Antimicrobial mechanism of action of marine algal compounds remains unclear still, and only limited information has been specified by some researchers. Different concepts have been proposed by some investigators. Most of the studies depicted that cell wall and cell membrane damage are primary targets of marine algal antimicrobial compounds (Wang, Xu, Bach, & McAllister, 2009; Smith, Desbois, & Dyrynda, 2010; Guedes, Barbosa, Amaro, Pereira, & Malcata, 2011; Hierholtzer, Chatellard, Kierans, Akunna, & Collier, 2012; Wei et al., 2016; Falaise et al.
, 2016). Mechanism of action of different natural antimicrobial compounds from marine algae on bacteria is shown in Fig.2.
Antibacterial mechanism of the marine algal polysaccharide is due to glycoprotein-receptors present on the cell-surface of polysaccharides which binds with compounds in the bacterial cell wall, cytoplasmic membrane, and DNA. This action resulted in increased permeability of the cytoplasmic membrane, protein leakage and damaging of bacterial DNA finally leading to cell death (He, Yang, Yang, & Yu, 2010; Pierre et al., 2011; Shannon and Abu-Ghannam, 2016).
An important mechanism was proposed by Shannon and Abu-Ghannam (2016) that the marine algal fatty acids target essential metabolic pathways in the bacterial cell, i.e., electron transport chain (ECP) and oxidative phosphorylation, which take place in cell membranes. These resulted in damages of adenosine triphosphate (ATP) energy transfer and hindered enzymes such as bacterial enoyl-acyl carrier protein reductase, essential for the production of fatty acids within the bacterial cell. Another report claimed by Guedes et al.
(2011) that the possible mechanism of action of fatty acids in cell leakage due to morphological damages of the outer cell membrane. It was corroborated by the recent research findings of El Shafay, Ali, Mostafa, & El-Sheekh (2016). They elucidated the mechanisms of action of fatty acids extracted from seaweeds, Sargassum vulgare and S. fusiforme on gram-positive bacteria Staphylococcus aureus and gram-negative Klebsiella pneumoniae using Transmission electron microscopy (TEM). The results showed that shrunken, ruptured and distorted shape of bacteria cells were observed due to puncture of the cell wall. Not all the marine algal bioactive compounds demonstrate potent effects on all groups of bacteria.
It depends on algal species, the efficiency of the extraction method and concentration of the compositions present in the extracts. For example, diethyl ether extract of red seaweed S. fusiforme showed high activity against gram-positive bacteria S.aureus; however, the methanol extract of same marine algae showed higher inhibition activity against gram-negative bacteria P.
aeruginosa (El Shafay et al., 2016). Another example, the phycobiliproteins, and exopolysaccharides from the red microalgae Porphyridium aerugineum and Rhodella reticulata respectively were active against gram-positive bacteria S.aureus and Streptococcus pyogenes but presented no effect against the gram-negative bacteria E.coli and Pseudomonas aeruginosa (Najdenski et al., 2013). The difference in sensitivities between bacteria may be due to their complex membrane permeability, making the penetration and the bactericidal action of the compound more difficult.
Lane et al. (2009) suggested that the mechanism of antibacterial activity of terpene compounds was due to the hydrophobicity and conformational rigidity of the tetrahydropyran structure.Wang, Xu, Bach, & McAllister (2009) accounted that mechanisms of action of phlorotannin were also similar to fatty acids documented by Shannon and Abu-Ghannam (2016). Moreover, they added that phloroglucinol units of phlorotannin contain Phenolic aromatic rings and hydroxyl group that makes the hydrogen bond with the amine group of bacterial proteins that cause cell lysis. It was further evidenced by Wei et al. (2016) that phlorotannin extracted from S.thunbergii injured the cell membrane of V.parahaemolyticus, leads to cytoplasm outflow and disassembly of cell inclusions.
However, a higher amount of phloroglucinol is required to destroy Gram-negative bacteria than the Gram-positive due to the multifaceted structure of former than the latter (Kamei & Isnansetyo, 2003). It was corroborated by the previous research reports of Hierholtzer et al. (2012). In their report, a mechanism of action of phlorotannin was directly evidenced by electron microscope images showing total cell membrane damages of bacterial cells. Paiva et al. (2010) explained that the antibacterial activity of lectins is due to the binding and damages of cell wall compositions of bacteria such as teichoic acids, peptidoglycans, and lipopolysaccharides and destroy them by leaking cell inclusions.
