Corn silk: A promising source of antimicrobial compounds for health and wellness

Abbreviations

CS

Corn silk

FMCSS

Fresh matured corn silk sample

DMCSS

Dried matured corn silk sample

CE

Consecutive extraction

IE

Individual extraction

QS

Quorum sensing

ATP

Adenosine triphosphate

HIV

Human immunodeficiency viruses

HSV

Herpes simplex virus

SARS

Severe acute respiratory syndrome

EPI

Epigenetics

PEDV

Porcine epidemic diarrhea virus

UTI

Urinary tract infection

MFC

Minimum fungicidal concentration

MIC

Minimum inhibition concentration

UAE

Ultrasound assisted extraction

MAE

Microwave-assisted extraction

TZP

Piperacillin-tazobactam

1 Introduction

Microorganisms pose major problems to public health and the infectious diseases caused by them are a significant cause of morbidity and mortality around the world. Although antibiotics play an important role in the treatment of infectious diseases, the incidence of antibiotic resistance has surged over the past decades [1]. The alarming severity of drug resistance has led many scientists to search for new and alternative antimicrobial sources. A wide range of medicinal plants are a rich source of antimicrobials and they possess different medicinal properties against microorganisms [2]. Consequently, strong efforts are being made to utilize those medicinal plants as medicines in treating microbial infections.

Corn silk (CS) (Stigma maydis L.) is the stigma and style of the maize flower which is obtained as a by-product of corn cultivation. It has numerous biochemical nutrient compounds like proteins, carbohydrates, vitamins, and mineral salts and is also rich in B complex vitamins, vitamin A, vitamin K, and minerals like sodium and potassium [3]. CS also contains several bioactive compounds such as steroids, alkaloids, anthocyanins, saponins, carotenoids, and phenolic compounds which possess cooperative effects on physical health [4]. Many countries like China, the United States, and France have been using CS for the treatment of prostate disorders, kidney stones, obesity, urinary infections, and bedwetting. Extracts of CS help in reducing the deteriorating effects of diabetes, hypertension, and high blood pressure [5]. Scientifically, it has been examined that CS inhibits the α-amylase activity and retard the digestion of starch as well as restrains the increase of post-meal blood sugar [6]. It helps to reduce high blood pressure by regulating the electrolyte balance through the release of potassium and sodium in the urine [7]. CS, rich in flavonoids and phenolic compounds, acts as an excellent source of antioxidants and enhances the scavenging activity of harmful free radicals [8]. The anthocyanins present in some colored CSs exhibit higher antioxidant, anticarcinogenic, and anti-inflammatory activity [9]. CS can also regulate cell death by modulating different signaling cascades thereby supporting the nervous system [10].

Besides the numerous health properties of CS, it is known to hold a strong antimicrobial activity against different microorganisms. Several studies have been conducted to expose its potential to inhibit microbial strains. The ethanolic extract of CS has been found to inhibit the growth of Pseudomonas aeruginosa and Staphylococcus aureus which are one of the most common infections causing microbes such as urinary tract, respiratory and gastrointestinal infections [11]. The antibacterial effects of maize silk extracts were shown to be attributed to the presence of components such as steroids, flavonoids, tannins, and saponins, which were examined by phytochemical analysis [12]. Furthermore, optimised ethanol extracts of CS showed hypoglycemic action and efficiency against Bacillus subtilis, boosting its potential for antibacterial and health-promoting properties [13]. CS when used in making wine and vinegar produced a number of volatile compounds such as 1-butanol, 3-methyl-acetone, 1-butanol, 3-methyl-hexanoic acid and almost all of them showed antimicrobial properties when tested [14]. In an in vitro study, it was found that using 10% of CS extract as mouth gargles significantly reduced the decay-causing bacteria (S. mutans) as compared to the commercially available mouth gargles [15].

