Antimicrobial activities of the extracts and secondary metabolites from Clausena genus – A review

2.1 Occurrence

Clausena is a genus of Angiosperms (flowering plant) belonging to the citrus family, Rutaceae [18]. Clausena has the widest geographical distribution of the orange subfamily genus [19] and is found in some countries in Africa, India, Taiwan, Australia, New Guinea, and Pacific islands [20,21]. Clausena’s taxonomy remains ambiguous, with the genus estimated to have 15–30 species names, of which about 24–27 names of the species are accepted with other names made synonyms [17,22]. About 17–18 of the accepted species names are reported to be phytochemically or pharmacologically examined. These species are C. anisata, C. lansium, C. indica, C. anisum-olens, C. lenis, C. harmandiana, C. smyrelliana, C. dunniana, C. emarginata, C. dentata, C. heptaphylla, C. vestita, C. hainanensis, C. suffruticosa, C. yunnanensis, C. odorata, and C. sanki [23]. Other species names include C. brevistyla, C. engleri, C. austroindica, Clausena poilanei, C. kanpurensis, C. henryi, C. inolida, and C. luxurians, which are also accepted but are yet to be phytochemically or pharmacologically examined.

2.2 Botanical presentation

The Clausena genus is well known to include shrubs and trees. However, the character of the growth and the height of these shrubs and trees varies greatly. They are also characterized by having berry-like fruits with pinnate leaves. The gynophore supporting the ovary has been the most characterized morphological feature in this genus, separating them from other related genera [21]. Secondary metabolites such as alkaloids, flavonoids, and terpenoids are reported to be abundant in Clausena plants [17,24,25], but the principal phytochemical components in Clausena plants are the carbazole alkaloids and coumarins, with the alkaloids being known to exhibit high antibacterial, anticancer, antidiabetics, and antiparasitic activities [25,26].

3.1 Clausena anisata (Wild.) Hook.f. ex Benth

C. anisata, popularly known as Iperepesi in the Xhosa language [27] or “Ana” by the people of Tamil Nadu [28], is a tropical shrub or tree with a strong scent that can reach a height of 10 m. It grows in and along the edges of evergreen forests. The plant is mostly found in Africa and Mozambique, known as “Horsewood.” It also grows in Indonesia, Malaysia, India, Sri Lanka, Queensland in north-eastern Australia, and certain Pacific islands in tropical and South-East Asia (Figure 1) [29,30].

Figure 1

Clausena anisata: (a) the leaves, and (b) the fruit. Source: (a) tropical.theferns.info; (b) growwild.co.za.

It has pinnately complex leaves with 10–17 alternating leaflets. The leaves have a pungent smell and are densely studded with glands when crushed. It has petite, white flowers with orange-yellow stamens [27,31].

The traditional medicinal values of different parts of C. anisata have been reported to be useful as effective treatments for parasitic infections, especially flatworm infections, such as taeniasis and schistosomiasis, and eye diseases; influenza and other respiratory illnesses [32]; heart disease; hypertension; and abdominal cramps. Moreover, constipation and gastroenteritis, a liver disease that causes breathlessness, malaria, fevers and pyrexia, boils, rheumatism, arthritis, other inflammatory diseases, headaches, body pains, toothaches, swollen gums, impotence, sterility, convulsions, and some mental disorders. The leaves of C. anisata are used to treat hypertension in South Africa, and fresh leaves are burned to repel mosquitoes in the Philippines [28,33,34].

3.2 Clausena lansium (Lour.) Skeels

This species, locally known as “wampee,” is an evergreen shrub or small tree that grows to a height of 6 m, with long, upward-slanting flexible branches, rough gray-brown bark, and smooth, dark green leaves. The plant is a highly prized fruit tree that grows primarily in Southern China. It is also grown extensively in tropical and subtropical areas. The white or yellow fruits are ovoid, roughly the size of a pigeon’s egg, with different sizes and shapes depending on the cultivated variety (Figure 2) [35,36,37].

Figure 2

Clausena lansum: (a) the flowers, and (b) the leaves and fruit. Sources: commons.wikimedia.org.

The fruit, leaves, and root of wampee have long been utilized to treat gastrointestinal disorders such as acute, chronic inflammation, and ulcers. Furthermore, the peels of C. lansium have been used as a stomachic for treating bronchitis, and as a vermifuge in India and China [38]. If the huge seed or seeds are discarded, a completely ripe, peeled wampee is pleasant to eat out of hand. The seeded pulp can be used to make fruit cups, gelatins, and other desserts, as well as pies and jams. The leaves are historically used to cure coughs, asthma, hepatitis, and dermatological diseases. The decoction of the leaves is also used as a hair wash to eliminate dandruff and maintain hair color [39,40,41].

