Keywords: Marine algae, Electrospinning, Nanofibers, Food pathogens, Biopolymers
In the modern world, the consumption of packaged foods by the people has been increased day by day. However, the most significant hurdle of the food industry is the limited shelf life of packaged food products due to contamination by food spoiling pathogens which results in a global public health issue, trade, and the economy. Given access to improper food preservation, bacteria and fungi rapidly colonize, increase in population that leads food spoilage (Hammond et al., 2015). The World Health Organization reports that unsafe food results in the illnesses of at least 2 billion people worldwide annually and can be deadly (Sharif et al., 2017). Addition of synthetic antimicrobial agents effectively controls the growth of food contaminants and extends the shelf life of foods (Irkin and Esmer, 2015). Though, recent toxicological studies indicate that specific concentrations of synthetic preservatives and their continuous use may be potentially mutagenic and genotoxic. For example, Sales et al. (2018) recently proved that artificial synthetic additives induced the formation of micronuclei in the bone marrow erythrocytes and believed as cytotoxic and genotoxic in the animal study. This apprehension rising consumers demand additional of natural preservatives that must be non-toxic as well as excellent defensive from microbial attack (Gyawali and Ibrahim, 2014; Brandelli and T.M. Taylor, 2015; Piran et al., 2017). Therefore, search for new alternatives to preserve foods is of great interest in the food industry. Natural antimicrobials attract considerable attention in the food industry because these substances would not cause any toxic or undesirable effect on the consumers (Brandelli and T.M. Taylor, 2015; Wang et al., 2018).
Natural bioactive compounds from many plants, bacteria and animal sources for food application have been extensively studied by various researchers and reported the possibility of commercial use. However, each source has own disadvantages and thus that bottleneck 100% usage for commercial purpose. Most of the essential oils from plants shows instability (Moghimipour et al, 2012) and very less effective against gram-negative bacteria (Tiwari et al., 2009; Naik et al., 2010; Nazzaro et al., 2013). Moreover, overuse of bacteriocins can lead to resistant pathogens (Cavera et al., 2015), loss of their activity by proteolytic enzymes (Bradshaw, 2003; Fahim et al., 2016). Some of the animal source antimicrobial peptides like lysozyme and pleurocidin are not showing strong effects on gram-negative bacteria (Aloui and Khwaldia, 2016) and was inhibited by magnesium and calcium in the foods which may limit the use (Tiwari et al., 2009). There is an urgent need of new and alternate to above mentioned antimicrobial agents. In the recent years, seaweeds have been recognized as one of the wealthiest and most unexplored new source of antimicrobial compounds and nanofibers for therapeutic and food preservation. This review focus on various antimicrobial compounds extracted from marine algae and their biochemical compositions, antimicrobial activities against food pathogens. Also, this article pivots the potential use of marine algae for nanofibers synthesis used for incorporating antimicrobial agents for greater delivery and their stability during food preservation.
2. Natural antimicrobial compounds from marine algae
Over the last few years marine algae has attracted many researchers worldwide to isolate high value bioactive compounds for food and pharmaceutical industries due to its broad spectrum of various biological activities such as antioxidant, antibacterial, antifungal, anticancer, anti-inflammatory and antidiabetic (Thomas and Kim, 2011; Eom et al., 2012; Hamed et al., 2015; El Shafay et al., 2016; Sathya et al., 2017; Davoodbasha et al., 2018). Algae are the fastest growing plant in the world and generally divided into macroalgae and microalgae based on morphology. The macroalgae or “seaweeds,” are more abundant, multicellular plants growing up to 60 meters in length growing in an ocean. Microalgae are microscopic, mostly existing as small cells of about 2–200 µm and habitat in fresh, sea and even wastewater systems (Sirajunnisa and Surendhiran, 2016). Generally, marine algae are categorized into four main groups namely Rhodophyceae (red algae), Chlorophyceae (green algae), Phaeophyceae (brown algae) for the attribution of different pigments like phycobilins, chlorophyll and fucoxanthin respectively (Kadam et al., 2013; Barbosa et al., 2014). Asian countries like China, Japan, and Korea, used seaweeds for medicinal and food purposes since prehistoric times (Thomas and Kim, 2011).
Many research reports revealed that marine algae could act as a potential alternative source of antimicrobial agents because of their functional groups with excellent antibacterial activity include phlorotannins, fatty acids, polysaccharides, peptides, terpenes, polyacetylenes, sterols, indole alkaloids, aromatic organic acids, shikimic acid, polyketides, hydroquinones, alcohols, aldehydes, ketones, and halogenated furanones, alkanes, and alkenes (Barbosa et al., 2014; Shannon and Abu-Ghannam, 2016; Sathya et al., 2017; Pina-Pérez et al., 2017; Zouaoui and Ghalem, 2017). Screening for bioactive compounds from marine algae with antimicrobial properties under exploited to be employed in food applications. Hence, the research has moved towards to find natural antimicrobial compounds against food pathogens and to replace synthetic compounds. Various primary bioactive compounds from marine algae are shown in Fig.1. Antimicrobial agents from terrestrial plants such as spices and herbs and their antimicrobial activity against food pathogens have already been well documented in many kinds of literature. There are more than 100,000 species of algae existing on earth (Sirajunnisa and Surendhiran, 2016). However, information about their potential activity against food pathogens is sparse since it is a recent field of research worldwide. Recently, some research reports have been published on antimicrobial potential of bioactive compounds extracted from marine algae against food pathogens and obtained notable positive results by various researchers globally. For instance, Rajauria et al. (2012) reported that methanol extract of polyphenolic compounds from the Irish brown seaweed Himanthalia elongates showed potent bactericidal activity against Gram-positive Listeria monocytogenes and Enterococcus faecalis and Gram-negative Pseudomonas aeruginosa and Salmonella abony at a concentration of 60 mg/mL. Dussault et al. (2016) reported that low concentrated algal extracts (?500 µg/ml) from Padina and Ulva sp. showed potential antimicrobial activity against Gram-positive foodborne pathogens such as Listeria monocytogenes, Bacillus cereus, and Staphylococcus aureus. A summary of antimicrobial agents from marine algae and their antimicrobial activities against food pathogens is shown in Table 1.
