Background

Nanotechnology is a nascent technology, gaining popularity globally due to its usefulness in various fields. Nanometals ranging from 1 to 100 nm in size have unique physical and chemical properties which can be exploited for various applications [1, 2]. Further these are promising novel therapeutic agents having antimicrobial and antibiofilm activity.

Development of microbial resistance to antibiotics is a major challenge in the medical field. Therefore, the search for drugs with new modes of action is of major interest in the pharmaceutical and research communities. Two potential sources of novel antimicrobial agents are medicinal plants and nanomaterials [3, 4]. The antimicrobial properties of nanomaterials including metal nanoparticles can be attributed to different mechanisms such as generation of reactive oxygen species, inactivation of cellular enzymes and nucleic acids of the microbes resulting in pore formation in the bacterial cell wall [3]. Among the metal nanoparticles TiO2 NPs are known to be cost effective, stable and safe for humans and the environment. A unique property of TiO2 NPs is the photocatalytic property resulting in enhanced microbicidal activity on exposure to light in the UV range [3, 5]. TiO2 NPs exist in three crystalline phases, where the anastase phase demonstrates high photocatalytic and antimicrobial properties [3].

Garcinia zeylanica is an endemic plant to Sri Lanka, which belongs to the family Guttiferae (Clusiaceae). Ragunathan et al. [6] reported antibacterial activity of pericarp of G. zeylanica extract against MRSA, while it had no antimicrobial activity against Candida albicans and Candida parapsilosis [7]. Others have reported antimicrobial activity of Garcinia species against Staphylococcus aureus, Streptococcus pyogenes and some Gram negative bacteria [8]. Garcinia species have many important phytochemicals with antimicrobial potential [9, 10]. The phytochemical analysis of G. zeylanica which is an endemic plant to Sri Lanka, is not yet documented. This study aimed to determine the antibacterial activity of TiO2 NPs modified with G. zeylanica aqueous extract. The combined synergistic effect of phytochemicals and TiO2 NPs were also investigated.

Methods

Preparation of Garcinia zeylanica aqueous extract

Dried pericarp of G. zeylanica was collected locally and authenticated at the Bandaranayaka Memorial Ayurveda Research Institute, Navinna, Maharagama, Sri Lanka. The pericarp was rinsed, dried (6 h at 42 °C) and aqueous extract was prepared using 30 g of plant material in 720 ml distilled water, then boiled under low heat to reduce the volume to 120 ml according to Ayurvedic protocol [11]. The plant extract was filtered using sterile Whatman No 1 filter paper. The filtrate was transferred to a sterile glass container and stored in the refrigerator (4 °C) up to 2 weeks.

Characterization and surface modification of TiO2 NPs with G. zeylanica extract

Surface modification of 21 nm TiO2 NPs (Sigma Aldrich) with G. zeylanica aqueous extract was done by refluxing 25 ml of G. zeylanica aqueous extract with 0.30 g of TiO2 (mainly anatase). Solid part was centrifuged and separated. Separated solid was washed with distilled water several times by centrifugation. Washed solid was separated air dried and placed in a vacuum desiccator for 48 h.

Scanning electron microscope (SEM) imaging was performed to understand the surface morphology of TiO2 of the coated petri dishes. SEM imaging was done using FE-SEM (JSM-6320F) at accelerating voltages of 10 kV. Powered X-ray diffraction (XRD) analysis was carried out for the identification of the phase of coated TiO2 using Ultima III (Rigaku) powder diffractometer (Cu-Kα/λ = 0.154 nm). Surface characterization of pure and modified NPs were performed using diffuse reflectance spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Diffuse reflectance spectroscopic studies were carried out using PerkinElmer Lambda 35 spectrophotometer equipped with integrating sphere. ATR-FTIR analysis was carried out using Thermo Scientific Nicolet iS10 FTIR spectrometer.

Phytochemical analysis of the aqueous G. zeylanica extract

Qualitative analysis of various phytocompounds present in the G. zeylanica aqueous extract was done using previously described protocol by Krishnamoorty et al. [12]. Flavanoids, terpenoids, phenols, tannins, cardiac glycosides, carbohydrates, saponins, amino acids, phlobatannin, sterols and alkaloids were detected in this study.

Microorganisms

A clinically confirmed isolate of Methicillin resistant S. aureus was obtained from the culture collection at the Department of Microbiology, University of Sri Jayewardenepura. The organism was cultured on Nutrient agar at 37 °C for 18 h. Suspensions of organisms were prepared in sterile normal saline to obtain a 0.5 MacFarland absorbance corresponding to 108 organisms/ml.

