Dioscorea bulbifera is a potent medicinal plant used in both Indian and Chinese traditional medicine owing to its rich phytochemical diversity. Herein, we report the rapid synthesis of novel nanoparticles by D. bulbifera tuber extract (DBTE). NPs synthesis was completed within 5 h showing a prominent peak at 540 nm. HRTEM analysis revealed 9 nm inner core of elemental gold covered by a silver shell giving a total particle diameter upto 15 nm. NPs were comprised of % gold and % silver of the total mass. NPs showed highest biofilm inhibition upto % against A. baumannii. Biofilms of P. aeruginosa, E. coli, and S. aureus were inhibited up to %, %, and %, respectively. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirmed unregulated cellular efflux through pore formation leading to cell death. Potent antileishmanial activity of NPs (µg/mL) was confirmed by MTT assay. Further SEM micrographs showed pronounced deformity in the spindle shaped cellular morphology changing to spherical. This is the first report of synthesis, characterization, antibiofilm, and antileishmanial activity of NPs synthesized by D. bulbifera.

1. Introduction

Recent advances in the field of nanotechnology encompass the development of safe and ecofriendly route towards the synthesis of nanoparticles for the benefit of human health. Gold nanoparticles (AuNPs) with unique physicochemical and optical properties are reported to have potent applications in photonics, chemical sensing as well as biomedical applications like photothermal therapy and drug delivery [14]. Silver nanoparticles (AgNPs) are considered to be superior to other nanostructured inorganic metal particles due to their well-known electrical conductivity, optical properties, and oxidative catalysis [5, 6]. AgNPs are used in different fields in medicine due to their broad spectrum antimicrobial effect [710]. Therefore, AgNPs have found their applications in various pharmaceutical products, such as water purification systems, burn dressings, and medical devices [11, 12]. Thus, combination of both gold and silver to synthesize bimetallic is of utmost scientific rationale. However, till date there are no well-defined biological routes for synthesizing with therapeutic potential.

Among various medicinal plants used in Ayurveda, Indian system of traditional medicine, Dioscorea bulbifera is noteworthy owing to its multiple therapeutic potential [13, 14]. It is reported to exhibit antimicrobial, plasmid curing, analgesic, anti-inflammatory, antihyperglycemic, antihyperlipidemic, antinociceptive, and antitumor activities [1519]. Recently, we have reported its antidiabetic and antioxidant potential [2022]. Additionally, we have reported the potential of D. bulbifera in synthesizing both AgNPs and AuNPs owing to its rich phytochemistry containing both reducing agents and stabilizing agents [7, 23]. However, there are no reports up to date on synthesis of bimetallic nanoparticles using D. bulbifera.

Multidrug resistant (MDR) Gram negative and Gram positive bacteria like Acinetobacter baumannii and Staphylococcus aureus,respectively, have emerged as highly infectious nosocomial pathogens owing to their remarkable resistance to antibiotics, metal salts, desiccation, and disinfectants [2428]. Such type of pathogenesis among the critically ill and immune-compromised patients is characterized by the biofilm formation that comprise of a complex, organized bacterial community adhering to the surface forming microcolonies composed of exopolymeric matrix of carbohydrates, proteins, and nucleic acids [2932]. Hereby, the bacteria gain ability to evade the host defenses, resistance to antibiotic therapy and deliberate delocalization of planktonic bacteria resulting in implantation, and colonization at newer sites, causing nascent acute infections in the host [33]. Currently, there are no reports on the application of bimetallic nanoparticles composed of gold and silver, in controlling the biofilm formation which can prove to be a powerful therapeutic strategy.

In view of the above background, we report for the first time the synthesis of by D. bulbifera tuber extract (DBTE) followed by characterization using UV-visible spectroscopy, high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), and dynamic light scattering (DLS). Bioreduced were checked for the biofilm inhibitory activity against A. baumannii, P. aeruginosa, E. coli, and S. aureus employing field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM).

