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. 2025 Jan 31;25:35. doi: 10.1186/s12906-025-04788-z

Determination of total phenolic and flavonoid contents, antioxidant and antibacterial potential of the bark extracts of Syzygium guineense (Wild.) DC.

Engeda Dessalegn 1, Mengisteab Mathewos 1, Hiwot Gebremeskel 2, Nigatu Tuasha 2,✉
PMCID: PMC11783908  PMID: 39891191

Abstract

Background

Syzygium guineense (Wild.) DC. is a wild indigenous tree widely used as a traditional medicine for various human ailments in Ethiopia. The purpose of this study was to quantify total phenolic (TPC) and total flavonoid (TFC) contents and determine the antioxidant and antibacterial activities of various solvent extracts of the bark of the plant.

Methods

The TPC and TFC were determined using Folin-Ciocalteu and aluminum chloride methods, respectively. The 2, 2-diphenyl-1–picrylhydrazyl (DPPH) scavenging, ferric-reducing power, and total antioxidant capacity assays were used to evaluate the antioxidant activities. Antibacterial properties were determined using the disc-diffusion and broth dilution assays.

Results

The ethanol extract of the bark was found to have high TPC (37.80 ± 3.70 mgGAE/g) and TFC (19.22 ± 1.44 mgQE/g). Similarly, the ethanol extract showed stronger DPPH scavenging activity (EC50 = 5.62 μg/mL). The ferric-reducing power and total antioxidant capacity were also strong (163.08±11.67 mgAAE/g and 143.72±2.86 mgBHTE/g of dried extract of 1 mg/mL, respectively). The lowest MIC was observed in acetone extract against S. aureus (1.56 mg/mL) and in ethanol extract against K. pneumoniae (1.56 mg/mL).

Conclusion

The ethanol extract of the bark of S. guineense possesses high TPC and TFC. In addition, it showed strong ferric-reducing power and total antioxidant capacity, asserting high antioxidant content. The extracts have shown antibacterial activities against both Gram-positive (S. aureus) and Gram-negative bacterial species. Thus, further in-depth investigations may warrant the isolation of powerful antioxidants and potent antimicrobial agents from the plant.

Keywords: Antimicrobial agents, Ethiopia, Free radical scavenging activity, Medicinal plants, Phytochemicals

Introduction

The use of medicinal plants as a fundamental component of the African traditional healthcare system is the oldest with a long track record and is widely acknowledged in all therapeutic systems. In many parts of rural Africa, traditional healers prescribing medicinal plants are the most easily accessible and affordable health resource available to the local community and at times the only treatment modality that exists [1]. Traditional medicine (TM) in Africa is holistic involving both the body and the mind and traditional healers offer information, counseling, and treatment to patients and their families in a personal manner [1]. Patients’ preferences, the low ratio of medical doctors to the total population, and the lack of effective modern medical treatment for some ailments in Africa are additional factors for the wider practice of TMs.

Ethiopia is rich in plant biodiversity and many of them are used as traditional medicinal plants. It is, therefore, imperative to assume that some of these plants have chemical compounds of therapeutic value that may be used in the treatment of major diseases such as malaria, cancer, and pathogenic microorganisms [2, 3]. According to the World Health Organization (WHO), 70–80% of Africans today depend either totally or partially on TM [4]. According to a recent report, 65% of the Ethiopian population also relies on TM and a significant proportion of the preparations are made from plants [5, 6]. The majority of Ethiopians who rely on TM are rural population and a segment of the urban population where there is little or no access to modern health care [7].