In general, the presence of certain bioactive compounds and the chemical composition of CS are mainly accountable for their healthcare applications and antimicrobial effect [16]. So, this review will highlight the different bioactive compounds found in CS and their antimicrobial mechanism against microorganisms. It will also reveal the antimicrobial efficacy of various CS extracts and will provide insight into how CS can become an operative antimicrobial agent in the preparation of new drugs.

2 Statement of novelty

The major focus of this review is toward utilization of agricultural waste that contains good amounts of biologically active components. Various studies have observed the antibacterial qualities of maize silk, even though numerous studies have examined its nutritional composition, therapeutic properties, and product development. This study outlines the mechanisms underlying the bioactive components in maize silk and covers the body of research on the material’s antimicrobial properties. Given the rising issue of microbial resistance, this study outlines research needs for future investigations and underlines the potential of CS for creating antimicrobial medications against bacteria resistance.

3 Bioactive compounds present in CS

Phytochemicals, or bioactive substances found in plants, are secondary metabolites that may have toxicological or pharmacological effects on humans [17]. These substances are produced along with primary metabolic substances and are essential for the growth, reproduction, and interactions of plants with the environment. Phytochemicals are also responsible for several microbiological and nutraceutical properties of food-based products and are commonly used for the preparation of dyes, flavors, fragrances, and insecticides [18]. Secondary metabolites are entirely accumulated in different parts of the plants and the amount present in them plays a pivotal role in treating many health-related diseases. Most profusely available classes of plant phytochemicals are alkaloids, tannins, saponins, flavonoids, terpenoids, steroids, and glycosides and are known to serve as an antioxidant, antiviral, anti-diabetic, anticancer, and anti-inflammatory agents [19]. CS is a major plant ingredient containing numerous biologically active compounds and has been used in various therapeutic methods for over thousands of years. It is rich in flavonoids, alkaloids, steroids, tannins, terpenoids, saponins, organic acids, and volatile oils [20,21]. The complete extraction of all the bioactive phytochemicals by using a single extraction method is very difficult and uncertain [22]. Therefore, various solvent extracts such as aqueous, alcohol, acetone, and hexane are used for extracting all the phytochemicals from CS as shown in Figure 1.

Figure 1

Phytochemical constituents found in various CS extracts. Source: Figure prepared by the authors.

A qualitative analysis of phytochemicals using ethanol, methanol, and chloroform-based CS extract was conducted by Morshed and Islam [23]. The main phytochemical constituents subjected to analysis were glycosides, terpenoids, flavonoids, tannins, steroids, phenols, saponins, amino acids, sugars, and carbohydrates. It was observed that flavonoids and glycosides were found in all three extracts whereas sugars and steroids were only found in ethanol and methanol-based extracts. However, terpenoids, phenols, saponins, amino acids, carbohydrates, and tannins were not found in any of the extracts. In another study conducted by Emmanuel et al. [24], the methanol extract of a fresh matured corn silk sample (FMCSS) and dried matured corn silk sample (DMCSS) were taken to check the occurrence of phenols, alkaloids, cardiac glycosides, flavonoids, terpenes, steroids, glycosides, tannins, anthraquinones, saponins, balsams, triterpenoids, phlobatannins, resins, and volatile oils. It was observed that phenol and volatile oil were not present in the fresh sample but were present in the dried sample and resin was only present in the fresh sample, whereas alkaloids, cardiac glycosides, saponins, flavonoids, steroids, tannins, glycosides, and balsams were present in both samples, and terpenoids, triterpenoids, phlobatannins, and cardenolides were not found in any of the samples. Solihah et al. [25] used the aqueous as well as methanolic extract of Malaysian CS for preliminary screening of phytochemical constituents. The results disclosed that phenols, flavonoids, alkaloids, saponins, phlobatannins, tannins, and cardiac glycosides were present in both extracts, whereas anthraquinones and terpenoids were noticed in methanolic extract alone. Besides these, sterols and protein-xanthoprotein were not found in any of the extracts as shown in Table 1.