2. Materials and Methods

2.1. Synthesis of Gold Core Silver Shell Nanoparticles (AucoreAgshellNPs)

DBTE was prepared as per our earlier report [7]. In brief, fresh tubers of D. bulbifera were washed thoroughly and chopped into slices and shade dried followed by grinding into fine powder. 5 g of the powder was boiled in 100 mL of distilled water for 5 min and decanted followed by filtration. Synthesis was initiated by addition of 5 mL of DBTE to 95 mL of aqueous solution with 1 mM HAuCl4 and 0.7 mM of AgNO3 followed by incubation at 50°C for 5 h. The progress of reduction was monitored by measuring the UV-visible spectra of the solution at regular intervals on a spectrophotometer (SpectraMax M5, Molecular Devices Corp, USA) operated at resolution of 1 nm. Bioreduced synthesized by DBTE were centrifuged at 10,000 rpm for 15 min at room temperature, followed by redispersal of the pellet in sterile distilled water to remove any uncoordinated biological molecules. Repetition of alternate centrifugation and redispersion in sterile distilled water for three times ensured better separation of free entities from the nanoparticles which were used for all further biological activities.

2.2. Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM), and Dynamic Light Scattering (DLS) Measurements

Shape and size of the bioreduced were determined using TEM (Tecnai 12 cryo TEM, FEI, Netherland) and was confirmed by JEOL-JEM-2100 higher resolution transmission electron microscope (HRTEM) coupled with elemental composition mapping under scanning transmission electron microscopic mode (STEM). Energy dispersive spectra of , recorded in the energy dispersive spectrometer (EDS) equipped in JEOL JSM 6360A analytical scanning electron microscope at an energy range 0–20 keV confirmed the synthesis of . Particle size was analyzed using the dynamic light scattering equipment (Zetasizer Nano-2590, Malvern Instruments Ltd, Worcestershire, UK) in polystyrene cuvette.

2.3. Antibiofilm Activity

5 μL, OD600 (0.05) adjusted overnight grown cultures of A. baumannii, P. aeruginosa, E. coli, and S. aureus were incubated in absence and in presence of at various concentrations ranging from 1 to 1024 μg/well in Mueller Hinton broth in 96 well microtitre plates. Negative controls (no cells) were included for each of the experimental preparations. The microtitre plates were then incubated overnight at 37°C under static conditions and processed thereafter. Nonadherent cells were removed from microtitre by aspiration. Wells containing biofilm matrices were washed thrice with sterile PBS followed by staining with 0.1% gentian violet (HiMedia, India) for 10 min at room temperature. Excess stain was removed by immersing in water trough followed by drying in laminar air flow. Finally, 200 μL of absolute ethanol was added to each well and shaken at 1020 rpm for 10 sec. Absorbance at 570 nm was recorded in a multiplate reader. Biofilm indices were calculated after normalizing with appropriate controls. All biofilm assays were repeated thrice.

2.4. Visualization of Biofilm Employing SEM and AFM

The biofilm inhibition was carried out on sterile grease free glass cover slips as per the above mentioned procedure with a final volume of 2 mL in 6 well plates. After 24 h, cover slips were washed with sterile PBS followed by fixation with glutaraldehyde which was further dehydrated sequentially using ethyl alcohol and analyzed using scanning electron microscope (SEM, JEOL JSM 6360A).