However, more research findings must be required, and various advanced techniques such as molecular techniques, protein, enzyme and DNA analysis and scanning and transmission electron microscope analysis should be carried out to support the direct evidence of the mechanism of action of bioactive compounds isolated from marine algae. Application of nanofibers in food preservationNatural bioactive compounds show ideal antimicrobial properties, too desired to be used in food preservation. Nonetheless, there is also a need for an efficient method for their delivery into foods (Balasubramanian, Rosenberg, Yam, & Chikindas, 2009). Nowadays nanotechnology has become the call of the century. It has a thriving application in several other sectors, and its application in the food industry has been a recent event (Pradhan et al.
, 2015). Many nanotechniques including nanoemulsions, nanoliposomes, and nanoencapsulation using nanofibers are employed in food preservation to incorporate natural antimicrobial agents with different supportive materials. Among various nanodelivery systems, nanofibers have more advantages than other methods due to its rapid, controlled release, high surface to volume ratios and show higher antimicrobial activity and stability than other nanomaterials.
These assets create the mats composed of electrospun fibers outstanding candidates for immobilization of natural antimicrobial compounds in food applications globally. Many different methods such as phase separation, self-assembly, drawing, and electrospinning can be used to produce nanofibers (Esentürk, Erdal, & Güngör, 2016; Akhgari, Shakib, Sanati et al., 2017) and chemical method (Saurabh et al., 2016; Berglund, Noël, Aitomäki, Öman, & Oksman, 2016; Martelli-Tosi et al., 2016; Xie et al.
, 2016). Among the other nanofiber production, electrospinning is the most cost-effective one with simple tooling, and is applied to produce ultrafine fibers with a simple step-up production for the encapsulation of various bioactive compounds (Esentürk et al., 2016; Wen, Wen, Zong, Linhardt, & Wu, 2017). Recently, the electrospun nanofibers have drawn significant interest to the food industry because of their high surface area-to-volume ratios. This property makes electrospun fibers potential materials for various applications, like edible films and additive delivery systems (Padilla, Soto, Iturriaga, & Mendoza, 2014).Electrospi nning The working principle of electrospinning is to use electrostatic repulsion of charged polymer jets to generate arbitrarily oriented or united nanofibers on the exterior of the collector.
In general, electrospinning set up consist two electrodes; one is attached with polymer mixer and second one associated to a collector. Electrically charged polymers produce a Taylor cone at the end of the needle and are evicted at a positive charge. Nanofibers are generated after the evaporation of solvents while the mixer of polymer solution goes faster towards rotary collector or an auxiliary electrical field (Brandelli & Taylor, 2015). Electrospinning is widely accepted and superior method for production of nanofibers, owing to its ease, less cost, good elasticity, the potential to massive scale production, the capability to produce nanofibers from most of polymers (Esentürk et al., 2016; Wen et al., 2017). Furthermore, both hydrophobic and hydrophilic compounds such as protein and amino acids could be directly encapsulated on nanofibers by electrospinning technique (Wen et al.
, 2017). Also, immense stability and high encapsulation efficiency of natural antimicrobial agents could be achieved by this method (Yang et al., 2017). Moreover, heat sensitive bioactive compounds could be efficiently immobilized in nanofibers during electrospinning method since it operates at ambient environment when compared to conventional techniques like spray drying which runs at high temperature (Wen et al.
, 2017). Basic electrospinning set up is illustrated in Fig.3.Efficient nanofiber formation and its physicochemical properties such as mechanical strength, structure, release features of drug including burst effect and biocompatibility are influenced by the selection of polymers to electrospun because that would impact the interactions of the mixer of bioactive compounds/polymer/solvent (Esentürk et al.
, 2016). Nowadays, considerable attention has been given to natural biopolymers due to their remarkable advantages including biocompatibility, biodegradability, renewability, and sustainability as carriers for encapsulation of bioactive compounds in the food industry (Sa?, Morshed, Ravandi, & Ghiaci 2007; Wen et al., 2017). Many researchers have been successfully synthesized nanofibers using electrospinning from biopolymers such as cellulose acetate (Dods, Hardick, Stevens, Bracewell, 2015; Mehrabi, Shamspur, Mostafavi, Saljooqi, & Fathirad, 2017; Liakos, Holban, Carzino, Lauciello, & Grumezescu, 2017), chitosan (Tripathi, Mehrotra, & Dutta, 2009; Liu, Wang, & Lan, 2018), gelatin (Agudelo et al., 2018), dextran (Fathi, Nasrabadi, & Varshosaz, 2017), pullulan (Liu, Li, Tomasula, Sousa, & Liu, 2016; Wen et al., 2017), pectin (Liu et al.
, 2016), hyaluronic acid (Zhao et al., 2016; Wen et al., 2017), collagen and silk fibroin (Zhao et al., 2016).
The basic concept of immobilization and delivery of natural antimicrobial agents through nanofibers for food preservation is represented in Fig. 4.