Determination of antimicrobial activity of 21 nm TiO2 NPs, and TiO2 NPs modified with G. zeylanica

TiO2 NPs was used at a concentration of 13.9 g/l in sterile miliq (MQ) water [13]. Suspension of TiO2 was prepared by sonication at 35 kHz for 1 h followed by autoclaving for 30 min at 121 °C. The pH of all solutions was adjusted to pH 5.5 prior to coating of the petri dishes.

A separate plate (A) was used as negative control which contained MQ water. Sterile 3 cm petri dishes were coated with (B) TiO2 only, (C) G. zeylanica aqueous extract only and (D) G. zeylanica extract modifies with TiO2. Each petri dish was coated by adding 1 ml of solutions of B, C and D to individual petri dishes. The petri dishes were then evaporated to dryness.

One milliliter of MRSA suspension (108 organisms/ml) was added to each petri dish. The inoculated petri dishes were kept for 1, 4 and 24 h, at room temperature. At the end of each time point 100 μl of suspension was collected from each petri dish and colony forming units/ml (CFU/ml) was determined by spread plate method on Nutrient agar. Further, to determine the enhanced antimicrobial activity due to the photocatalytic activity of TiO2 NPs, one set of petri dishes (tests and control) were incubated for 30 min in sunlight after addition of MRSA suspension and the number of colonies were counted as described above. All experiments were done in triplicates.

Statistical analysis

Colony forming units/ml was calculated by multiplying the number of colonies obtained by plating 100 μl of suspension by the dilution factor. This was further multiplied by 10 to obtain CFU/ml. The percentage reduction was calculated as follows:

$${\text{Average}}\;{\text{reduction}}\% = \frac{{{\text{CFU/ml}}\;{\text{in}}\;{\text{MQ}} - {\text{CFU/ml}}\;{\text{in}}\;{\text{TiO}}_{2} }}{{{\text{CFU/ml}}\;{\text{in}}\;{\text{MQ}}}} \times 100$$

The paired t test was used to compare the significant differences between test and control. Significance was tested at p = 0.05.

Results and discussion

SEM and XRD analysis

A scanning electron microscope (SEM) image of the surface of TiO2 coated petri dish is shown in the Fig. 1. Petri dish surface was evenly coated with TiO2. Figure 2 shows the XRD pattern of the coated TiO2. The pattern recorded closely resembles the previously published XRD pattern of the anatase phase and rutile phase of TiO2 [14–16].

Fig. 1
figure 1

SEM image of TiO2 coated on a petri dish. Inset 10 nm magnification

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Fig. 2
figure 2

XRD pattern of TiO2 NPs

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Diffuse reflectance, UV–visible and ATR-FTIR study

Diffuse reflectance spectra of TiO2 and TiO2 modified with G. zeylanica aqueous extract are shown in Fig. 3. Alteration of the diffuse reflectance spectrum of TiO2 noticeably indicates the characteristic change of TiO2 surface followed by the adsorption of G. zeylanica extract. The diffuse reflectance spectra were analyzed using [17] the Kubelka–Munk transformed reflectance spectra according to,

$$\alpha_{KM} = \frac{{\left( {1 - R_{\infty } } \right)^{2} }}{{2R_{\infty } }}$$

where α KM is the equivalent absorption coefficient, R ∞ is the reflectance of an infinitely thick sample with respect to a reference at each wavelength. Kubelka–Munk transformed reflectance spectra are shown in the inserted image of Fig. 3. Surface modification of TiO2 NPs with the adsorption of G. zeylanica extract, causes to decrease the band gap energy of TiO2 NPs. Band gap energy of bare TiO2 and G. zeylanica extract adsorbed TiO2 were found to be 3.24 and 2.61 eV, respectively. Lowering the band gap energy of TiO2 is leading to enhancement of photocatalytic activity under visible light [18] which is reflected by change in the colour of the TiO2 surface to buff colour. UV–visible absorption spectrum of dilute solution of G. zeylanica aqueous extract is shown in the image of Fig. 4.

Fig. 3
figure 3

Diffuse reflectance spectra of a TiO2 modified with G. zeylanica extract and b TiO2. Inset Kubelka–Munk transformed reflectance spectra

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Fig. 4
figure 4

UV–Vis absorption spectrum of aqueous extract of G. zeylanica

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ATR-FTIR spectra of dried pulp of G. zeylanica extract, G. zeylanica extract adsorbed TiO2 and TiO2 are shown in Fig. 5. ATR-FTIR spectrum of dried pulp of G. zeylanica extract closely resembles the previously published FTIR spectrum of dried pulp of G. pedunculata [19]. Adsorption of surface anchoring compounds in G. zeylanica extract on to TiO2 is confirmed by the presence of IR peaks of G. zeylanica extract, for G. zeylanica extract treated TiO2. FTIR frequencies suggested that the presence of –OH group (3351 cm−1 for O–H stretching), alkane side chains (2942 cm−1 is characteristic for C–H stretching), carbonyl group (1724 cm−1 for the C=O stretching), and carboxylic group (1402 cm−1 is for (COO−) asymmetric stretching) [19–21]. IR absorption peak at 1724 cm−1 is decreased by the adsorption of G. zeylanica extract into TiO2, which may be due to the deprotonating of carboxylic group [20].