Sarepoua et al. [26] estimated the total phenolic, flavonoid, and anthocyanin content in the silk of purple waxy corn, white waxy corn, and super sweet corn at various stages of maturity. Methanolic extracts of CS samples were prepared using a modified method and it was found that purple waxy corn had higher than 100 µg GAE/g of total phenolic content whereas in white waxy corn and super sweet corn, the total phenolic content had less than 100 µg GAE/g. The highest total flavonoid concentration was found in purple CS, while super sweet CS with an intermediate level, and white waxy CS had the lowest. Similarly, super sweet CS and white waxy CS had lower anthocyanin contents, whereas purple waxy CS had the greatest (23.9–46.0 µg C3G/g). Moreover, the phytochemical analysis of two baby CS varieties Pacific 271 and Zeba SG 17 hybrid was done in a study conducted by Limmatvapirat et al. [4]. The ethanolic and aqueous extracts of both varieties were made and the result showed the presence of flavonoids and tannins in both the extracts. However, terpenoids and steroids were only found in ethanol extracts and alkaloids were not detected in any of them. Furthermore, the study presented that the amount of total phenolic contents and the total flavonoid contents in 40% v/v ethanol extracts were significantly higher than those of aqueous extracts. Nawaz et al. [22] conducted a study to obtain a high yield of phytochemicals using the best extraction method. CS variety (P.1543) was used for extraction by various methods such as consecutive extraction, individual extraction (IE), and consecutive extraction of crude methanolic extraction. The presence of flavonoids, tannins, phenolic acids, cardiac glycosides, and ascorbic acid was confirmed through phytochemical screening but saponins were absent in all three extracts. The amount of total phenolic content (0.11 ± 0.02–2.34 ± 0.3 g/100 g dw), tannins (0.031 ± 0.013–2.276 ± 0.12 g/100 g dw), flavonoid (0.03 ± 0.005–1.65 ± 0.12 g/100 g dw), and ascorbic acid (0.008 ± 0.001–0.164 ± 0.017 g/100 g dw) and their extraction yield was found to be comparatively higher in IE method.

Apart from this, the amount of total phenolics, flavonoids, anthocyanins, and proanthocyanidins of purple, green, pink, and yellow CSs was estimated at different maturity stages in a study led by Žilić et al. [27]. The observation depicted that flavonoids were present in all the varieties of CS and anthocyanins were only present in purple and pink CS which also contributes to their color, whereas proanthocyanidins were only found in purple-colored corn. Also, the phenolic and flavonoid content was found to be 2–4 folds higher in fresh silks as compared to the mature ones. Another study presented by Ammor et al. [28] showed the presence of phytoconstituents such as alkaloids, flavonoids, anthocyanins, heterosid sterodic, coumarins, leucoanthocyans, cardiac glycosides, and tannins in both aqueous and hydro ethanolic extracts of stigmata of Zea mays. While, terpenoids, sterols, oses, and holosides were only found in hydro-ethanolic extract, and mucilage was found only in aqueous extract. Nevertheless, saponosids, gallic tannins, and triterpens heterosids were not found in any of the extracts. A qualitative phytochemical screening was done on the ethanolic CS extract by Azevedo et al. [29] to identify its phytoconstituents including, alkaloids, flavonoids, phenols, tannins saponins, carbohydrates, terpenoids, and anthraquinones. The result showed positive for flavonoids, tannins, phenols, and terpenoids but negative for other constituents.

4 Antimicrobial properties of phytochemicals present in CS

CS being a major source of bioactive metabolites possesses various antimicrobial and antioxidant functions and several CS extracts have shown antibacterial activity against common pathogens and food spoilage bacteria [30]. The phytochemical composition of CS is directly linked to its antimicrobial effect and several researchers are trying to investigate the efficiency of those bioactive compounds as antimicrobial agents and how they combat the growth of different microbial strains [31]. In most cases, the plant extracts contain complex bioactive ingredients that act synergistically in one way or the other to kill the microorganism. The cytoplasmic membrane is the main target site of bioactive compounds and they disrupt its structural integrity, functionality, and permeability in different ways. Certain phytochemicals can also inhibit the efflux pump and normal cell communication or quorum sensing (QS) [32]. QS is related to cell-cell interactions and through which the bacterial cells determine their cell density [33]. Besides this, some phytochemicals can inhibit cell wall construction, inhibit the formation of biofilm, and inhibit microbial DNA replication or energy synthesis and some can even induce reactive oxygen species production in the microorganism [34]. Figure 2 shows the antimicrobial mechanism of certain bioactive compounds that are found in CS.