The seeded fruits are served with meat meals in China. A bottled, carbonated beverage equivalent to champagne is manufactured in Southeast Asia by fermenting the fruit with sugar and filtering out the juice. The Chinese believe that eating wampee will help overcome the negative consequences of eating too many lychees [36,42].

3.3 Clausena harmandiana (Pierre) Guillaumin

C. harmandiana is a 1–1.5 m tall evergreen shrub with leaves up to 20 cm long and three leaflets ranging in size from 2 to 4 cm. C. harmandiana is a robust shrubby plant native to Asia that has a citrus scent and contains fragrant oil. It is referred to as “Song-fa” in Thailand. It produces an egg-shaped berry with a diameter of 3–5 mm. It is dark red when ripe and contains one or two seeds. It is mostly found across much of Asia [36,43]. The plant is believed to grow in the understory of deciduous and evergreen woods on various soil types or along streams across Laos, but primarily on poor sandy soils [44] (Figure 3).

Figure 3

Clausena harmandiana: (a) the fruits, and (b) the leaves and flowers. Source: (a) flickr.com; (b) shutterstock.com.

The young leaves of C. harmandiana are reported to be edible, and traditionally, C. harmandiata roots, young leaves, bark, and flowers have been combined with different herbs to relieve intestinal gas and food poisoning. The roots are also mentioned to be used traditionally to deal with headaches, fever, bronchitis, and eye pain. The leaves are used to feed cattle, buffalo, and humans [43,45].

3.4 Clausena anisum-olens (Bl.) Merr

C. anisum-olens is a fragrant evergreen shrub or small tree that can reach a height of 15 m. It is a plant native to the wet lowlands of the tropics, mostly distributed within the Philippines islands, where it thrives at elevations below 500 m. When crushed, all portions of the plant emit strong anise or anise-like odor, and it thrives in annual seasonal rainfall of roughly 2,000 mm, with high temperatures of around 30°C and lowest temperatures of around 20°C throughout the year (Figure 4) [36,46].

Figure 4

Clausena anisum-olens: (a) the leaves, (b) the flowers. Source: (a) cloud.diversityoflife.org; (b) flickr.com.

The plant has long been used as a food, beverage, and medicine. The leaves of C. anisum-olens are well-known in Philippine traditional medicine, where they are used as condiment local dishes and drinks. The leaves are filled into pillows for a sedative effect, utilized in rheumatism treatments, and used in a decoction to treat nausea at some point of pregnancy. An infusion of the roots and fruit is used to cure a cough accompanied by a fever. The essential oil extracted from the leaves can be used as a substitute for anise oil. Fermentation of the leaves should be avoided because it leads to the production of unpleasant notes in the essential oil, and these essential oils are said to have potential use as a cheap source of natural anethol [36,47,48]. C. anisum-olens (Bl.) Merr. was classified as a synonym of C. sanki (perr.) Molino [22].

3.5 Clausena dentata (Willd.) M. Roem.

C. dentata (Willd.) Roem is a small tree plant that grows 2–6 m tall and bears delicious fruit. It has odoriferous leaves that are heavily spotted, membranaceous, odoriferous, and erose near the tip. It is predominantly found in Karnataka’s Coorg, N. Kanara, and Shimoga districts, growing in evergreen and semi-evergreen woods. It is well known among the hill tribes where it is locally called “Mor Koorangee” [49].

C. dentata (Willd.) Roem is a small tree well-known for its usage in traditional Chinese medicine. It is primarily found in India, Sri Lanka, and China. It produces tasty fruit blooms in April and ripens in late June. “Mor Koorangee” is the local name given to the tree by the hill tribes (Figure 5) [50].

Figure 5

Clausena dentata (Willd.) M. Roem: (a) the flowers, (b) the leaves. Source: indiabiodiversity.org.

3.6 Clausena excavata Burm.f

C. excavata is a strong-smelling shrub known locally as “Cherek hitam,” “Chemama,” and “Kemantu hitam.” Its gynophore is formed like an hourglass, usually found in China and Malaysia. It has a tall, slender tree that could attain a height of 10 m. Twigs have a fine hairy appearance. Pinnate leaves have 10–15 pairs of dark green narrowly oval oblique leaflets that are 3.5–7 cm long and have pointy tips. It has small white flowers bloom in terminal clusters, accompanied by translucent pink berries measuring 7–10 mm in diameter and holding one to two seeds each [18,23,51].

In traditional medicine, C. excavata has been used to relieve abdominal pain, snakebite, and detoxify agents. The pounded root is used as a poultice for lesions, such as nose ulcers, and the leaves are also used as a poultice [52].

Colic can be treated with a decoction made from the flowers and leaves of C. excavata, and a decoction made from the leaves is given after childbirth. This plant’s leaves treat colds, stomachaches, malaria, and dysentery. The dried and powdered rootstock can be used to cure decayed teeth, while the stem can be used to treat colic with or without diarrhea (Figure 6) [53,54].