Highly diverse species of plants are used in Ethiopian TM practices and one of them is Syzygium guineense (Wild.) DC. Syzygium guineense (Family Myrtaceae) named “Duuwancho” in Sidamu-Afoo is a wild tree with edible fruits. Its distribution ranges from tropical Africa, sub-tropical and tropical Asia to Australia [8]. Traditionally, the bark of the plant is pounded, macerated, and drunk for the treatment of a health condition locally referred to as ‘ARRISHSHO’ and it cures after inducing diarrhea. ‘ARRISHSHO’ is an acute ailment characterized by pain in the hip, irritation during urination, sweating, and loss of appetite as described by the traditional healers [3]. The ripened fruits of the plant are also eaten in small amounts to treat obesity [3]. Traditionally, it is also used for the treatment of inflammation, various allergic disorders, bronchitis, dysentery, and ulcers [8]. Two bioactive 3-β-hydroxylupane-type isoprenoids, betulinic acid methylene diol ester, were isolated from the stem bark of the plant and were reported to have anti-tuberculosis activity [9]. The crude leaf extract of the plant was also reported to have in vivo antimalarial activities [10]. Various studies reported the bioactivities of S. guineense extracts including antioxidant [11], anti-diabetic [12], antibacterial [13], antihelmintic activities [14], analgesic and anti-inflammatory [15], antidiarrheal, antifungal, antiviral and anticancer activities among others [16]. Therefore, the main purpose of this study was to evaluate the total phenolic and flavonoid contents and antioxidant and antibacterial activities of various solvent extracts of the bark of this widely used traditional medicinal plant.

Materials and methods

Sample preparation and extraction

After permission was obtained from the local authorities, the specimen of S. guineense was collected from the wild habitat of Dalle district, 47 km away from Hawassa, the regional capital city of the Sidama National Regional State, in February 2022. The formal identification of the plant material was done by Melaku Wondafrash, a seasoned plant taxonomist, given the voucher specimen identification number: NT061 and is deposited in publicly available National Herbarium (ETH), Addis Ababa University, Addis Ababa, Ethiopia [17]. After the bark of the plant dried and pounded, chloroform, ethyl acetate, water, acetone, and ethanol extracts all were prepared by dissolving 10 g of the air-dried bark fine powder separately in 100 mL of each solvent. All extractions were conducted in triplicate. After keeping in an orbital shaker (Buchi, Switzerland) for 8 h at room temperature, the extract was filtered using Whatman № 1 filter paper (Whatman LTD, England) and evaporated to dryness under vacuum at 45 ºC under reduced pressure using a rotary evaporator (Buchi, 3000 series, Switzerland). Finally, the dried extracts were stored in a sealed plastic container at 4 ºC until used for the experiments.

Phytochemical analysis

Total phenolic content

The total phenolic content (TPC) was estimated by the Folin-Ciocalteu method with slight modifications [18]. A 0.1 mL of the extract (1 mg/mL) was mixed with 1 mL of diluted Folin-Ciocalteu reagent. The mixture was left for 5 min and then 1mL of sodium carbonate (7.5 g/100mL solution) was added. After incubating at 25 ºC for 90 min, the absorbance of the resulting blue color was measured at 765 nm using a UV-visible spectrophotometer (Spectronic 20, UK). The TPC was expressed in terms of milligram gallic acid equivalent per gram of dried extract (mgGAE/g) using a gallic acid calibration curve (y = 0.024x – 0.014, R2 = 0.996), and the results were calculated using the equation:

graphic file with name M1.gif

Where; C = TPC (mgGAE/g), c = the concentration established from the gallic acid calibration curve (μg/mL), V = volume of extract solution in milliliter, m = weight of dried extract in gram.

Total flavonoid content

The total flavonoid content (TFC) was determined as described by Ayoola et al. [19]. , with minor modifications. The quantification was based on the formation of the yellow color of the flavonoid-aluminum complex. Briefly, a volume of 2 mL of 2% aluminum chloride was mixed with the same volume of the extracts (1 mg/mL). Then Absorbance reading at 415 nm was carried out after 1 h of incubation at room temperature against a blank sample (2 mL of sample solution and 2 mL of ethanol without aluminum chloride). The total flavonoid content was determined using a standard curve of quercetin and values were calculated as milligrams of quercetin equivalent per gram of dried extract (mgQE/g) based on the quercetin calibration curve (y = 0.011x + 0.132, R2 = 0.97) and the results were calculated using the equation:

graphic file with name M2.gif

Where; C = TFC (mgQE/g), c = the concentration established from the quercetin calibration curve (μg/mL), V = the volume of extract solution in milliliter, m = the weight of the dried extract in gram.

