Phytochemical Insights and Antimicrobial Potential of Acacia Nilotica (L.) Delile (Babul)

Acacia nilotica (L.) Delile, commonly known as Babul, the Indian Gum Arabic tree, or Kikar, is a member of the family Fabaceae (subfamily Mimosoideae). It is a medium-sized, thorny, perennial tree widely distributed throughout tropical and subtropical regions of Africa, the Middle East, and the Indian subcontinent. The tree is characterized by bipinnate compound leaves, fragrant yellow spherical flowers, and curved pods containing numerous seeds. It thrives in dry and semi-arid regions and plays an important ecological role by enriching soil fertility through nitrogen fixation [1]. Beyond its environmental importance, Acacia nilotica holds a prominent place in traditional medicine systems such as Ayurveda, Siddha, and Unani, where it is valued for its astringent, anti-inflammatory, and antimicrobial properties. Historically, different parts of Acacia nilotica—including bark, pods, leaves, seeds, and gum—have been used to manage ailments such as diarrhoea, cough, wounds, toothache, and infections of the skin and mucous membranes. The plant is also used in formulations for oral hygiene and wound healing due to its ability to inhibit microbial growth and promote tissue regeneration [2]. Phytochemical studies have revealed that Acacia nilotica is rich in bioactive compounds such as tannins, flavonoids, phenolic acids, alkaloids, glycosides, saponins, and terpenoids, which collectively contribute to its broad-spectrum antimicrobial activity. Tannins like catechin and gallic acid are known to precipitate microbial proteins and disrupt cell wall integrity, leading to bacterial death. Flavonoids such as quercetin and rutin interfere with nucleic acid synthesis and energy metabolism, while saponins and terpenoids increase cell membrane permeability, resulting in leakage of intracellular materials [3]. These synergistic effects make Acacia nilotica extracts effective against both Gram-positive and Gram-negative bacteria, including Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa, as well as fungal species such as Candida albicans and Aspergillus Niger. Several studies have demonstrated that ethanolic and methanolic extracts of Acacia nilotica bark exhibit significant zones of inhibition comparable to standard antibiotics, suggesting their potential as a natural antimicrobial agent [4]. Furthermore, the plant’s antioxidant properties enhance its ability to combat oxidative stress during infections, while its astringent and anti-inflammatory actions promote wound healing [5]. Recent advances in phytochemical characterization using chromatographic and spectroscopic techniques (HPLC, GC-MS, LC-MS) have facilitated the identification of its active constituents, paving the way for formulation development in modern herbal therapeutics. In view of the increasing prevalence of antibiotic resistance, the exploration of Acacia nilotica as a natural source of antimicrobial compounds holds great pharmacological and clinical promise. Its rich phytochemical profile, proven traditional efficacy, and wide biological spectrum highlight its potential as a safe, affordable, and sustainable alternative for the treatment of infectious diseases [2].

Fig – acacia nilotica plant, leaves and flowers

Fig-Acacia nilotica plant, leaves, and flowers. Bark, seeds.

Taxonomical identification

• Domain: Eukaryota
• Kingdom: Plantae
• Sub-kingdom: Viridiplantae
• Phylum/Division: Tracheophyta (Angiospermae)
• Sub-phylum: Euphyllophytina
• Class: Magnoliopsida (Dicotyledons)
• Sub-class: Rosidae
• Order: Fabales
• Family: Fabaceae (Leguminosae)
• Subfamily: Mimosoideae
• Genus: Acacia
• Species: Acacia nilotica (L.) Delile
• Synonym: Acacia arabica (Lam.) Wild [1].

MACROSCOPY

Acacia nilotica (L.) Delile, commonly known as Babul, is a moderate-sized, thorny tree characterized by its spreading crown and rough, fissured bark. It typically attains a height of 5–20 meters and is well adapted to arid and semi-arid climates. The entire plant exudes a gum resin when injured, which is of considerable medicinal and industrial value [6].

Stem and Bark: The stem is erect, woody, and covered with dark grayish-brown to blackish bark that becomes deeply fissured with age. The bark thickness ranges from 0.5 to 1.5 cm, showing an external rough texture and an inner reddish-brown surface. When cut or scratched, the bark exudes a sticky, amber-coloured gum (commonly called Babul gum or gum Arabic). It possesses an acidic and slightly bitter taste. The younger branches are slender, angular, and pubescent, bearing a pair of short, sharp, straight thorns (5–7 mm) at each node [7].

Leaves: The leaves are bipinnate, alternately arranged, and bright green in colour. Each leaf measures 4–7.5 cm in length, consisting of 3–10 pairs of pinnae, each bearing 10–20 pairs of small leaflets. The leaflets are oblong or linear, about 2–4 mm long and 1 mm broad, with a smooth margin and a rounded apex. The leaves are exstipulate or occasionally have minute stipules, and the rachis often bears minute glands [8].