Figure 6

Clausena excavata: (a) the leaves and flowers, (b) the fruit. Source: pinterest.com.

3.7 Clausena indica (Dalzell) Oliv.

C. indica (Dalzell) Oliv. is a small evergreen shrub or tree that grows to a height of 3–7 m. It has alternate leaves that are up to 25 cm long and have 7–13 leaflets that are alternate and opposite, measuring 3.5–7.5 cm × 2–3.5 cm. It bears globose flowers in buds that are carried on short (2 mm long) with sharp or rounded sepals and ciliate. Its petals are oblong and acute, measuring about 3.5 mm × 1.2 mm, with a filament subulated above and dilated below, measuring about 2 mm long. C. indica is primarily found in Vietnam, Southeast Asia, Southern India, and Sri Lanka. The fruit has a yellowish, globose berry with one or two seeds roughly 13 mm in diameter. In Malayalam it is called as “Gorakotta,” whereas in Tamil it is locally called as “Kariveppilei” [22,55,56].

C. indica (Dalzell) Oliv. is a plant that has been used in conventional medication for hundreds of years. It is also collected in the wild for local consumption. It bears beautiful flowers that bloom in February and fruits in June. The leaves and roots are used to cure colds, flu, headaches, colic, and rheumatism; the smashed leaves are used to treat joint dislocations and bone fractures; and the essential oils are used for a massage [55,57,58] (Figure 7).

Figure 7

Clausena indica (Dalzell) Oliv.: (a) the fruit and flowers, (b) the leaves. Source: commons.wikimedia.org.

3.8 Clausena lenis Drake

C. lenis, also known as “Clausena kerrii Craib,” is a little evergreen tree that grows to a height of 2–3 m. Its leaves are 9–15 foliolate; leaflet blades are ovate to lanceolate, asymmetric, 2–5 or 1.5–3.5 cm at the rachis’ base, ca. 18–11 cm at the middle and apex, margin crenate, with 4–5 petals, white but reddish to dark yellow at the base. Flowers ovoid in the bud, 8–10 stamens; filaments short; anthers oblong Fruit is globose, with a diameter of about 1 cm and 1–3 seeds. C. lenis is native to China, Vietnam, Thailand, and Laos. It normally blooms between April and June and bears fruits around September to October [23,59,60]. No traditional use has been reported for this plant (Figure 8).

Figure 8

Clausena lenis Drake: the leaves and flowers. Source: mdpi.com.

3.9 Clausena smyrelliana P.I Forst.

C. smyrelliana, also known as Smyrell’s Clausena or Native wampi, is a small evergreen shrub in the Rutaceae family that grows 6–10 m tall and is native to the sub-Tropical Rain Forest. It has 10 cm long dark green aromatic leaves glossy on top and spectacular white fragrant flowers with long style, no gynophore, straight staminal filaments, subcylindrical buds, and corymbiform inflorescence. Its fruits are white [22,23,61]. C. smyrelliana, commonly known as Greg’s wampii, is a rare plant found exclusively in southeast Queensland of Australia, Maryborough, and Hervey Bay. Under Queensland’s Nature Conservation Act 1992, the species thrives in damp Sandy soil and has been designated as “Endangered” [62]. No traditional use was reported for this plant (Figure 9).

Figure 9

Clausena smyrelliana P.I Forst: the whole plant tree. Source: ppnn.org.au.

3.10 Clausena dunniana H.

C. dunniana, a synonym for Clausena anisata, is deciduous trees that grow 2–5 m tall. It has 5–15 foliolate leaves with oblong to lanceolate leaflet blades, 4–10 cm × 2–5 cm, glabrous or villous base, and terminal inflorescences. Flowers bloom between June and July and are 4 (or 5)-merous and globose in the bud, with 8 (or 10) stamens; filaments geniculate in the center, subulate toward the apex. The ovary is globose, and the style is shorter than the ovary. When ripe, the fruit is blue-black, globose, 1–1.5 cm in diameter, and 1- or 2-seeded, and it appears in October–November [22,23,63].

C. dunniana is native to Montane forests, moist areas in the mountains, and is found in Guangdong, Guangxi, Guizhou, West Hubei, Hunan, East and Southeast Sichuan, and South Yunnan [23]. C. dunniana is a synonym of C. anisata [64]. No traditional use was reported for this plant (Figure 10).

Figure 10

Clausena dunniana H.: (a) the leaves, (b) the flowers. Source: flickr.com.

3.11 Clausena emarginata

C. emarginata is an evergreen tree that reaches a height of 4–15 m. It has grayish black branchlets. Leaflet blades are subsessile, obliquely lanceolate to oblong, 2–6 cm × 1–3 cm, dark brownish-black when dried, base oblique, edge crenate, apex obtuse; leaves are 5–11 foliolate. Bracts subulate, inflorescences terminal or axillary, 3–7 cm. The lobes of the calyx are roughly oval [63].