2.5. Atomic Force Microscopy

Imaging of the cell surface was performed using JPK Nano-wizard II (Germany) AFM setup in intermittent contact (IC) mode in air. Images were recorded in height and deflection (amplitude) modes where deflection mode gives higher contrast in morphological details and height image provide sample surface topography. Silicon nitride tips were used with the tip radius < 10 nm, spring constant of 40 N/m and resonant frequency of 300 KHz for IC mode [34]. Line rate of 0.5−1 Hz was used for topographical (height) and deflection images. Elastic properties like Young’s Modulus of the treated and untreated biofilm samples were evaluated where the cantilever served as soft nanoindenter allowing local testing of small and inhomogeneous samples. Hertzian model of contact theory for indentation with spherical tips was used to evaluate Young’s Modulus [3539]. The cantilever was modified by gluing (Dymax-OPV 30 UV curable glue) 20 μM glass bead to the tip. The spring constant of the cantilever was determined using thermal noise analysis in liquid while glass was used as rigid surface to calibrate the photo detector sensitivity. Force curves were recorded at the rate of 5 μM/sec with typical piezodisplacement of 1 μM–10 μM. Ensemble of 64 force curves was obtained on different locations of the cell surfaces. Analysis of the force curves was done using JPK data processing software. The mean values of Young’s modulus were calculated as the average, weighted over the data obtained by analysis of 64 curves from each independent experiment. Young’s modulus is calculated by the equation [36, 38, 40]where is the force applied by the cantilever, is Young’s Modulus, is the indentation depth, is the radius of the bead (10 um), is the contact radius, and is Poisson’s ratio (0.5 for biological samples).

2.6. Antileishmanial Activity

Promastigotes of Leishmania donovani (MHOM/IN/83/Ag83) were cultured in Medium 199 (Sigma, St. Louis, MO) supplemented with 10% heat inactivated foetal bovine serum (FBS) (Sigma, St. Louis, MO) at 25°C. The parasites were maintained in vitro by subpassaging them every fifth day. Antileishmanial activity of was investigated using MTT (3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) assay. Logarithmic phase L. donovani promastigotes (2 × 105 parasites/mL) were seeded in a 96-well microtitre plate and treated with the nanoparticles followed by incubation at 25°C for 72 h.

2.7. SEM and AFM Analysis

72 h treated L. donovani promastigotes were adhered to poly-L-lysine coated cover slips and fixed with 4% paraformaldehyde for 15 min at room temperature which were subjected to SEM and AFM analysis as per the procedure described in the earlier section.

2.8. DNA Fragmentation Assay

The assay was carried as described in earlier report by Herrmann et al., 1994 with a slight modification [41]. Briefly, L. donovani promastigotes were treated with nanoparticles for 72 h. The pellets were treated with lysis buffer (1% NP-40 in 2 mM EDTA, 50 mM Tris-HCl, pH 7.5) followed by centrifugation at 1600 ×g for 5 min. The supernatant was brought to 1% SDS and treated with RNAase (5 μg/μL) for 2 h at 56°C. Proteinase K (2.5 μg/μL) treatment was given for 3 h at 37°C followed by precipitation of DNA with 1/2 volume 10 M ammonium acetate and 2.5 volume ethanol. The DNA was dissolved in TE buffer (Tris 10 mM, EDTA 1 mM, pH 8.0) and analyzed by agarose gel electrophoresis.

3. Results

3.1. UV-Visible Spectroscopy

DBTE mediated reduction of silver and gold ions in aqueous solution was confirmed by the gradual change in color from orange red to dark reddish violet as a function of reaction time. UV-vis spectra recorded from 0 to 5 h revealed a distinct and clear increase in intensity of the gold surface plasmon band, while a well-defined band corresponding to AgNPs was not observed in case of (Figure 1(a)). Although no significant synthesis was observed from 0 to 0.5 h, but thereafter the rate of reduction took a rapid pace, almost completing the reduction within 4 h, with a slight increase in the intensity till 5 h. UV-vis spectra confirmed the synthesis of which showed a peak at 540 nm unlike that of individual AgNPs at 440 nm (Figure 1(b)). However, a decrease in the peak intensity was observed in case as compared to that of individual AuNPs at 540 nm which might be attributed due to formation of the silver shell onto a gold core.