Fig. 5
figure 5

ATR-FTIR spectra of a dried G. zeylanica extract, b TiO2 modified with G. zeylanica extract, and c TiO2

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Phytochemical screening of the aqueous extract of G. zeylanica

Qualitative analysis of G. zeylanica extract revealed the presence of tannins, cardiac glycosides, carbohydrates, coumarin and saponins (Table 1). Tanins are a group of polyphenolic compounds and their antimicrobial activity against fungi, bacteria and viruses have been reported [22]. Coumarins which are reported to be present in plant extracts including Garcinia species, have antimicrobial and anti-inflammatory activities [23]. Saponin is a glycoside and are present in plants with reported antibacterial and antifungal activity [24].

Table 1 Phytochemical screening of the aqueous extract of G. zeylanica
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Antibacterial activity of TiO2

The colony forming units of MRSA reduced significantly (p = 0.0001) after 30 min in the presence of TiO2 following sunlight exposure compared to the control having only MQ water exposed to sunlight. When MRSA suspension (108 organisms/ml) was added to TiO2 coated plates and incubated for 1, 4 and 24 h (without exposure to sunlight), there was a significant reduction in the colony counts (p = 0.0002, 0.0022, 0.0322 respectively) when compared to the control (Fig. 6). The average percentage reduction of MRSA was seen to be 99.1% after 30 min sunlight exposure when compared to the control. The percentage reduction of colony counts seen after 1, 4 and 24 h, were 48.3, 59.2 and 32.9% respectively. These results demonstrate that TiO2 itself has antimicrobial activity which is enhanced in the presence of sunlight. TiO2 has photocatalytic properties which have been reported to be useful as a microbicide [3]. Our study shows that in the presence of sunlight the antimicrobial activity of TiO2 is enhanced against MRSA. Several groups have evaluated the antimicrobial activity of TiO2 against both Gram negative bacteria such as Escherichia coli [3], Salmonella typhimurium [4], Pseudomonas aeruginosa [4, 25], Bacteroides fragilis [4] and Gram positive bacteria such as S. aureus [25], Enterococcus faecalis [26], Streptococcus pneumoniae [26], MRSA [26], fungi such as C. albicans [27], Aspergillus niger and Trichoderma reesei [28] and viruses such as HSV-1 [29] and influenza virus [30]. The advantage of TiO2 as an environmental disinfectant is mainly due to its photocatalytic activity in the presence of UV irradiation. TiO2, when exposed to light in the UV range (λ < 400 nm) result in generation of redox reactions that produce reactive oxygen species, such as hydroxyl radical (·OH), superoxide radical (·O2 −) and singlet oxygen (1O2). These free radicals contribute to the biocidal activity by destruction of cellular organic compounds [26]. Hence close proximity of the microorganisms to the TiO2 NPs is needed for good bactericidal activity.

Fig. 6
figure 6

Antibacterial activity of TiO2 against MRSA

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The antimicrobial activity of TiO2 even in the absence of photo activation has been well reported [26]. TiO2 carries a positive charge while the surface of microorganisms carry negative charges resulting in an electromagnetic attraction between microorganisms and the TiO2 NPs which leads to oxidation reactions. TiO2 deactivates the cellular enzymes and DNA by coordinating to electron-donating groups, such as: thiols, amides, carbohydrates, indoles, hydroxyls etc. The resulting pits formed in bacterial cell walls lead to increased permeability and cell death [26].

TiO2 NPs are reported to be non carcinogenic and nontoxic [31] and are used extensively in food packaging [5], textile industry [32], self-cleaning ceramics and glass [33], in the paper industry for improving the opacity of paper [33], cosmetic products such as sunscreen creams [33] etc. Further, TiO2 NPs are used in commercial products such as water purification plants [34]. The antimicrobial activity of TiO2 NPs are exploited in medical devices, in order to prevent biofilm formation and sepsis [35–37].