At anthesis, the petals are around 4 mm long, with 10 stamens; filaments geniculate, slightly enlarged in the basal half, and longer than anthers. The disk has been lengthened. It has a glabrous ovary. Fruits are pale yellow, 8–10 mm in diameter, and have one or two seeds. The species grows in valley forests at elevations of 300–800 m blooms in March–April and bear fruit in June–July [63,65].

C. emarginata was classified as a synonym of Clausena sanki, which was later renamed as C. anisum-olens [64]. C. emarginata Huang (Rutaceae), a bush prevalent across southern China, has long been used as a folk remedy for coughs, headaches, rheumatoid arthritis, gastrointestinal problems, and other ailments [66].

3.12 Clausena heptaphylla (Roxb.) Wight and Arn.

C. heptaphylla is a small bushy shrub or small tree that grows 1–4 m tall. They have broad compound leaves ca. 45 cm long and alternating, dark green, and glossy top with light green bottom. Flowers ca. 5 mm long; petals ca. 3–3.5 cm × 1–1.5 mm, greenish-yellow, imbricate, and oblong, with 8 stamens, subequal in length; filaments ca. 2 mm long, subulate above, dilated below. The plant which is propagated by seed flowers in March–April and bears fruit in May–August. The species, which grows well in an open forest at heights of up to 2,500 m, is indigenous to E. India, Nepal, Bangladesh, Myanmar, Thailand, Cambodia, Laos, Vietnam, Malaysia, Indonesia, and Philippines. It is locally called Panbilash, Karanphul, or Pomkaphur by the bengalis [57,67].

C. heptaphylla is a species of tree in the Rutaceae family that grows in a self-supporting manner. The plant is mostly harvested from the wild for local use of its essential oil, where the essential oil from the leaves is used to flavor alcoholic beverages in the same way as those of C. anisum-olens [67,68] (Figure 11).

Figure 11

Clausena heptaphylla (Roxb.) Wight and Arn: (a) the leaves, (b) the fruits. Source: (a) tropical.theferns.info; (b) efloraofindia.com.

3.13 Clausena vestita D. D. Tao

C. vestita is a Chinese tree that grows up to 4 m tall. The plant, which is distributed mainly in Yunnan province, has leaves that are 5–7-foliolate; petiolules 2–4 mm; leaflet blades alternating, generally ovate, elliptic, or orbicular, 3–11 1.5–8 cm, apex rounded, obtuse, or acute; leaflet blades alternate, broadly ovate, elliptic, or orbicular, 3–11 1.5–8 cm, apex rounded or acute; apex Calyx has four parts and stays put in the fruit. Fruits are bluish-black, globose to broadly ellipsoid in shape, 1.2–1.6 cm in diameter, glabrous, and have 1–3 seeds, 8–12 mm × 6–9 mm, ovoid to broadly ellipsoid seeds. The species, which is abundant in dry, hot river valleys, ripens in early May [63,69]. C. vestita is a synonym of C. anisata [64]. No traditional use was reported for these species (Figure 12).

Figure 12

Clausena vestita D. D. Tao: the leaves and fruit. Source: ScienceDirect.com.

3.14 Clausena hainanensis

C. hainanensis Huang ex Xing is a Chinese plant exclusively found on the Chinese island of Hainan. It has shrubs or trees up to 5 m tall. It has leaves that grow at about 25–37 foliolate with leaflets alternate or opposite, asymmetrically elliptic, ca. 21 cm, midvein slightly depressed, base oblique and obtuse, margin repand, apex obtuse. Flowers are unknown. Infructescences are terminal and narrowly paniculate, measuring around 5 cm in length. Fruit is pale yellow ellipsoid and it ripens around July to August [22,70]. No traditional use was reported for this species (Figure 13).

Figure 13

Clausena hainanensis: the leaves and fruit. Source: ScienceDirect.com.

3.15 Clausena suffruticosa

C. suffruticosa, also known as Sadi urisha, is an understory shrub found in mountainous areas of Bangladesh’s Chittagong and Sylhet districts, the Eastern Himalayan regions, India’s Kashi Hill, and Burma [27,71].

C. suffruticosa, also known as Kalomoricha in Chakma, has been utilized in traditional Chakma herbal medicine for a long time. Different portions of this plant have been utilized to treat chronic disorders such as paralysis, tumors, kidney, liver, discomfort, bleeding, and fever. It is also used to treat mumps, viral pneumonia, and cerebrospinal meningitis, among other conditions. Rheumatoid arthritis has traditionally been treated with a paste made from the root of C. suffruticosa (Figure 14) [72,73].