Flowers: The flowers of A. nilotica are small, golden-yellow, and fragrant, arranged in dense globular heads or spikes approximately 1–1.5 cm in diameter. Each inflorescence arises from the leaf axis or on terminal branches. The flowers are bisexual, actinomorphic, and pentamerous, possessing numerous bright yellow stamens that impart a soft appearance. The calyx is small and cup-shaped, while the corolla is funnel-shaped, with petals shorter than the stamens. Flowering usually occurs between February and June in India, depending on climatic conditions [9].

Fruits (Pods): The fruit is a flat, straight, or slightly curved pod, 7–15 cm long and 1.5–2 cm wide, containing 8–15 dark brown seeds separated by narrow constrictions between them, giving the pod a beaded appearance. When mature, the pods turn dark brown or black and do not split open easily. The pod surface is smooth, leathery, and slightly glossy [10].

Seeds: The seeds are hard, oval, or compressed, about 6–8 mm long, smooth, and dark brown in colour. Each seed is enclosed in a tough testa and is attached to the pod by a short funicle. The seeds contain fixed oil and proteins and are used traditionally in various therapeutic formulations [11].

Gum Exudate: The gum collected from incisions in the bark is a transparent to amber-coloured, brittle solid that swells in water to form a viscous solution. It is nearly tasteless and odourless, soluble in hot water, and widely used as a demulcent, emulsifying agent, and stabilizer in pharmaceuticals.

Odor and Taste: The bark and pods have a characteristic astringent odour and bitter, slightly pleasant, astringent taste due to high tannin content.

Traditional and Ethnomedical Uses 

Acacia nilotica is an important medicinal plant in Ayurvedic, Unani, and folk medicine, valued for its wide range of therapeutic properties.

1. Ayurvedic Medicine: Known as Babula; classified under Kashaya rasa (astringent). Used to balance Kapha and Pitta doshas. Bark decoction is used for diarrhoea, dysentery, sore throat, and leucorrhoea.

2. Unani Medicine: Called Kikar or Babool, regarded as a blood purifier and gum tonic. Gum (Gond Babool) is used as a demulcent and cooling agent in urinary and digestive disorders [14].

3. Folk and Tribal Uses: Bark and leaf paste applied to wounds, ulcers, and burns. Leaf decoction is used for cough, fever, and eye infections. Pods are used as an anthelmintic for intestinal worms [15].

4. Oral and Dental Care: Twigs used as natural toothbrushes (chewing sticks) for gum strengthening and prevention of gingivitis.

5. Dermatological Applications: Bark and gum are used externally for eczema, boils, and leprosy due to their antiseptic and astringent actions.

6. Respiratory and Reproductive Disorders: Bark infusion is used for bronchitis, throat infections, and menorrhagia [16].

Phytochemistry and Antimicrobial Activity Relevance of Acacia nilotica (L.) Delile

Acacia nilotica (L.) Delile, commonly known as the Babul or Indian Gum Arabic tree, is a member of the family Fabaceae (Leguminosae) and is recognized as one of the most pharmacologically important medicinal trees in tropical and subtropical regions. Every part of the plant—bark, leaves, pods, seeds, and gum—contains a diverse array of bioactive phytochemicals that contribute to its therapeutic properties, especially its broad-spectrum antimicrobial potential [1]. Traditionally used in Ayurvedic and Unani systems for treating infections, wounds, diarrhoea, and skin diseases, the plant has attracted extensive phytochemical and pharmacological research attention in recent decades [17]. The antimicrobial potential of A. nilotica is closely associated with its polyphenolic content, particularly tannins, flavonoids, and phenolic acids. These compounds not only inhibit microbial growth but also demonstrate antioxidant, anti-inflammatory, and cytoprotective activities, making the plant a strong candidate for development into natural antimicrobial formulations [2, 18].

1. Phytochemical Constituents:

Extensive phytochemical analyses of A. nilotica have revealed a wide range of secondary metabolites with distinct biological activities. The major classes of compounds include tannins, flavonoids, phenolic acids, alkaloids, saponins, terpenoids, sterols, and carbohydrates. The concentration and diversity of these compounds vary with the plant part, solvent, and geographical origin [7, 19].

1.1 Tannins: Tannins are the most abundant class of compounds in A. nilotica, particularly in the bark and pods. The plant contains up to 32% condensed tannins (proanthocyanidins), which are responsible for its characteristic astringent taste. These tannins include gallic acid, ellagic acid, catechin, epicatechin, and procyanidins. Tannins possess strong antimicrobial, antioxidant, and wound-healing properties. Their antimicrobial mechanism involves precipitation of microbial proteins, inhibition of extracellular enzymes, and complexation with polysaccharide layers of microbial cell walls, leading to cell lysis [2, 7, 20].