DPPH scavenging activity

The DPPH scavenging activity of the extracts and references was determined as described by Engida et al. [18], . Various concentrations ranging from 10 up to 100 μg/mL) of the extracts and commercial antioxidant (ascorbic acid) were taken in different test tubes. The volume of 2 mL of freshly prepared DPPH solution (0.06% w/v) prepared in ethanol was added to each of the test tubes containing 1 mL of the extract. The extract mixtures and the reference standards (ascorbic acid and BHT) were vortexed and left to stand at room temperature in the dark for 30 min. The absorbance of the resulting solution was then measured at 517 nm. Ethanol was used as a blank. The ability to scavenge the DPPH radical was calculated as:

graphic file with name M3.gif

Where Ac is the absorbance of the control and As is the absorbance in the presence of a sample of the extract.

The IC50 value is defined as the effective concentration (μg/mL) of extracts that scavenges the DPPH radical by 50%. All the tests were performed in triplicate and the graph was plotted with the average of three observations. Using the dose-response curve the IC50 values of the commercial antioxidant and crude extracts were calculated.

Ferric-reducing antioxidant power (FRAP)

The presence of antioxidants in the extract causes the reduction of the yellow ferric/ferric cyanide complex to the ferrous form which can be monitored by measuring the formation of Perl’s Prussian blue at 700 nm and was determined as described in Abebie et al. [20]. , .

Different concentrations (0.5 mg/mL and 1 mg/mL) of the bark extract of S. guineense in ethanol were mixed with 2.5 mL potassium phosphate buffer (0.2 M, PH 6.6) and 2.5 mL of 1% potassium ferricyanide. Then, the mixture was incubated at 50 0C for 20 min, and then 2.5 mL of 10% trichloroacetic acid was added to the mixture. Finally, 2.5 mL of the supernatant solution was mixed with 2.5 mL of distilled water and 0.5 mL FeCl3 (0.1%). Absorbance was measured to determine the amount of ferrous cyanide (Prussian blue) at 700 nm against a black in a UV/vis spectrophotometer. An increase in the absorbance of the reaction indicates the reducing power of the samples. The reducing power was expressed as milligram ascorbic acid equivalents per g of the extract (mg AAE/g) using the following equation (y = 0.0049x + 0.231, R2 = 0.98) based on the ascorbic acid calibration curve.

Total antioxidant using phosphomolybdenum assay

The total antioxidant capacity (TAC) was evaluated as described earlier [18] with slight modifications. The solution of different concentrations of 0.3 mL (5 and 10 mg/mL) in ethanol was prepared in different test tubes. The reagent solution was prepared by mixing 10 mL of 0.6 M sulphuric acid, 10 mL of 28 mM sodium phosphate, and 10 mL of 4 mM ammonium molybdate into a beaker, and 3 mL reagent solution was added to all the tubes. A volume of 0.3 mL of ethanol served as a blank. All the tubes were incubated at 95 0C for 90 min. The tubes were measured at 695 nm using UV-visible spectrophotometers. The TAC was expressed as a milligram of Butylated hydroxytoluene equivalent per gram of the dried extract (mg BHTE/g) based on the calibration curve (y = 0.01x + 0.15, R2 = 0.99).

The test microorganisms

Gram-negative bacteria (Escherichia coli (ATCC-25922), Salmonella typhimurium (ATCC-14028), Klebsiella pneumoniae (ATCC-13883), Pseudomonas aeruginosa (ATCC-43495)) and Gram-positive bacteria Staphylococcus aureus (ATCC-25923) were obtained from former Southern Nations, Nationalities and Peoples’ Region (SNNPR) Health Bureau Public Health Institute, Hawassa, Ethiopia in September 2022. Each microbial culture was maintained by sub-culturing on the appropriate nutrient medium and stored at 4 ºC until use [21].

The growth media preparation

The medium was prepared according to the manufacturer’s instructions. Briefly, 38 g Mueller Hinton Agar was added to a flask containing 1000 ml of distilled water and gently heated until the medium was completely dissolved. The medium was sterilized by autoclaving at 121 °C for 15 min. After cooling to about 50 ºC, approximately 15 ml of the sterilized medium was aseptically poured into 90 mm diameter sterilized Petri dishes and allowed to cool. The sterility of the prepared media was checked by incubation of blindly selected plates at 37 °C for 24 h. All chemicals, reagents, standards, and media used for the experiments were purchased from Sigma-Aldrich (St. Louise, MO, USA).