Figure 14

Clausena suffruticosa: the leaves. Source: lietuvai.lt.

3.16 Clausena yunnanensis C. C. Huang

C. yunnanensis is a tree species in the Rutaceae family. The grey trees are 3–8 m tall. The species is well known to have a big Pith and stout branchlets. Leaves 5–11 mm; leaflet blades oblong to ovate-elliptic, 10–40 cm × 5–16 cm, with a papery, margin denticulate. It has inflorescences terminal, paniculate to 40 cm with bracts narrowly ovate and apex acute. Flowers are globose in the bud with Pedicel 1.5–3 mm, Sepals 1 mm, Petals 2–3 mm, Stamen 10 mm. Fruit mostly orange in color are 1 or 2 seeded. The species which grows in mountain forests are reported to be flowers in June and fruits around September to October [63]. C. yunnanensis was synonymized with C. engleri Tanaka [64]. No any traditional use was reported for these species.

3.17 Clausena odorata C. C. Huang

C. odorata is a woody shrub in the Rutaceae family with dark purple-red stems that grow to a height of 2 m. It bears 19–25 foliolate leaves with 1–2 mm petiolules and leaflet blades that are oblong to lanceolate, asymmetric, 4–7 2–3.5 cm, base oblique, border crenulate to subentire, apex acuminate, and often reused. Flowers which bloom in April are aromatic and 5-merous, 3–4 mm with white petals. Stigma capitate is formed like a shield [22,23]. C. odorata is a synonym of Clausena anisata [64]. No traditional use was reported for these species.

4.1 Antibacterial activities

Antibiotic resistance has become a global pandemic and one of the most serious global concerns due to the global epidemiology of bacterial illnesses. Because of the accelerated resistance of microorganisms to antimicrobial medications, new cost-effective antimicrobial remedies, both herbal and artificial, are needed [74].

Lawal et al. [78] investigated the antibacterial activities of acetone extract from the leaves of C. anisata where it was reported to be active against both Gram-positive and Gram-negative bacteria with minimum inhibitory concentration (MIC) ranging from 0.1 to 0.5 mg/mL. The potential of C. anisata, ethanol stem extract against Propionibacterium acnes was also reported by De Canha et al. [79] with an MIC value of 31.25 μg/mL. Moreover, Agyepong et al. [80] investigated the antimicrobial activity of C. anisata against four Gram-positive bacteria and two Gram-negative bacteria. The result shows that C. anisata was active against all the tested organisms with MIC range of 0.5–7.0 mg/mL against Gram-positive bacteria and 2.5–1.0 mg/mL against Gram-negative bacteria. The antibacterial activities of silver nanoparticles synthesized from C. anisata have also been reported by Arsia [81], where the silver nanoparticle leaf and root extracts showed antibacterial activity at concentration ranging from 50 to 150 µg/mL. Senthilkumar and Venkatesalu [83] and Dorcas et al. [88] have all reported the activities of the C. anisata leaf essential oil with the results all indicating a promising MIC against one or more of the tested bacteria species. C. anisata root [82,84], leaf [84,85], and stem bark [87] have all been investigated and was shown to be active against one or more bacteria using different extracts. Makirita et al. [86] evaluated the antimicrobial activities of C. anisata against seven Gram-negative bacteria, with all extracts indicating antibacterial activity on at least three to five of the tested seven bacteria with MIC value ranging from 0.7812 to 12.5 mg/mL. The highest activity was demonstrated by the twig’s ethyl acetate extracts with MIC value of 0.7812 mg/mL against Pseudomonas aeruginosa.

Jagadeesan and Elumalai [89] reported the hexane, ethyl acetate, and methanol leaf extract activities of C. excavata against both Gram-positive and Gram-negative bacteria with the results showing a promising zone of inhibition ranging from 12.29 to 189.6 mm. Moreover, the ethanolic extracts from the leaves of four Rutaceae species, including Acronychia pedunculata, Clausena excavata, Glycosmis pentaphylla, and Luvunga scandens, were tested for antibacterial efficacy against six bacteria species [90]. The results show that C. excavata leaf extract was found to resist four of the tested bacteria, including B. cereus, Staphylococcus aureus, E. coli, and P. aeruginosa, but fails to show resistance against Salmonella enteritidis and Salmonella typhimurium.