1.2 Flavonoids: Flavonoids are another dominant group of phytoconstituents. Major flavonoids reported in A. nilotica are quercetin, kaempferol, luteolin, catechin, epigallocatechin, and rutin. These polyphenolic compounds contribute to the plant’s antioxidant, anti-inflammatory, and antimicrobial effects. Flavonoids act by disrupting microbial membranes, chelating metal ions, and inhibiting nucleic acid synthesis. Their ability to scavenge free radicals also enhances immune response and tissue regeneration in infected tissues [19, 21].

1.3 Phenolic Acids and Polyphenols: The plant is rich in phenolic acids such as gallic acid, caffeic acid, chlorogenic acid, and ferulic acid, which have proven antimicrobial activity. These compounds interfere with microbial enzyme systems, leading to oxidative damage and suppression of pathogen growth [15, 22].

1.4 Alkaloids and Saponins: Acacia nilotica also contains moderate amounts of alkaloids (N-methyltyramine, tryptamine, and hordenine) and saponins, which exhibit surface-active and membrane-disrupting effects against bacteria and fungi. Saponins reduce surface tension and increase permeability of microbial membranes, enhancing leakage of intracellular components [23].

1.5 Terpenoids and Sterols: Terpenoids such as lupeol, β-amyrin, and betulin, and sterols like β-sitosterol are also present. These compounds contribute to the plant’s anti-inflammatory, antifungal, and wound-healing properties by modulating inflammatory mediators and strengthening host defence mechanisms [2, 24].

1.6 Carbohydrates and Gum Exudate: The gum obtained from the bark is composed mainly of arabinose, galactose, rhamnose, and uronic acids, which have mild antimicrobial and demulcent properties. The gum forms viscous solutions that act as protective films on mucous membranes, thereby preventing microbial colonization and aiding wound healing [2, 25].

2. Analytical studies:

Advanced analytical techniques, including HPLC, GC–MS, LC–MS, FTIR, and NMR, have been employed to identify and quantify bioactive compounds in A. nilotica. HPLC analysis confirmed high concentrations of gallic acid and catechin in bark and pods. GC-MS studies revealed volatile compounds such as octadecanoic acid, hexadecanoic acid, and methyl esters, which exhibit antimicrobial and antioxidant effects. FTIR spectra indicated the presence of hydroxyl, carbonyl, and aromatic functional groups typical of polyphenols and flavonoids, confirming their structural diversity. These analytical studies support the correlation between phytochemical composition and biological activity, establishing A. nilotica as a chemically rich and pharmacologically potent plant [26,27].

3. Antimicrobial Activity:

The antimicrobial efficacy of A. nilotica has been extensively investigated against a variety of Gram-positive and Gram-negative bacteria, as well as fungal pathogens. Extracts prepared using different solvents—methanol, ethanol, acetone, and water—have shown varying degrees of inhibition depending on the polarity and solubility of active constituents.

3.1 Antibacterial Activity: Numerous studies report that A. nilotica extracts exhibit strong antibacterial activity against both Gram-positive bacteria, such as Staphylococcus aureus, Bacillus subtilis, and Streptococcus pyogenes, and Gram-negative bacteria, including Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Salmonella typhi. The methanolic bark extract has shown significant inhibition zones comparable to standard antibiotics like ciprofloxacin and ampicillin. Minimum inhibitory concentration (MIC) values generally range from 50 to 250 µg/mL, indicating strong bactericidal potential. The activity is primarily attributed to the synergistic action of tannins and flavonoids, which disrupt bacterial membranes, interfere with energy metabolism, and cause leakage of essential ions and proteins. Phenolic acids further inhibit bacterial enzymes and suppress nucleic acid replication [2, 28, 29].

3.2 Antifungal Activity: A. nilotica has also demonstrated potent antifungal activity against pathogenic fungi such as Candida albicans, Aspergillus Niger, Trichophyton mentagrophytes, and Fusarium oxysporum. The mechanism involves disruption of fungal cell walls, inhibition of ergosterol synthesis, and oxidative stress induction through the generation of reactive oxygen species (ROS). The presence of tannins and saponins plays a crucial role in altering fungal membrane permeability, leading to cytoplasmic leakage and cell death [29, 30].

3.3 Antiviral and Antiparasitic Effects: Although less studied, some reports suggest that A. nilotica extracts exhibit antiviral activity against RNA viruses and antiparasitic effects against helminths. The polyphenols may inhibit viral replication enzymes and prevent parasite adherence to host tissues [31].