3.2. TEM, HRTEM, and EDS Analysis

TEM images confirmed the size, shape, and nature of bioreduced nanoparticles. TEM images distinguished the shell from the core with prominent boundary between the two layers (Figure 2(a)). The diameter of the inner core was in a range between 5 and 11 nm while the entire particle showed a diameter of 13 to 19 nm. Particles were found to be almost monodispersed, the majority being spherical with a core represented by dark contrast surrounded by a shell with a lighter shade. Aggregation was not observed indicating the stable nature of the particles. HRTEM analysis was also in agreement with the TEM analysis where the diameter of the core and the entire particle was measured to be 6 to 10 nm and 13 to 18 nm, respectively (Figure 2(c)). Lattice fringes were clearly observed on the surface of the nanoparticles confirming the crystalline nature (Figure 2(c)). Additionally, elemental mapping clearly indicated the location of elemental silver and gold in the core shell nanoparticles (Figure 2(d)). It was distinctly evident that gold formed the core while silver formed the shell of the nanoparticles confirming the fact that the bioreduced nanoparticles were in nature. EDS results confirmed the presence of both elemental silver as well as elemental gold in the nanoparticles. were comprised of % gold and % silver of the total mass (Figure 2(b)). Particle size analysis exhibited a variation mostly between 7 and 18 nm, the majority being 10 nm (Figure 2(a)).

3.3. Biofilm Inhibitory Activity

Biofilm inhibition studies revealed a high efficacy and selectivity of towards controlling bacterial pathogens. , at a concentration of 100 μg/mL, could inhibit biofilm formation up to % against A. baumannii (Figure 3). However, showed a comparatively lower activity equivalent to % against P. aeruginosa at the same concentration. Biofilm formation in case of E. coli was inhibited up to % which was found to be superior to P. aeruginosa while inferior as compared to A. baumannii. It is important to note that the biofilm formation was inhibited up to % even against Gram positive bacteria like S. aureus.

3.4. Scanning Electron Microscopy for Biofilm Inhibition

Scanning electron micrographs revealed the effective inhibition of biofilm by against A. baumannii. Untreated biofilm were represented by confluent lawns (Figure 4(a-i)) with intact cellular morphology. Treatment with exhibited marked biofilm inhibition with discrete cells on the glass surface instead of lawns (Figure 4(a-ii)). The treated cells showed irregular morphology owing to cell damage by the . A similar observation was achieved in case of P. aeruginosa as well. In case of the untreated cells a profound bacterial biofilm surrounded by exopolymeric slimy matrix was seen (Figure 4(b-i)). Treated cells exhibited biofilm inhibition characterized by prominent membrane disintegration leading to efflux of cellular materials (Figure 4(b-ii)). Size of E. coli cells were found to increase abruptly followed by efficient biofilm inhibition on treatment with (Figure 4(c-ii)). Surface attachment was greatly compromised on treatment unlike the untreated cells which formed a compact biofilm due to effective adherence onto the glass surface (Figure 4(c-i)). In case of S. aureus, the untreated biofilm was characterized by groups of closely packed spherical cells (Figure 4(d-i)) which on treatment showed discontinuous biofilm consisting of sparsely packed cells in discrete aggregates. The cells in treated biofilm showed surface invaginations and deformities (Figure 4(d-ii)).

3.5. Atomic Force Microscopy

AFM images were recorded to confirm the effect of treatment on bacterial biofilms which illustrated a packed lawn in case of untreated biofilms. The interrupted biofilms without packed morphology owing to compromised cell adhering capability in treated biofilm (Figure 5(a-ii)) were readily differentiated from the untreated bacterial biofilms (Figure 5(a-i)). A. baumannii showed complete elimination of biofilm characterized by discrete single cells attaching onto the glass surface. The measurements of elastic moduli showed that the treated cells (104.59 KPa) were stiffer as compared to untreated cells (0.82 KPa). P. aeruginosa biofilm exhibited cells embedded in the exopolymeric matrix while in case of treated biofilm, discrete cells were observed (Figures 5(b-i) and 5(b-ii)). Similar trend in the increase of stiffness in the treated cells (75.72 KPa) was observed compared to untreated cells (40.02 KPa). Untreated E. coli biofilm was characterized by evenly distributed uninterrupted confluent bacterial lawns while biofilms treated with showed uneven cellular distribution with discrete solitary cells on the glass surface at large distances (Figures 5(c-i) and 5(c-ii)). Stiffness (57.64 KPa) in the treated biofilm increased as compared to untreated biofilm (22.84 KPa). Inhibition of S. aureus biofilms was clearly evident from the AFM micrographs showing the irregularly clumped cellular masses separated by large valleys unlike the untreated biofilm which exhibited profound uniform lawn (Figures 5(d-i) and 5(d-ii)). Stiffness in treated biofilm increased up to 197.45 KPa which was significantly higher compared to 48.63 KPa found for the untreated biofilm.