Dong et al. [94] reported the isolation of a terpenoid compound, Nerol oxide-8-carboxylic acid (1), and one new flavonoid glycoside, claulansoside A (2), together with six known compounds, clausenamide (3), quercetin (4), isorhamnetin (5), dihydromyric (6), 2″,3″-dihydroxyanisolactone (7), and (E,E)-8-(7-hydroxy-3,7-dimethylocta-2,5-dienyloxy)psoralen (8), from the peels of Clausena lansium (Lour.) Skeels. Only compounds 1 and 7 exhibited antibacterial activity against S. aureus with the diameter zones of inhibition of 11.5 and 14.2 mm. A further investigation on the stems and leaves of C. lansium undertaken by Lui et al. [93] led to the isolation and characterization of four terpenoid compounds (1–4). The bacterial activity of all isolated compounds was tested. Only compound 4 demonstrated a weak antibacterial activity against B. cereus, with IC50 values of 74.6 µM. These data show that terpenoids derived from C. lansum tend to demonstrate a very low antibacterial activity as shown from their weak zone of inhibition, with the coumarin compound reported to have a moderate potency. The activities of the trunk [91] and Twings [92] of C. lansium against one or more bacteria were also reported, with all showing a promising inhibition against the tested bacteria species.

Maneerat et al. [96] isolated 15 alkaloid compounds from the twigs of C. harmandiana. The isolated compounds were evaluated for their antibacteria activities against MRSA SK1, S. aureus TISTR 1466, E. coli TISTR 780, and S. typhimurium TISTR 292. All the tested compounds were reported to show weak or no activity against Gram-negative bacteria. However, compound 6 was found to show potent antibacteria activity against MRSA SK1 with an MIC value of 0.25 μg/mL which is higher than a standard drug, vancomycin (MIC value = 1 μg/mL), while compounds 5 and 14 showed strong antibacteria activity against MRSA SK1 with MIC values of 8 and 4 μg/mL, respectively. Also, compound 14 displayed strong antibacteria potency against S. aureus TISTR 1466 with an MIC value of 4 μg/mL. However, the γ-lactone (1) and δ-lactone (4) carbazoles reduced anti-bacterial activity against MRSA SK1 when compared to that of free prenyl carbazole (14), but the activity was found to be enhanced when the dehydration (C-3′/C-5′) of compound 5 was formed. Moreover, the selective O-methylation of compound 6 makes it to have higher antibacterial activity than normal drugs. Overall, the data indicated that alkaloid compounds derived from C. harmandiana were found to be highly effective against bacterial species, particularly compound 6 that yielded a promising result and compound 14 that shows dual activities. The activities of C. indica leaves essential oil [95] were equally reported. Chatchawanchonteera et al. [97] conducted a research on the antibacterial efficacy of C. harmandiana stem, root bark, and leaf extracts against bacteria isolated from canine otitis externa. The results reveal that C. harmandiana stem, root bark, and leaf extracts have antibacterial activity against bacteria isolated from dog’s otitis externa with minimal bactericidal concentration values ranging from 6.93 to 111.1 mg/mL. The stem bark and leaves [99], leaf essential oil [100], and root [101] of C. suffruticosa were all tested and found to be efficacious against one or more of the tested bacteria species.

Maneerat et al. [105] reported the isolation of four new carbazole alkaloids, clausenawallines (1−4), along with 18 known compounds (5−22) from the roots of Clausena wallichii. All compounds were evaluated for their antibacterial activity against Gram-positive and Gram-negative bacteria, where all the carbazole alkaloids were reported to have weak (MIC 64−128 μg/mL) or no antibacterial activity against E. coli TISTR 780 and S. typhimurium TISTR 292, except for compound 19 that shows moderate activity against S. typhimurium TISTR 292 with an MIC value of 32 μg/mL. Compounds 9 and 3 exhibited good antibacterial activity against MRSA SK1 (4 μg/mL) and S. aureus TISTR 1466 (8 μg/mL), respectively. Compound 22 showed good antibacterial activity against S. aureus (MIC 8 μg/mL) and weaker activity against MRSA SK1 (MIC 16 μg/mL). The other compounds had either weak (MIC 64−128 μg/mL) or no antibacterial activity against both S. aureus and MRSA SK1. Although the activities of some of the alkaloid compounds isolated from C. wallichi are promising, they all tend to have activity toward a single target bacterium only without having effect on the other. C. lenis stem bark [59], C. dentata leaf [102] and stem bark [103], C. heptaphylla leaves [68] and stem bark [67], and the nutlets from C. anisum-olens have all been reported with promising activities against one or more of the bacteria species.

The leaf, stem bark, root, rhizomes, twigs, and fruits of various Clausena species have been tested against various bacteria in the hunt for antibacterial drugs. C. anisata was the most studied Clausena species tested against Gram-positive and Gram-negative bacteria using different solvents (Figure 15), this may be due to the abundance of phytochemicals found in various parts of the plant [75]. S. aureus was the most tested Gram-positive bacteria (Figure 16), while E. coli and P. aeruginosa from the Gram-negative bacteria (Figure 17). In general, much research has been done on the antibacterial properties of Clausena species, with the leaves of the Clausena species being the most extensively tested portion (Figure 18). The activities of all portions of the Clausena species reported against various bacteria species have justified the claim of using Clausena species in the traditional treatment of antibacterial diseases [54,76,77].