Figure 2

Antimicrobial mechanism of action of phytochemicals that are conspicuously found in CS. Source: Figure prepared by the authors.

5 Antimicrobial activity of CS extracts

The high phytochemical profile of CS and its efficient bactericidal action reveals that it has great potential to kill several microorganisms. Many studies have been investigated to know the antimicrobial action of CS against most common pathogens and food spoilage microbes. Thus, Tables 2 and 3 show how the different solvent extracts of CS exhibit a wide range of antimicrobial activity against the various strains of bacteria and fungi respectively.

According to Abirami et al. [62], the ethanol extract of CS was checked against (UTI) Urinary Tract Infection causing bacteria and some isolated fungi. The inhibitory action of the extract was observed against all UTI-isolated bacteria, where the highest inhibition zone was found against Proteus mirabilis (15 mm) followed by K. pneumoniae (12 mm), E. coli (10 mm), and S. aureus (9 mm), whereas the lowest minimum bactericidal concentration value was found to be 700 μg against K. pneumoniae and P. mirabilis. Further, the antifungal activity of ethanolic CS extract was tested against three isolated fungi and the result showed that the percentage of inhibition of mycelial growth was highest in Aspergillus brasiliensis, followed by A. niger, and A. flavus, at MFC 1.6 mg/20 mL, 1.8 mg/20 mL, and 2 mg/20 mL respectively. In another study by Feng et al. [63], an ethanolic extract of CS was tested against Bacillus cereus, P. aeruginosa, Bacillus coli, S. aureus, and C. albicans in the presence of Andrographis paniculata as a standard antibiotic. The result showed that the maximum antibacterial activity of the silk extract was found against B. cereus with the inhibition zone of (28 mm) and MIC of 62.5 mg/mL, followed by S. aureus with inhibition zone of (19 mm) and MIC of 500 mg/mL. Whereas, the extract was unable to inhibit B. coli and P. aeruginosa even at the maximum concentration of (500 mg/mL). However, it showed a good antifungal activity against C. albicans with the inhibition zone of (21 mm) and MIC of (250 mg/mL). These findings state that the ethanolic extract of CS is quite effective against several UTI bacteria and Fungi and it can be used against some specific class of microorganisms.

Various organic solvent extracts of CS were evaluated for their antimicrobial properties in a study by Nessa et al. [64]. All the different extracts (pet-ether, chloroform, and methanol) were evaluated for their antimicrobial activity against four gram-positive bacteria (S. aureus, B. subtilis, P. aeruginosa, B. cereus), eight gram-negative bacteria (E. aerogenes, S. typhi, S. paratyphi, E. coli, Shigella sonnei, S. flexneri, P. vulgaris, P. mirabilis) and one yeast (C. albicans). The gentamicin was taken as a reference antibiotic. The result revealed that the pet-ether extract (25 mg/mL) and the methanolic extract of CS (25 mg/mL) were effective against eleven out of twelve bacterial strains as shown in Table 2, whereas the chloroform extract (25 mg/mL) was only sensitive against the five bacterial strains and none of the extracts showed any sensitivity against the yeast (C. albicans). Similarly, Morshed and Islam [23] have also published a study in which they have determined the antimicrobial action of diverse microorganisms with ethanol, methanol, and chloroform extract of CS. They tested the four gram-positive bacteria (B. subtilis, S. aureus, B. cereus, P. aeruginosa), eight gram-negative bacteria (S. sonnei, S. flexneri, P. vulgaris, P. mirabilis, E. aerogenes, Salmonella typhi, S. paratyphi, and E. coli) and one yeast strain of C. albicans by taking streptomycin as a positive control. Methanol and ethanol extracts showed a sensitive response against all the bacteria except E. coli. While, chloroform extract showed sensitive response only against five (B. cereus, B. subtilis, S. aureus, S. sonnei, and E. aerogenes) bacterial strains. However, C. albicans did not show any response in all the three CS extracts. Different types of CS extracts have different impact on the outcomes and alcohol-based extracts are more effective than the others like pet-ether and chloroform.

