 mm inhibition zone) against all five foodborne pathogenic bacteria. When Ag NPs were mixed with standard antibacterial or anticandidal agent, they displayed strong synergistic antibacterial (10.62–12.80 mm inhibition zones) and anticandidal activity (11.43–14.33 mm inhibition zones). The findings of Patra and Baeke highlighted the potential use of maize industrial waste materials in the synthesis of Ag NPs and their utilization in various applications, particularly as an antibacterial substance in food packaging, food preservation to protect against various dreadful foodborne pathogenic bacteria together with its biomedical, pharmaceutical-based activities. Chahardooli et al. (2014) displayed moderate antibacterial activity (9.26–11.57 mm inhibition zone) against all five foodborne pathogenic bacteria using green synthesis of Ag NPs using Protium serratum leaf extract.

Table 16.2 shows the effect of Ag NPs against other microbes. Emamifar et al. (2011) showed antimicrobial effect and improved preservation of orange juice storage for more than 112 days against bacteria strain, Lactobacillus plantarum. Min et al. (2009) showed sclerotial germination and growth inhibition against plant pathogens, Sclerotinia minor and Sclerotinia sclerotiorum. Mahdizadeh et al. (2015) showed mycelial growth inhibition caused by plant pathogen, Macrophomina phaseolina. Lara et al. (2010) exhibited bactericidal effect against multidrug-resistant bacteria, namely, Listeria monocytogenes, Escherichia coli O157:H7, Salmonella typhimurium, and Vibrio parahaemolyticus. Zarei et al. (2014) showed antibacterial effects on four important foodborne pathogens, namely, Listeria monocytogenes, Escherichia coli O157:H7, Salmonella typhimurium, and Vibrio parahaemolyticus. Abalaka et al. (2017) demonstrated the antibacterial against both foodborne and human pathogens such as Escherichia coli, Salmonella typhi, and Pseudomonas aeruginosa.

Table 16.2 Effect of plant-mediated silver nanoparticles against plant and foodborne pathogens
Full size table

16.4.1 Mechanisms of Action of Silver Nanoparticles on Microbial Pathogens

Silver ion is known to have microbial effect on a broad range of microorganisms (Bragg and Rainnie 1974; Brown and Smith 1976; Liau et al. 1997); however, the mechanisms of action of Ag NPs on microbes to cause the microbicidal effect is only partially understood (Richards 1981; Russell and Hugo 1994; Matsumura et al. 2002; Morones et al. 2005; Kim et al. 2007; Feng et al. 2008; Dallas et al. 2011; Nasrollahi et al. 2011). Some hypotheses suggest that Ag NPs attached to cell wall cause cell lysis, resulting in structural damage, and cause disturbance to its proper functioning that can lead to the termination of microbial cell life (Rai et al. 2009). Figure 16.3 shows different bactericidal and fungicidal mechanisms using Ag NPs. The study by Morones et al. (2005) showed bactericidal effects employing Ag NPs with size range between 1 and 10 nm against E. coli, P. aeruginosa, and S. typhus. The study showed the accumulation of Ag NPs on the surface of the cell membrane leads to the formation of free radicals, which increased membrane permeability and make it porous resulting into cell death. In addition, Ag NPs were able to penetrate inside the bacteria and bind to DNA bases and disrupt bacteria functions such as DNA replication, respiratory chain, and cell division ultimately leading to cell death (Dallas et al. 2011; Aziz et al. 2014, 2015, 2016). Another study by Prabhu and Poulose (2012) showed another mechanism by which the cells die using Ag NPs; in this study, cell death was showed to be influenced by Ag NPs that bind to ribosome and cause inhibition of protein. Also, it was proposed that Ag NPs enter the cell membrane and interact with respiratory enzyme, cause release of reactive oxygen, and lead to cell damage (Inoue et al. 2002; Choi and Hu 2008). In another study by Kumar et al. (2017), it was proposed that Ag NPs enter into fungal cell membrane and nucleus through phagosome process, then binds to chromosomes and cause chromosomal damage. Many studies showed that the microbial properties of Ag NPs to be more effective and influenced by nanoparticle size. Nanoparticles with a diameter of 1–10 nm are shown to have direct interaction with the cellular components such as cell membrane, DNA, protein, enzymes, and chromosomes, and also have an effect on microbial growth ultimately leading to cell death.

Fig. 16.3
figure 3

Mechanism of action of Ag NPs on microbes (Prabhu and Poulose 2012)

Full size image

16.5 The Potential Benefits of Using Green Synthesis of Silver Nanoparticles in Agriculture and Food Sectors

Applications of nanotechnology in agriculture and the food sector have many benefits. Nanotechnology has many potential benefits in improving food quality and safety, reduction of agricultural inputs, and enrichment of absorbing nanoscale nutrients from the soil (Prasad et al. 2017; Sangeetha et al. 2017a, b). Furthermore, Prasad et al. (2017), Gruère (2012), Mukhopadhyay (2014), Prasad et al. (2014) explain that agriculture, food, and natural resources are a part of those challenges like sustainability, susceptibility, human health, and healthy life; therefore, the importance of nanomaterials in agriculture is to reduce the amount of spread chemicals, minimize nutrient losses in fertilization, and increase yield through pest and nutrient management. In the food industry, Ag NPs promises to overcome the followings setbacks: disinfection during food processing, packaging materials, preservation, food additives, and supplements. In agriculture, nanomaterials can also be used to control agricultural pathogens and protect crops and seeds (Bhattacharyya et al. 2016; Ismail et al. 2017; Gupta et al. 2018). In healthcare, nanomaterials such as Ag NPs promised to impact in medical health device, diagnostic devices, and pharmaceuticals (Zandi et al. 2013). Microbial resistance to current antibiotics and disinfection agents resulting into food spoilage due to foodborne pathogens pose threat to the public health and food security. Therefore, there is an urgent need to develop a new generation of antimicrobial agents that are effective against foodborne pathogens (Parihar et al. 2008; Rajeshkumari and Malarkodi 2014; Zarei et al. 2014).