Antibacterial effect of G. zeylanica aqueous extract

Antimicrobial activity of G. zeylanica alone and TiO2 modified with G. zeylanica showed a significant reduction in colony forming units at all time points tested as shown in Fig. 7. When MRSA was treated with the aqueous extract of G. zeylanica (0.25 g/ml) and exposed to sunlight for 30 min, a significant reduction of MRSA colony counts were observed, compared to the control (p = 0.0001). Further, when MRSA was incubated without sunlight for 1, 4 and 24 h, a significant reduction (p = 0.0002, 0.0007, 0.0044 respectively) of colony counts was seen compared to the control. This shows that the plant extract itself exhibits strong antimicrobial activity against MRSA. The average percentage reduction of MRSA was seen to be 99.96% after 30 min sunlight exposure when compared to the control. The percentage reduction of colony counts seen after 1, 4 and 24 h, without sunlight were 99.96, 99.93 and 99.84% respectively. The TiO2 modified with G. zeylanica aqueous extract demonstrated remarkably enhanced antimicrobial activity compared to the antimicrobial activity of TiO2 alone. Dried pericarp of G. zeylanica and other Garcinia species is widely used as a flavouring and preserving agent in traditional culinary practices in Sri Lanka and other Asian countries. In Ayurvedic practices, Garcinia is used in treatment of skin and soft tissue infections. Further, it is included as a component of Ayurvedic wound wash. In this study, the aqueous extract of the pericarp of an endemic plant, G. zeylanica was investigated for synergistic microbicidal activity when combined with TiO2 NPs. While the antimicrobial activity of other Garcinia species have been reported in detail, reports on the antimicrobial activity of G. zeylanica is not available. Recent study by Ragunathan reports that the aqueous extract of G. zeylanica pericarp showed antibacterial activity against MRSA while no activity was detected for Candida species [6]. The G. zeylanica aqueous extract was used after adjusting the pH to 5.5 throughout the experiments, which is compatible for use as a wound wash.

Fig. 7
figure 7

Antibacterial activity of G. zeylanica aqueous extract and TiO2 modified with G. zeylanica aqueous extract

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Garcinia zeylanica extracts from other species have been reported to contain hydroxy citric acid, xanthones, flavonoids and benzophenone derivatives such as garcinol [38]. Previous reports have investigated the antimicrobial activity of Garcinia Cambogia [39], and Garcinia indica [40].

Antibacterial effect of TiO2 modified with G. zeylanica aqueous extract

When the TiO2 was modified with G. zeylanica extract, there was significant antimicrobial activity in the presence of sunlight (p value = 0.0001) compared to the control. When the modified extract was incubated with MRSA for 1, 4 and 24 h, the antimicrobial activity was seen to be further enhanced with increasing incubation time (p = 0.0002, 0.0007, 0.0044). The percentage reduction of colony counts at all four time points were >99.99%. These results show that the antimicrobial activity of TiO2 was significantly enhanced when modified with G. zeylanica both in the presence and absence of sunlight as shown in Fig. 7. Exposure to sunlight and prolong contact was seen to further enhance the antimicrobial activity.

On comparison of antimicrobial activity of G. zeylanica extract only and TiO2 modified with G. zeylanica aqueous extract, a significant enhancement of microbicidal activity was observed in the presence of TiO2 modified with G. zeylanica aqueous extract (exposed to sunlight or without sunlight exposure). Further, prolonged contact with TiO2 modified with G. zeylanica aqueous extract showed a significant reduction in colony counts compared to G. zeylanica alone as shown in Table 2. Figure 8 shows a representative experiment where colony counts were obtained after 1 h contact of MRSA (108 cells/ml) with the control (a), TiO2 coated plate (b), G. zeylanica aqueous extract coated plate (c) and TiO2 modified with G. zeylanica aqueous extract coated plate (d). A clear reduction in colony counts were observed in plates c (99.96%) and d (99.99%) when compared to the control. The antimicrobial activity of TiO2 modified with G. zeylanica aqueous extract is thought to be due to multiple mechanisms of the phytochemicals and TiO2 NPs. Garcinol which is an important phytochemical, is reported to competitively inhibit histone acetyltransferases in cells [10]. It has also been reported to regulate gene expression in HeLa cells. Further, garcinol is able to induce apoptosis in cells making it a potential therapeutic agent in cancer treatment [10]. The combination of G. zeylanica and TiO2 as a potential antimicrobial agent in medicine may be an important future direction due to the widely reported emergence of multidrug resistance among microbes, which is a major challenge in medicine.

Table 2 Comparison of antimicrobial activity of G. zeylanica extract and TiO2 modified with G. zeylanica aqueous extract
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Fig. 8
figure 8

MRSA colonies with 1 h incubation a MQ water, b TiO2, c G. zeylanica aqueous extract, and d TiO2 modified with G. zeylanica aqueous extract

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Conclusions

Anatase 21 nm TiO2 NPs shows antimicrobial activity against MRSA following photoactivation by sunlight. G. zeylanica aqueous extract itself has antimicrobial activity against MRSA. Enhanced antimicrobial activity was observed when the TiO2 was modified with G. zeylanica aqueous extract. Activity against MRSA was further enhanced when TiO2 was modified with G. zeylanica aqueous extract with the exposure to the sunlight.