The disc diffusion assay

The disc diffusion susceptibility test was carried out as described earlier [21]. Briefly, the discs of a 6 mm diameter were prepared from Whatman № 1 filter paper using a cleaned paper puncher and were sterilized by autoclaving and dried in an oven at 120 ºC. Then, the discs were placed in a container and stored at 4 ºC until further use.

To determine the inhibition zone, the sterile discs were soaked aseptically by applying 10 μL of the crude extract at concentrations of 100 mg/mL using a sterile micropipette and then allowed to dry at room temperature for 15 min. Plates with gentamycin (10 μg) served as the positive control and discs with distilled water without the plant extracts were used as a negative control. After inoculation, for each test bacteria, the previously soaked disc with the plant extracts was placed at the center of each of the inoculated plates. The antibacterial activities of the plant extract were evaluated by measuring the diameter of the inhibition zone in each of the plates at the end of the incubation period. The diameter of the inhibition zone including the diameter of the disc was measured using sliding digital micro caliper [21].

The preparation of bacterial inoculum

The disk diffusion method was performed according to the standard procedures stated in [21]. Three to five colonies from pure cultures of each of the selected microbe species were transferred with the help of a sterile wire loop into a separately labeled test tube containing 5 mL of nutrient broth and incubated to grow at a temperature of 37 0C for 2 h. The turbidity of the actively growing broth culture was adjusted by adding sterile nutrient broth to obtain turbidity that was comparable to the 0.5 McFarland turbidity standards [22]. The microbial suspension prepared was used to inoculate the sterile MHA plates by streaking with a sterile cotton swab all over the plate. Streaking was repeated by rotating the plate approximately 60 degrees each time to ensure an even distribution of the inoculum. All of the tests were conducted in triplicate and the averages of the three measurements were used to present the results. All the plates were incubated at 37 0C for 24 h.

Determination of minimum inhibitory concentration

The minimum inhibitory concentration (MIC) was determined using the broth dilution test tube method as described earlier [21]. A total of 100 μL of pure nutrient broth was added to each test tube. The total of 10μL stock solution of 100 mg/mL extract dissolved in distilled water was subjected to two-fold serial dilutions ranging between 50 mg/mL to 1.6 mg/mL. Again, 10μL of 0.5 McFarland bacterial suspensions were added to each test tube containing 100 μL pure nutrient and 10 μL plant extracts and then the tubes were incubated at 37 0C for 24 h. After 24 h incubation, the solution was further inoculated in agar plates and incubated. The MIC was taken as the highest dilution of the extract which inhibited the growth of the bacteria. The lowest concentrations of the extracts that inhibited the bacterial growth after 24 h of incubation at 37 0C were recorded as the MIC.

Statistical analysis

The data were subjected to analysis of variance (ANOVA) using SPSS version 20 (IBM SPSS Inc. Chicago, USA). Linear regression analysis was used to calculate the EC50 value. Duncan’s multiple range tests were used to assess mean separation and the statistical significance was considered at a level of P < 0.05.

Results

The total phenolic content and total flavonoid content

The TPC of the bark of S. guineense determined by the Folin–Ciocalteu method using gallic acid as the standard showed that the ethanol fraction had a higher TPC of 37.80 ± 3.70 mgGAE/g. Meanwhile, the highest TFC value was obtained for the ethanol followed by acetone extract (19.22 ± 1.44 and 11.70± 1.11, respectively) (Table 1). Generally, the TPC and TFC contents decreased in the order of ethanol > acetone > water > ethyl acetate > chloroform.

Table 1.

Total phenolic and flavonoid contents of various solvent extracts from the bark extract of S. guineense

Extract TPC (mgGAE/g) TFC (mgQE/g)
Chloroform 10.30± 1.23a 1.88± 0.64a
Ethyl acetate 12.04± 1.10a 4.12± 0.80b
Water 16.61± 0.54b 5.38± 0.66b
Acetone 25.56±2.26c 11.70±1.11c
Ethanol 37.80± 3.70d 19.22±1.44d
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Values are expressed as mean ± SD (n = 3) from triplicate experiments. Means with the same letter in a column were not significantly different at the level of p > 0.05