Green synthesis of Ag NPs are very much stable, environment friendly, biocompatible to the human cell line, and have a low systemic toxicity to humans (Ahmed et al. 2016; Aziz et al. 2019). In food industries, Ag NPs gain attention due to the excellent antimicrobial and antioxidant properties and have been used in food process, food packaging materials, preservation, and also as disinfectant agents. As a disinfectant agent, Ag NPs showed significant effect against foodborne pathogens such as Escherichia coli O157:H7, Listeria monocytogenes, Salmonella typhimurium, and Vibrio parahaemolyticus (Zarei et al. 2014). However, L. monocytogenes, Gram-positive bacteria, show some resistance against Ag NPs when compared to Gram-negative bacteria such as E. coli O157:H7. The study agreed with Kim et al. (2007) that suggested that the effect of Ag NPs as antibacterial agent could be influenced by differences in bacterial cell walls; Gram-negative bacteria have thin cell walls and Gram-positive bacteria have thick cell walls. In addition, Ag NPs could be used for cleaning equipments and surface areas in food-related environments. In the food packaging industry, the presence of Ag NPs showed to have bactericidal effect against foodborne pathogenic bacteria such as Bacillus subtilis, Klebsiella planticola, Klebsiella pneumoniae, Serratia nematodiphila, and Escherichia coli (Rajeshkumari and Malarkodi 2014). The study conducted by Rajeshkumari and Malarkodi (2014) showed the bactericidal effect against foodborne pathogenic bacteria such as Bacillus subtilis, Klebsiella planticola, Klebsiella pneumoniae, Serratia nematodiphila, and Escherichia coli at a high (750 μl) concentration of Ag NPs. As a result, Ag NPs could be used during surface coating of packing material to improve the lifespan of food products and ensure food safety by eliminating bacterial pathogens in food products. Zandi et al. (2013) showed improvement in food quality postharvest, food preservation, and increased storage life for more than 10–12 days (Zandi et al. 2013). Emamifar et al. (2011) showed antimicrobial effect on the inactivation of Lactobacillus plantarum in orange juice stored in packaging materials made up of nanocomposite films containing Ag NPs over a period of 112 days. A recent study by Menon et al. (2017) showed the antifungal activity employing green-synthesized Ag NPs using plant extract against the food pathogens such as Aspergillus niger, Aspergillus flavus and Aspergillus fumigatus. The study showed significant maximum inhibition of Aspergillus fumigatus at 75 μl of Ag NPs. Due to antioxidant properties and fungicidal activity, green synthesis of Ag NPs have the potential to prevent food spoilage caused by foodborne pathogens such as fungi.

16.6 Nanotechnology in Agriculture and the Food Sector: Potential Economic Impact

As silver possesses antibacterial and antifungal properties, Ag NPs have been incorporated into more than 200 consumer products, including clothing, cosmetics, ceramics, car paints, and medical products. According to the Lux Research report 2015 estimates that nanoproducts to have markets value of USD 2.84 trillion (Lux Research 2004; Smith 2008). However, nanoproducts in the agrifood sector are facing challenges when it comes to consumer acceptance; this could be due to risks not clearly communicated to the consumers and uncertainty around nanomaterial toxicity.

In SSA, nanotechnology ensures to impact on low- and middle-income countries, particularly in agriculture and the food sectors. Majority of SSA countries’ economics are heavily dependent on agriculture and food production. However, agricultural pathogens affecting plant health and foodborne pathogens are associated with food wastage, which posed threat to food production, quality, food security, and Africa’s economic growth (Cozzini et al. 2008; Kiaya 2014). The report compiled from national workshop by Nanotechnology Research Group at Nigeria’s Ladoke Akintola University of Technology indicates that inadequate funding on nanotechnology research is a major challenge for nanotechnology development in Africa. In addition, the report also indicates that African countries are too slow in embracing nanotechnology despite the potential benefits it has toward the continent (Rateng 2017). Therefore, African countries need urgently to look at their nanotechnology strategies in order to strengthen Africa’s competitive position in nanotechnology and in order improve the food quality and improve their economy.

16.7 Conclusion

Plant and foodborne diseases pose immense threat to the agricultural food production, food quality, and economic growth for developing countries, particularly for regions such as SSA. Nanotechnology is an emerging technology that is promising to the development of a new generation of antimicrobial agents to fight and prevent disease at atomic scale and molecular level. The use of Ag NPs as a new generation of antimicrobial agents enables the inhibition of harmful pathogens to grow, multidrug-resistant bacteria, antioxidant property, improve food quality, packaging, storage, and plant disease control. The generation of Ag NPs using plant extracts promises to offers inexpensive, rapid, environment friendly, and biocompatible to human cell line and low systemic toxicity to humans. In addition, green synthesis approaches are easy to scale up for large-scale production of nanoparticles. Although, green synthesis promises nanotherapy with low toxicity, there is a need for the analysis of commercial viability and economic value associated with the use of nanoparticles generated through plant extracts. Further, in-depth study on the mechanism of action and the properties of Ag NPs against antibacterial and antifungal agents in the mammalian immune system is required. SSA countries need to begin prioritizing on nanotechnology in order improve their agricultural food production and food quality and for their economic growth.