Figure 15

Clausena species screening reported against different bacteria species.

Figure 16

Gram-positive bacterium tested against Clausena species.

Figure 17

Gram-negative bacterium active against Clausena species.

Figure 18

Parts of Clausena species reported for antibacteria screening.

Compounds isolated from different parts of the Clausena species which were reported to have antibacterial activities against one or more bacteria species include nerol oxide-8-carboxylic acid (1), 2″,3″-dihydroxyanisolactone (2), (+)-(E)- α -santalen-12-oic acid (3), clausamine A (4), B (5), and F (6), clausenawalline B (7) and E (8), and 2,7-dihydroxy-3-formyl-1–(3′-methyl-2-butenyl) carbazole (9) (Table 1).

Compared to the thousands of bacteria identified worldwide, more research on different portions of other Clausena species is needed to explore their strength against the bacteria species that are dangerous to humans.

4.2 Antifungal activities

Candida albicans is the most prevalent cause of oral candidiasis, although other Candida species, such as Candida tropicalis, Candida glabrata, Candida parapsilosis, Candida krusei, and Candida guilliermondii, have also emerged as important infections [107]. Patients with candidiasis and other fungal diseases face several challenges, including antifungal resistance, treatment toxicity, and high antifungal drug costs [108]. As a result, the range of powerful antifungal drugs available decreases, necessitating the development of recent antifungal treatments.

The antifungal activities of the acetone, methanol, petroleum ether, and chloroform leaf extract of C. anisata were all reported by Makirita et al. [86], where the results show that acetone leaf extract has more antifungal activities with MIC values ranging from 0.02 to 0.63 mg/mL than the other solvents used, having MIC values ranging from 5.0 to 10.0 mg/mL. Similarly, Ying-Hua Yang et al. [117] described that two new monoterpene coumarins were isolated from the leaves and twigs of C. anisum-olens where they were found to exhibit activities against C. albicans, C. tropicalis, and C. krusei but had no detectable inhibitory activity against the tested species. Yan et al. [115] conducted a research involving the isolation of seven amide compounds from the seeds of C. lansium. The results of the antifungal assay revealed that lansiumamide B and C had an excellent antifungal activity against Sclerotinia sclerotiorum with EC₅₀ values of 4.95 and 13.24 μg/mL, which were both lower than that of carbendazim (EC₅₀ = 0.64 μg/mL). Further in vitro investigation of these compounds antifungal properties against S. sclerotiorum were conducted. Lansiumamide B was discovered to cause cell rupture and mycelial abnormalities of S. sclerotiorum, and its curative efficacy (75.17%) against S. sclerotiorum infection was better than that of carbendazim (56.57%). This study reveals that amides isolated from C. lansium possessed the potential to be used as botanical fungicides for commercial application or can be used as templates for developing new fungicides with novel action.

Moreover, the essential oil of C. anisata [111]; C. lansum from Hanan, China [114] and Guangxi province, China [142]; C. excavata, C. emarginata, C. dunniana [142]; C. harmandiana [116]; and C. suffruticosa [100] were all reported to have activities against one or more fungi species with different solvents used with santalane sesquiterpene been the major compound found in the essential oil. A report by Makirita et al. [86] on the antifungal activities of the leaves, stem bark, fruits, and twigs of C. anisata against C. albicans and C. neoformans using chloroform, ethyl acetate, and methanol as solvents shows promising antifungal activity against the two fungi species. In addition, the antifungal activities of the stem bark of C. anisata against six different fungal species reported by Hamza et al. [109] show strong activity against all the tested species. Four coumarins, dentatin (1), nor-dentatin (2), clausenidin (3), and xanthoxyletin (5), and six carbazole derivatives, 3-formylcarbazole (6), mukonal (7), 3-methoxycarbonylcarbazole (8), murrayanine (9), 2-hydroxy-3-formyl-7-methoxycarbazole (10), and clauszoline J (11), were isolated from the rhizome of Clausena excavata [112]. The results show that compounds 6, 7, 8, and 10 showed antifungal activity with IC₅₀ values of 13.6, 29.3, 9.5, and 2.8 µg/mL, respectively. The active carbazole alkaloid compounds which were obtained from the same structure have all their R1 to be hydrogen, except for compound 11 which has its R1 to be hydrogen but shows no activity, this could be due to the presence of the double methoxy group at R2 and R4. Compound 9, on the other hand, which was derived from the same structure but has its methoxy group as R1 shows no activity. All the coumarins isolated also fail to show potency against any of the tested bacteria.