DPPH scavenging activity

DPPH has a deep purple color in solution and changes to yellow when it encounters a proton-donating substance such as an antioxidant and a radical species which can be quantified by its absorbance reduction at wavelength 520 nm [23]. As shown in Fig. 1, the DPPH scavenging (%) effect of samples increased when the concentration of the extract increased. At 1000 μg/mL, DPPH free radical scavenging activities of the extract and ascorbic acid increased in the following order: chloroform (66.42 ± 1.44%) < ethyl acetate (71.82 ± 0.74%) < water (80.26 ± 0.86%) < acetone (94.50 ± 0.10% ) < ethanol (97.30 ± 0.73% ) < ascorbic acid (98.58 ± 0.20%). This implied that the S. guineense extracts contained a compound that can donate electron/hydrogen easily and stabilize free radicals.

Fig. 1.

Fig. 1

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DPPH radical scavenging activity (%) of various solvent extracts from dried bark of S. guineense and control (L-ascorbic acid) at different concentrations (μg/mL). The values are the average of triplicate measurements (mean ± SD)

In simple terms, the scavenging activities of the bark extracts that correspond to their concentrations, inhibiting 50% of the DPPH radicals were reported. In the present study, it was found that the ethanol extract resulted in the lowest EC50 value of 5.62 μg/mL which is very close to the positive control, ascorbic acid (5.27 μg/mL) (Table 2; Fig. 1).

Table 2.

The EC50 (μg/mL) values of DPPH scavenging activity of the bark extracts of S. guineense with various solvents

Extract EC50 (μg/mL)
Chloroform 50.39 ± 5.04c
Ethyl acetate 38.90 ± 7.26b
Water 12.90 ± 0.82a
Acetone 8.04 ± 0.47a
Ethanol 5.62 ± 0.08a
Ascorbic acid 5.27 ± 0.10a
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Ferric-reducing antioxidant power (FRAP)

The ethanol extract of the bark of the S. guineense was found to have strong ferric-reducing antioxidant power (163.08 ± 11.67 mgAAE/g, treatment dose = 1 mg/mL) increased in a concentration-dependent manner whereas the more nonpolar solvent, chloroform fraction, was found to have weak ferric-reducing antioxidant power (Table 3; Fig. 2A).

Table 3.

The Ferric-reducing power and total antioxidant capacity of various solvent extracts of the bark of S. guineense

Extract Ferric-reducing power Total antioxidant capacity

mgAAE/g

(at 0.5 mg/mL)

mgAAE/g

(at 1 mg/mL)

mgBHTE

(at 0.5 mg/mL)

mgBHTE/g

(at 1 mg/mL)
Chloroform 18.54 ± 0.50a 54.22 ± 2.40a 37.16 ± 1.26a 62.56 ± 2.47a
Ethyl acetate 38.78 ± 2.91b 63.73 ± 3.26a 44.94 ± 1.67ab 69.97 ± 2.88a
Water 30.16 ± 2.80ab 55.31 ± 5.13a 38.42 ± 1.22b 63.80 ± 1.23b
Acetone 62.78 ± 1.14c 134.06 ± 5.13b 51.50 ± 2.07c 112.17 ± 5.34c
Ethanol 82.83 ± 6.60d 163.08 ± 11.67c 80.63 ± 1.60d 143.72 ± 2.86d
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Fig. 2.

Fig. 2

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Ferric-reducing power (mgAAE/g) (A), and total antioxidant capacity (mgBHTE/g) (B) of various solvent extracts from dried bark of S. guineense. The values are the average of triplicate measurements (mean ± SD). The values within the same concentration with the same letter in the histogram bar were not significantly different at P > 0.05

Total antioxidant capacity (TAC)

It was determined that the ethanol extract of the bark of the S. guineense was found to have strong TAC (143.72 ± 2.86 mgBHTE/g, treatment dose = 1 mg/mL) increased in a concentration-dependent manner whereas the more nonpolar solvent, chloroform fraction, was found to have less TAC in a similar trend as that of the ferric-reducing antioxidant power (Tables 3 and Fig. 2B).

Antibacterial activities

Different solvent extracts of the bark of S. guineense were tested against selected bacteria strains (S. aureus, S. typhimurium, K. pneumoniae, and E. coli) for their antibacterial activities. The inhibition zones were compared with gentamicin 10 μg which was used as positive control.