Marine algae contain many different kinds of polysaccharides as their storage compounds and show good antibacterial, antiviral and antioxidant property. Many of them are soluble dietary fibers (Chojnacka et al., 2012) and could be converted into nontoxic bioactive oligosaccharides by simple hydrolysis (Pina-Pérez et al. 2017). For example, sulphated polysaccharides from seaweed, Chaetomorpha aerea containing alginates, fucoidans and laminaran showed potent antimicrobial activity against food pathogens E. coli and Staphylococcus aureus at the MIC concentration of 50 mg/mL of extract (De Jesus Raposo et al., 2015). Kadam et al. (2015) recorded the remarkable effect of ultrasound assisted extraction of laminarin from two Irish brown seaweeds Ascophyllum nodosum and Laminaria hyperborean against essential food pathogens such as Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, and Salmonella typhimurium. Another report published by Pierre et al. (2011) that carrageenans and the sulphated exopolysaccharide from the red microalga Porphyridium cruentum are effectively inhibiting one of the most important foodborne pathogen, Salmonella enteritidis. Treating Helicobacter pylori, is one of the most dangerous foodborne pathogen, is big challenge till now which affecting 50–80% of the worldwide population and responsible for gastric ulcer (Pina-Pérez et al., 2017). Chua et al. (2015) successfully inhibited the growth of H.pylori using sulphated polysaccharide fucoidan isolated from edible brown algae Fucus vesiculosus at the concentration of 100 µg /mL. Moreover, Araya et al. (2011) demonstrated that fucoidan shows no toxic effects on human trials with the daily intake of 6 grams which reveals the possibilities of applying into food industries.
2.3. Phenolic compounds
Phenolic compounds also known as Polyphenols are a group of tannin compounds that contain hydroxyl (?OH) substituents on an aromatic hydrocarbon moiety. Polyphenols in marine algae include phenolic acids, flavonoids, isoflavones, cinnamic acid, benzoic acid, quercetin, lignans, catechins, anthraquinones, phlorotannins (Chojnacka et al., 2012; Kadam et al., 2013; Pina-Pérez et al., 2017). Among various polyphenols, phlorotannins show excellent, potent free radical scavenging properties than polyphenols derived from terrestrial plants due to eight interconnected phenol rings (Sathya et al., 2017). Phlorotannins are found in many brown seaweeds such as Ecklonia cava, E. kurome, E. stolonifera, Eisenia aborea, Eisenia bicyclis, Ishige okamurae, Pelvetia siliquosa have medicinal and pharmaceutical benefits and have shown strong anti-oxidant, antiinflammatory, antiviral, anti-tumor, anti-diabetes and anti-cancer properties (Eom et al., 2012). Phlorotannins are polymers of phloroglucinol units (1,3,5-trihydroxybenzene) with molecular weights of 126 Da to 650 kDa (Kadam et al., 2013). In recent studies depicted that phlorotannin has excellent antimicrobial activity against food pathogens and gives a roadmap to replace synthetic chemicals for food preservation.
A research group led by Kim et al. (2017) investigated the antimicrobial activity of phlorotannin extracted from edible brown seaweed, Eisenia bicyclis and act against Listeria monocytogenes, is one of the essential food contaminants in the meat processing industry. They evidenced that phlorotannins have excellent anti-listerial activity in the ranges of 16–256 µg/ml. Choi et al. (2010) recorded the potent antimicrobial activity of eckol rich phlorotannin from E.cava against food pathogens methicillin-resistant S. aureus (MRSA) and Salmonella sp. in the range of 125–250 µg/mL. Moreover, some research results concluded that phlorotannin showed no cytotoxic effects on animal models with oral administration (Nagayama et al., 2002; Eom et al., 2012) which is highly suitable for food applications. A team led by Al-Saif et al. (2014), investigated the effects of flavonoids include rutin, quercetin, and kaempferol extracted from marine alga G.dendroides. They recorded the antibacterial activity of these compounds against some critical food contaminants such as E. coli, S. aureus and E. faecalis at the concentration of 10.5 mg/kg (rutin), 7.5 mg/kg (quercetin) and 15.2 mg/kg (kaempferol). Besides, marine algae can be directly added into human foods such as breads, pizza, cheese, pasta and meat products (Pina-Pérez et al., 2017) and used for edible coatings to preserve food products (Sánchez-Ortega et al., 2014; Pina-Pérez et al., 2017) which would add additional benefits of using marine algae in food industries.