3.6. Antileishmanial Activity

The effect of on L. donovani promastigotes was determined quantitatively by MTT. The parasites were killed effectively on treatment with (MIC = 32 μg/mL). SEM micrographs of untreated cells showed intact spindle shaped morphology tapering at both ends with a long flagellum which is typical for L. donovani healthy promastigote (Figure 6(a)). However, cells treated with showed a pronounced deformation characterized by complete loss of spindle shaped morphology finally becoming spherical (Figure 6(b)). Similar results were also obtained in case of AFM micrographs (Figures 6(c) and 6(d)) which confirmed the antileishmanial potential that were further supported by DNA fragmentation (Figure 6(e)).

4. Discussion

D. bulbifera is considered as most important medicinal plant in both Indian and Chinese traditional medicine due to its multidimensional therapeutic potential owing to diverse phytochemicals. In this study, we found a route for rapid and efficient synthesis of by DBTE within 5 h. In our earlier report, AuNPs were found to be formed rapidly within 90 min using D. bulbifera; however, synthesis of AgNPs was found to be completed at 5 h [7, 23]. Thus, the rapidly synthesized AuNPs acts as nanocrystalline seeds on which slowly formed AgNPs are uniformly deposited forming a complete coat on 9 nm Au core. Our results are in close correlation with the recent report on 11 nm Au seed mediated chemical synthesis of 13 to 50 nm core shell nanoparticles using ascorbic acid as reductant and cetyltrimethylammonium chloride (CTAC) as capping agent [42]. Hereby, it is immensely important to note the promises of DBTE mediated rapid synthesis of exotic as a novel route without involvement of any toxic chemicals. Phenolics and flavonoids might be the key players in the rapid synthesis while starch might stabilize the bioreduced nanoparticles which were composed of a gold core and a silver shell. Similarly citric acid and diosgenin may be responsible for the shape evolution. D. bulbifera tubers are reported to possess a rich phenolic and citric acid contents supporting the claim that fresh yam tubers are rich source of antioxidant that play a key role in prevention of oxidative stress mediated emergence of cancer and cardiovascular diseases [7, 43, 44]. Diosgenin and flavonoids and are also reported in D. bulbifera to promote antitumor and anticancer activity, respectively, which strongly support the potential of DBTE to synthesize [19, 22, 45]. Synthesis of was characterized by a peak at 540 nm which was similar to the report of Salunke et al. on synthesis of bimetallic nanoparticles (AgAuNPs) by Plumbago zeylanica indicating Au core and Ag shell [46]. DBTE synthesized showed concentric spherical morphology strikingly different from both the polyhedral particles synthesized by polyol process and blunt ended nanopolygonal particles formed in preliminary reports on biological synthesis [4648].