The study revealed that all bacteria strains were inhibited by the bark extracts of S. guineense with different degrees of inhibition zone (range: 25.39 ± 0.8 mm to 8.49 ± 0.26 mm in diameter). As the result indicates, the highest zone of inhibition was recorded against S. aurues with the acetone extract (25.39 ± 0.80) (Table 4; Fig. 3).

Table 4.

Inhibition zones (mm) of various solvent extracts of the bark of S. guineense

Solvents S. aureus S. typhimurium K. pneumoniae E. coli
Chloroform - - - -
Ethyl acetate 14.24± 0.60aC 12.72 ± 2.36aB 15.60 ±1.38aC 8.49 ± 0.38aA
Water 22.24 ± 1.30bC 23.87 ± 0.21bC 16.64 ± 0.92abB 8.98± 0.95aA
Acetone 25.39 ± 0.80cC 23.32 ± 0.52bC 18.26 ± 0.90bB 11.54 ± 0.98bA
Ethanol 23.35 ±1.84bcB 21.76 ± 0.23bB 24.39 ± 0.60cB 8.49 ± 0.26aA
Gentamicin 10 μg 24.08 ± 1.42cA 26.88 ± 1.08cB 29.06 ± 1.41dC 25.63 ± 0.84cA
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The values are expressed as mean ± SD from triplicate experiments. Means with the same letter in a column (lower case) and in a raw (upper case) were not significantly different at P > 0.05

Fig. 3.

Fig. 3

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Zone of inhibitions of the bark extracts of S. guineense in comparison with gentamicin (positive control) against S. aureus. Inhibition zone of the bark extracts of S. guineense with acetone – A; ethyl acetate – EA; ethanol – E; water –W; and chloroform – C extracts

In addition to the inhibition zone determination, the MIC was determined for five extracts of the bark of the plant against four different bacterial strains. Staphylococcus aureus was relatively the most sensitive strain against acetone and ethanol extracts of the bark of S. guineense with MIC values of 1.6 mg/mL and 3.12 mg/mL, respectively. Klebsiella pneumonia was also found to be highly sensitive to the ethanol extracts of the bark of S. guineense (MIC = 1.56 mg/mL). The bacterial strain E. coli was found to be less sensitive even at higher doses of 50 mg/mL of solvent extracts (Table 5; Fig. 4). Gentamicin was used as a positive control and completely inhibited the growth at 10 μg while distilled water without extracts was used as a negative control and thus normal growth was recorded.

Table 5.

The MIC of various solvent extracts (mg/mL) of the bark of S. guineense

Solvents S. aureus S. typhi K. pneumoniae E. coli
Acetone 1.56 3.12 6.25 50
Ethyl acetate 12.5 12.5 12.5 50
Chloroform - - - -
Ethanol 3.12 6.25 1.56 50
water 6.25 3.12 6.25 50
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(-) = No inhibition at 100 mg/mL

Fig. 4.

Fig. 4

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The MIC of the solvent extracts (mg/mL) of the bark of S. guineense against four different bacterial strains

Discussion

In this study, the dominant biologically useful phytochemicals and antibacterial activities of the bark of S. guineense, a traditionally useful plant in most rural parts of Ethiopia were quantitatively evaluated. It is well established that the information derived from various TM systems all over the world is utilized in the process of discovering lifesaving drugs [24]. The TM practiced today is maintained based on perceived safety profiles by the indigenous people which is based on its version of clinical trials, where the TMs have been used only when they have shown to be effective and safe [25]. It is agreeable that the practice of trial may take place for long periods and this might have helped mankind to retain a very detailed knowledge and understanding of many natural medicinal plants. The TM in use, however, needs rigorous validation studies for safe practice and wider application.

The fact that S. guineense is widely used in the TM all over the world [26, 27] is a remarkable indication that the plant has biologically active secondary metabolites as also revealed by the present study. Different parts of the plant reported in TM use include leaves [10, 28], roots [8], fruits [29], and the bark [11]. Among the diseases traditionally treated from preparations of different parts of S. guineense include diabetes [30], chronic diarrhea [31], malaria [10, 32], chest pain, stomachache and ringworm [33], hookworms [34], wounds [26], opportunistic infections [35], infertility, paresthesia and cancer [36].