A recent study put forward by Boeira et al. [31] showed the antibacterial and antifungal activity of CS extract by using UAE. Twenty-three microorganisms were collected and amongst them, five were gram positive, ten were gram negative bacteria and nine were clinically isolated yeasts. Chloramphenicol and ampicillin were used as a standard for bacterial analysis, whereas fluconazole and nystatin were used to control the sensitivity of the antifungal test. The UAE CS extract displayed a significant inhibition action against both the gram-negative and gram-positive bacteria, with MIC range between (62.5 and 250 µg/mL) as shown in Table 2. It also showed the antifungal potential by inhibiting the growth of fungi, with MIC values between (125 and 500 µg/mL) as shown in Table 3. The study also revealed that UAE CS extract showed better bactericidal potential than the positive control for B. cereus, B. subtilis, Enterococcus fecalis, E. coli, S. sonnei, S. typhimurium, and Morganella morganii bacteria at MIC of (62.5 µg/mL). Another study performed by Carvalho et al. [65] with Z. mays silk hexane extract against S. aureus, E. coli, and P. aeruginosas strains was done to determine the minimum inhibitory concentration. The result showed that the extract did not present a significant antibacterial effect even at the MIC values (≥1,024 µg/mL). However, the association of the hexane extract was then done with certain antibiotics and this modulation apparently decreased the MIC value for the antibiotics.

Another study was conducted by Nurani et al. [66] to assess the antibacterial effect of CS’s ethanolic extract against three acne-related bacteria S. aureus, S. epidermidis, and Propionibacterium acnes at increasing concentrations (10 to 100%). Clindamycin was taken as a standard antibiotic and its effectiveness was also taken into account. The result showed that the antibacterial activity of P. acnes and S. epidermidis was getting increased at the higher concentrations and that for S. aureus it was getting decreased (>70%). The diameter of the inhibition zone for P. acnes, S. epidermidis, and S. aureus was found to be 19.6 ± 0.09, 11.4 ± 0.4, and 2.8 ± 2.8 mm respectively at 100% concentration. The antimicrobial activity of aqueous extract of CS was estimated against different bacterial isolates in a study investigated by Saleh et al. [67]. The result showed that the extract was able to inhibit the growth of 11 gram-negative and 8 gram-positive bacterial pathogens according to the inhibition zone and the antibiotic ciprofloxacin was taken as standard. For gram-positive bacteria such as S. aureus, S. saprophyticus, S. epidermidis, S. pneumonia, S. progenies, S. agalactia, S. mutants, and E. faecalis, the inhibition zone ranged between (26 and 32 mm). Whereas, for gram-negative bacteria, the extract showed the maximum inhibition zone against S. typhimurum (35 mm), followed by S. typhi (34 mm) and the lowest value was shown by K. pneumonia (17 mm). From the above-mentioned results of different studies, it is identified that CS extracts are more effective against gram-positive bacteria than gram-negative bacteria. This variable susceptibility could be attributed to the difference in the structural composition of the cell walls of both bacteria.