Bioreduced nanoparticles have found profuse utility in clinical studies and for designing novel therapeutic strategies. Adhesion and colonization lead to most emerging problem of biofilm associated microbial infections. Chronicity of nosocomial infections owing to enhanced persistence is a result of protective biofilms [49]. Hereby, DBTE synthesized , exhibiting potent bacterial biofilm inhibition is a very significant finding towards development of new therapeutic strategy against microbial infections. This is the first report on antibiofilm activity of , synthesized by DBTE against A. baumannii which was found to be considerably superior as compared to bimetallic nanoparticles synthesized by P. zeylanica [46]. Enhanced antibiofilm activity can be attributed by high surface to volume ratio [10, 50, 51]. Our results are concurrent with earlier reports which showed, in nanoparticulate form, metallic silver exhibits antibiofilm activity against A. baumannii at concentrations as high as 1024 μg/well [9, 46]. However, inhibited the biofilm at a lower concentration of 100 μg/mL against A. baumannii which is one of the emerging most potent nosocomial multidrug-resistant pathogen. It poses threat due to its frequent outbreak causing urinary tract infection, meningitis, bacteremia, wound infections, and pneumonia. The ability of the pathogen to survive in diverse ecological niche and environmental conditions forming resistant biofilm facilitates its persistence on both inanimate and animate surfaces for extended periods of time [29, 52, 53]. Similarly it has been reported to gain resistance against multiple metal and metal ions as well in addition to antibiotics leading to compromised drug sensitivity [2628, 54, 55]. Multidrug resistance associated bacteremia, osteomyelitis, and respiratory infections in patients suffering from traumatic injuries have high mortality rate in ICU, ranging from 26% to 68% [52, 56]. It is speculated that the mortality can be attributed to antimicrobial resistance, lower effectiveness of empirical therapy and scarcity of available therapeutic options. Hereby, the observed selectivity of the bioreduced towards inhibition of A. baumannii biofilm is an immensely significant perspective which can be further exploited for designing powerful therapeutic strategy against A. baumannii infections.

showed antibiofilm activity against P. aeruginosa, E. coli and S. aureus leading to damage of cell wall causing efflux of cellular materials. This may be attributed to the property of silver component to adhere to cell surface of bacteria, further generating reactive oxygen species leading to catalytic oxidation of cell membrane causing deformation and damage to the cell surface [7, 57]. Although, AgNPs are reported to inhibit the biofilms formed by P. aeruginosa, this is the first report on inhibition by spherical synthesized by DBTE [58]. It is important to note that abrupt increase in bacterial cell length of E. coli was observed on treatment with which is well in agreement with earlier report justifying gold to be responsible for the observed phenomenon [59]. inhibited the biofilm formation in Gram positive S. aureus as well. Potential of the bioreduced nanoparticles might be attributed due to mechanisms like binding to cellular enzymes and proteins, specifically to SH-groups, interference with bacterial cell integrity and production and conservation of energy, as reported earlier against Staphylococcus epidermidis [8, 6063]. DBTE synthesized showed antileishmanial activity against L. donovani leading to complete disintegration of cellular morphology probably due to the silver component of the nanoparticles. Allahverdiyev et al. reported that AgNPs are leishmanicidal due to their capacity to produce ROS, owing to their large surface areas, smaller size, and ability to bind sulfur- and phosphor-containing groups [64].

5. Conclusion

This is the first report on synthesis of spherical using DBTE which is a novel biological route for synthesis of nanoparticles with unique morphology with concentric Au core and Ag shell using medicinal plant, D. bulbifera. It showed efficient biofilm inhibition against Gram negative bacteria, particularly A. baumannii and Gram positive bacteria, S. aureus. The mechanism behind the biofilm inhibition was observed to be intense damage to the cells leading to disintegration of cell wall causing efflux of cellular materials. Similarly, it showed effective antileishmanial activity against L. donovani. Hence, the present study provides a strong scientific rationale for the use of DBTE synthesized as potent antibiofilm and antileishmanial agent.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.


The authors acknowledge the use of TEM and HRTEM facilities in Chemical Engineering and CRNTS funded by the DST through Nanomission and IRPHA schemes. The authors thank Dr. M. Jayakannan, Indian Institute of Science Education and Research (IISER), Pune, for DLS facility. The authors also thank Mr. Sanjay Patil, Thematic Program Executive, Jawhar Unit, BAIF Development Research Foundation, Pune, India, for imparting knowledge about tribal usage and preparation of D. bulbifera as well as collection of plant samples. S. Ghosh thanks Council of Scientific and Industrial Research (CSIR, Government of India) for Senior Research Fellowship (09/137(0516)/2012-EMR-I). The authors acknowledge financial support for UPE Phase II for 2012–2017 by UGC, New Delhi, India.

Supplementary Materials

Representative force curve for A. baumannii biofilm treated with AucoreAgshellNPs is available in the supplementary material. Characterization of AucoreAgshellNPs synthesized by DBTE employing transmission electron microscopy at various magnifications are presented here.

  1. Supplementary Material