The extract of the bark of the plant had high TPC and TFC especially the extract with more polar organic solvents. This is in agreement with previous studies in that different parts of the plant extracted with more polar solvents showed high TPC and TFC [26]. The TPC and TFC reported in the present study were found to be high when compared to the reports elsewhere [37]. The antioxidant properties are correlated with the phenolic content of any plants which act as hydrogen donors, and reducing agents, and are capable of scavenging free radicals [38, 39]. The TAC, increasing in a concentration-dependent manner, high DPPH activities, and FRAP are high in more polar organic solvent extracts in the present study. This finding is corroborated by previous studies [40–42].

Free radicals are generated in our body by various endogenous systems and a balance between free radicals and antioxidants is necessary for a proper physiological state. Recently, interest in flavonoids and other polyphenolic compounds has increased due to their potent antioxidant and free-radical scavenging activities [43, 44]. The high contents of TPC and TFC, and the TAC reported here make the S. guineense the potential source of antioxidants.

In the present study, the remarkable DPPH scavenging activity with the lowest EC50 recorded value of 5.62 μg/mL was very close to the positive control, ascorbic acid (5.27 μg/mL). This result further signifies that the plant can be a good source of free radical scavenging compounds and is supported by earlier reports [11]. The organo-protective effect of the bark extract reported elsewhere is also attributed to the high antioxidant activities of the plant and thus agrees with the present finding [45].

The results of the present research should be interpreted cautiously in that high doses of S. guineense extracts of different parts were reported with adverse indications elsewhere. For instance, the administration of high doses of the hydroethanolic extract of its leaves to the pregnant dams showed teratogenic effects and toxicity to the reproductive system in female rats [46, 47]. However, the effective doses in our finding are much less than the suggested high dose that resulted in adverse effects.

The present finding revealed that both Gram-positive (e.g., S. aureus), as well as Gram-negative bacteria strains (e.g., K. pneumonia) were found to be sensitive against the bark extracts of S. guineense with more polar solvents such as water, ethanol and acetone. Both groups of bacteria were found to be non-sensitive to less polar solvent extract (e.g., chloroform) which might indicate the biologically active principle is soluble in more polar solvents [48]. From very nature, Gram-negative bacteria have structural organizations that make them more resistant than Gram-positive bacteria making them the most important public health threat resulting in high morbidity and mortality worldwide. Therefore, it can be argued that the plant can be a potential source of lead materials for new drugs in a quest to fight drug resistance, an ever-present enormous global health crisis [49].

Conclusion

The present results indicate that the ethanol extract of the bark of S. guineense possesses high TPC and TFC endowing it with high antioxidant activities exhibited through DPPH and FRAP assays. The total antioxidant content was also high. The polar solvent extracts of the bark have shown antibacterial activities against both Gram-positive and Gram-negative bacterial species. Therefore, carefully designed further investigations, targeting a lead compound isolation and antioxidant compound characterization are recommended.

Abbreviations

AAE

Ascorbic acid equivalents

BHT

Butylated hydroxytoluene

BHTE

Butylated hydroxytoluene equivalents

DPPH

2,2-diphenyl-1-picrylhydrazyl

FRAP

Ferric-reducing antioxidant power

MHA

Mueller Hinton Agar

MIC

Minimum inhibitory concentration

TAC

Total antioxidant capacity

TFC

Total flavonoid content

TM

Traditional medicine

Author contributions

Conceptual framework: ED, MM, HG, NT. Plant material collection, and extraction: MM, NT. Methodology: ED, MM, HG, NT. Phytochemistry experiments, spectrophotometry, and formal analysis: ED, MM, HG. Data curation – NT. Writing – original draft: NT. Writing – review & editing: ED, MM, HG, NT. All authors read and approved the final manuscript.

Funding

The authors declare that some costs incurred during this research were covered by the modest financial support obtained from the Hawassa College of Teacher Education (HCTE).

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

The study was approved by the research ethics committee of the Hawassa College of Teacher Education (Ref. №: HCTE/RIC/021/21). In addition, permission was obtained from local authorities to collect the plant material for this study. All the reagents used in this study were prepared, used, and disposed of according to the set laboratory guidelines and the material safety and data sheets (MSDS).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

References

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

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