Many studies have shown that the phytochemical composition of CS changes with the different maturity stages as well as with the different processing techniques. Later, Emmanuel et al. [24] recapitulated a study in which the antimicrobial activity of the methanolic extract of fresh matured CS and dried matured CS was done against bacteria by taking ciprofloxacin as a positive control. The result showed a positive effect of CS extract against E. coli, P. aeruginosa, K. pneumoniae, S. typhi, and S. aureus with the inhibition zone ranging between (10–16 ± 0.50 mm) for a fresh sample and (12–20 ± 2.50 mm) for the dried samples respectively. Qaiser et al. [68] evaluated the antagonist action of CS extract toward various fungal and bacterial pathogens. The MAE of CS was tested against the two gram-negative (Acenobacter baumanni and E. coli) and two gram-positive (Entarococcus and S. aureus) bacteria by taking piperacillin-tazobactam (TZP) as a positive control. The result showed that the maximum inhibition zone (20 and 25 mm) was obtained against the gram-positive bacteria followed by the gram-negative bacteria (19 and 23 mm). Alternatively, the (MAE) CS extract was also tested against fungus (Aspergillus nidulans, A. flavus, and Aspergillus fumigatus) by taking axicon as a positive control. The results revealed that the extract was able to reduce the growth of fungal pathogens. The antimicrobial activity of CS ethanolic extract was evaluated using four bacteria strains in a study directed by Azevedo et al. [29]. A more significant bactericidal effect was shown against gram-positive bacteria (S. aureus and S. epidermidis) with a minimum inhibition concentration of (12.5 mg/mL each) than in gram-negative bacteria (E. coli and P. aeruginosa) having MIC of (50 mg/mL) and (25 mg/mL) respectively. Quercetin was taken as a positive control and the analysis was done using the minimal bactericidal concentration of CS extract at (50 mg/mL).

6 Future prospective

CS can become one of the natural, cheap, and safe ingredients to be used for the formulation of new food products. It can be used as a natural antimicrobial and antioxidant ingredient in the food industry. Once thought of as an agricultural byproduct, maize silk is becoming recognized as an important, natural, and affordable ingredient with a wide range of uses in the culinary and medicinal sectors. The antibacterial and antioxidant qualities of this natural resource make it extremely promising. CS can act as a natural antibacterial agent in the food industry, preventing the growth of pathogenic microbes. Furthermore, because of its strong antioxidant properties, it is an ideal component of bioactive packaging materials that increase food products’ shelf-life by protecting them from oxidative damage. CS’s strong antioxidant and free radical-scavenging properties also make it an appropriate choice for use in animal feeds and nutritional supplements, especially in conditions with high oxidation-prone fat and lipid concentrations. CS’s high antioxidant activity is advantageous for both improving health and preserving food. It might boost the immune system as an immunostimulatory agent, adding protection against dangerous microbial diseases like coronavirus disease 2019 and monkeypox. Numerous studies on different CS extracts have shown how therapeutically they can be used. Its characteristics have been discovered to have potential use in the treatment of several chronic disorders, including diabetes, liver diseases, cancer, kidney diseases, hypertension, and parasite infections. This wide range of possible uses highlights CS’s potential as a natural therapeutic agent. Furthermore, the chemical components of CS extract have the potential to be used in the development of innovative pharmaceuticals. These include antifungal lotions for skin infections, novel antibiotic medications that can fight resistant bacterial strains, and other medicinal formulations. Despite its exceptional properties, more study is required to fully understand and utilize the potential of CS ingredients in application in various food and medical industries.

7 Conclusion

Microbial diseases are becoming more predominant in India due to antibiotic resistance. To resolve this problem, some alternative chemotherapeutic agents are being searched. This study has shown that CS is a good source of antimicrobial agents that can prevent the extension of microbial infections. CS contains many different bioactive compounds such as phenols, flavonoids, tannins, saponins, and terpenes which not only provide therapeutic effects but also act as a good antimicrobial agent. The antibacterial and antifungal effect of CS is majorly dependent on its variety, phytochemical composition, extraction solvent, and method of extraction. CS has proven to be an effective natural compound because, in the above-mentioned studies, it has shown good antimicrobial activity under lower values of minimum inhibition concentrations. It could be used for the formulation of new antimicrobial drugs which will eventually decrease the burden of antibiotic resistance. Thus, CS is worthy of further research which will elucidate its impact on human health.