Agricultural Plants and Their Antimicrobial Activities - A Mini Review

Introduction

Agricultural plants are major sources of natural fibers. Plants contain many bioactive compounds and are a great source of antimicrobial agents [1]. Microbial contamination poses a serious threat to human health Plants with medicinal values have always been effective to cure different diseases caused by microorganisms. Centers for Disease Control and Prevention (CDC) has reported that 90 % of foodborne human illness is attributed to bacteria. The most important bacterial diseases reported through transmitted water are cholera, typhoid fever, and bacillary dysentery [2].

Some common pathogens such as E. coli, S. aureus, and B. subtilis exhibit increased resistance to different antibiotics [3]. World Health Organization (WHO) has reported about 80% of the population from developing countries relies on traditional medicinal plants [4]. The emergence of multiple drug-resistant bacterial strains and fungal pathogens due to the indiscriminate use of antibiotics has generated a considerable interest among the researchers to investigate potent and alternative plants derived drugs [5,6]. Hemp, kenaf, jute, sisal, and flax are some of the agricultural plants containing lignocellulosic fibers, also called natural fibers. The main objective of this mini review is to explore the utilization of the above natural fibers based on their antimicrobial efficacy.

Fiber Source and Fiber type

Agricultural plants are producers of natural fibers and are classified into two broad categories viz primary plants and secondary plants. Plants that are cultivated due to their fiber content are known as primary plants while plants that are grown due to their by-product are called secondary plants. Hemp, kenaf, jute, sisal, and flax are some examples of primary plants while coir, pineapple oil, palm fall under the category of secondary plants. Commonly there are six types of natural fibers- bast fibers (hemp, kenaf, jute, flax), leaf fibers (pineapple, sisal), core fibers (hemp, kenaf, jute), reed fibers (wheat, rice, corn), and other types of woody fibers [7,8].

Bioactive Constituents of Fiber Plants

Agricultural plants produce a whole series of bioactive components termed secondary metabolites. These are organic molecules that appear as intermediate or end products of secondary plant metabolism that exhibit antimicrobial activity [1,9,10]. Bioactive molecules can inhibit the growth of pathogens as well as have no or very little toxicity to the host cell. Phenols, polyphenols, terpenoids, alkaloids, cannabinoids, lectins, essential oils tannins, quinones, and polypeptides are some of the fundamental and essential antimicrobial phytochemicals present in plants [1,11]. Phenolic acid, flavones, flavonoid, and tannins are some of the beneficial bioactive molecules which are included in phenols and polyphenols [1,12].

Phenols and polyphenols are the simplest bioactive phytochemicals. Quinones are highly reactive and are the source of stable free radicals which has high potential inactivation towards bacteria. The flavonoid, compounds have the phenolic structure and it has shown inhibitory effects not only towards bacteria also towards viruses like respiratory syncytial virus (RSV) and HIV [1,13,14,15,16]. The general formula of Terpenoids is C10H16 and they are highly active against Bacillus subtilis and Staphylococcus aureus [17,18]. Tannins are polymeric phenolic substances and they are found in almost every plant part. Several studies have reported that tannins are toxic to filamentous fungi, yeasts, and bacteria [19,20].

Antimicrobial Property of Natural Fibers and their Applications

Hemp (Cannabis sativa L.) is an eco-friendly and multifunctional plant and it is the most predominantly investigated natural fiber plant. It is a dioecious woody, an herbaceous annual plant which can grow in a wide range of climate and soil condition. Cannabinoids are essential bioactive constituents that are present in hemp. Researchers have reported the antibacterial activity of hemp towards B. subtilis, E. coli, and S. aureus [21,22]. Hemp can be mixed with polymers such as chitosan, polylactic acid, polysulfone, polyethylene glycol, and can be used in various biomedical applications like in the form of implants and medical devices [23]. Cannabinoids and alkaloids in hemp can be used to prepare antibacterial drugs.

Kenaf, Jute, flax, and sisal have also been explored and exhibited some antibacterial activities. Kenaf (Hibiscus cannabinus), an annual herbaceous crop belongs to the Malvaceae family, is a valuable medicinal crop and are rich sources of cellulose. The plants possess a wider range of adaptation to environments, climates, soils, and distributed in Asia and Africa. Kenaf has significant medicinal properties including anticancer and antioxidants, and the kenaf leaves are used for the treatment of diabetes and cough. The antibacterial activity of Kenaf leaves and seed extracts against S. aureus and E. coli were reported [24]. Results revealed that kenaf seed extracts were very effective against both Gram-positive and Gram-negative bacteria [25,26]. The essential oil of kenaf had antifungal activity against Colletotrichum fragariae, Colletotrichum gloeosporioides, and Colletotrichum accutatum [27].

Jute (Corchorus olitorius L.) is one of the traditional plants that have great potential for medicinal purposes. Jute leaves are used as a diuretic, antipyretic, analgesic, and antimicrobial activities. The antibacterial activity of jute was demonstrated against Staphylococcus aureus, Micrococcus luteus, Bacillus cereus, Escherichia coli, Klebsiella pneumonia, Salmonella enterica, Salmonella typhi, and Enterobacter sp [28].

Flax (Linum usitatissimum) is an annual herb that belongs to the Linaceae family and cultivated worldwide. The inhibitory activity on bacteria of flaxseeds protein was reported against Enterococcus focal, Salmonella typhimurium, Escherichia coli, and S. aureus [29,30].

Sisal (Agave sisalana Perrin) is a monocotyledonous plant belonging to the Agavaceae family. It is composed of 78 % cellulose and 10% hemicellulose and used as herbal medicine to treat various diseases. Sisal extracts have demonstrated antimicrobial activity against Candida. albicans, Escherichia. coli, Staphylococcus aureus and Pseudomonas aeruginosa [31,32].

Conclusions

Medicinal plants have always been part of human culture and have the potential to cure different diseases caused by microorganisms. Plants’ secondary metabolites contain essential antimicrobial agents, exhibit a greater inhibitory effect against Gram-positive and Gram-negative bacteria. Plant derivatives have a major application in water treatment, textiles, food packaging, polymer composites, cosmetic industries, and biomedical applications. An extensive investigation focused on the identification, characterization and, antimicrobial efficacy of bioactive compounds of natural fibers is required.

Acknowledgements

This work was supported by the University of North Texas, Toulouse Graduate School -Graduate Research Award.

Conflicts of Interest

The authors declare there is no conflict of interest regarding the publication of this research manuscript.

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