Kumar et al. [113] reported the isolation of a new γ-lactone coumarin, named as excavarin-A from the leaves of C. excavata and was tested against 15 fungal strains pathogenic of plants and human. Antifungal activity of the isolated compound was found to be stronger than that of the standard antibiotic nystatin against the clinically pathogens, A. fumigatus, C. tropicalis, and M. circinelloides. The plant pathogenic fungi, C. gloeosporioides, C. eragrostidis, F. oxysporum, L. theobromae, R. solani, R. stolonifer, and S. sclerotiorum were more sensitive toward the isolated compound than the standard fungicide bavistin. R. solani and F. oxysporum were found to be the most sensitive strains with an MIC value of less than 0.019 mg/mL. Fungicidal action against C. curvatus and T. cutaneum was observed to be similar to the standard antibiotic. A. niger and A. fumigatus were found to be most resistant (MIC 0.625 mg/mL). The antagonistic potential of the isolated coumarin was found to be considerably strong which could be owing to the presence of α-methylene-γ-lactone moiety in the side chain. The ethanol extract of the root and stem bark of C. suffruticosa [101] and C. heptaphylla [67] has also been reported to have antifungal activities with the later reported to have a promising zone of inhibition ranging from 37.5 to 64.0 μg/mL. Clausena species were shown to be the most tested against C. albicans, as shown in Figure 19. This supports the claim that some C. species are used in the traditional treatment of C. albicans infections [109].

Figure 19

Fungal species tested on Clausena plants.

It also suggests that Clausena species could be used in the future to develop medications that are effective against C. albicans. C. anisata, as in antibacterial and antiparasitic, has the highest number of antifungal activities compared to the other species (Figure 20). This could be due to the beneficial phytochemicals found in the species [104].

Figure 20

Antifungal activity screening of Clausena species.

Several compounds have been isolated from distinctive elements of the Clausena species, but those reported to possess antifungal activities are mukonal (10), 3-formylcarbazole (11), 3-methoxycarbonyl carbazole (12), 2-hydroxy-3-formyl-7-methoxy-carbazole (13), Gamma-lactone coumarine (14), lansiumamide B (15), and C (16) (Table 2).

4.3 Antiprotozoal activities

Pathogenic protozoa, sometimes known as parasites, are a diverse collection of unicellular eukaryotic organisms that can cause a variety of clinical diseases [132]. Protozoal infections are parasitic diseases that harms human health and affect all people regardless of their region. Some parasitic protozoans appear to be harmless, while others can cause life-threatening illnesses [118]. Plasmodium, Leishmania, Trypanosoma, and Toxoplasma are protozoan parasites that cause diseases known to be among the most lethal and widespread in the world, primarily affecting populations in developing countries. Moreover, protozoan infection has also been linked to cardiac and pulmonary involvement [132,133].

Depending on the patient’s signs and symptoms, as well as any underlying medical conditions, a variety of lab tests are available to diagnose parasite infections. The ova and parasite test (O&P) and endoscopy/colonoscopy are two regularly used tests for detecting parasites that cause diarrhea, lose or watery stools, cramps, flatulence (gas), and other stomach symptoms. Other tests include a blood test, which can be serology or a blood smear, X-ray, MRI scan, computerized axial tomography scan which is used to detect parasitic diseases that can cause organ lesions, and the most recent is the polymerase chain reaction technology, which works by targeting the DNA in parasites and worms [134,135].

Okokon et al. [119] reported the antimalarial and analgesic activity against chloroquine-sensitive Plasmodium berghei. The extract and its fractions considerably diminished parasitemia in preventive, suppressive, and healing models in a dose-dependent manner. These effects may be associated with a few secondary plant metabolites that have been suggested to have antiplasmodial activity with different mechanisms of movement. The extract was also reported to inhibit acetic acid, formalin-induced inflammation as well as hot plate-induced pain in mice. Similarly, Williams et al. [120] reported the screening of ethanolic extracts from 29 medicinal plants used in Africa (Ghana) for in vitro anthelmintic properties against A. suum, where extracts from C. anisata were identified among the most potent with an EC₅₀ value of 74 μg/mL. This result encourages further investigation on the usage of C. anisata similarly as complementary remedy alternatives for Ascariasis. A recent report by Kamte et al. [121] on the evaluation of C. anisata essential oils, which was active against Trypanosoma brucei TC221 with an MIC value of 100 µg/mL contains sesquiterpene hydrocarbons, monoterpene hydrocarbons, and oxygenated sesquiterpenes. Murungi [122] also reported the antimalarial activities of hexane, chloroform, and methanol extracts of C. anisata on Plasmodium berghei ANKA in in vivo Swiss mouse model. The result shows that C. anisata extracts examined tested a dose-dependent chemosuppression of 78.56% at 500 mg/kg/day. The chloroform extract shows tremendous antimalarial activity and increased average survival time of the mice. This indicates that the plant has antimalarial properties that can be explored to manage malaria. Irungu et al. [123] conducted a study on chloroform extracts of C. anisata against Plasmodium berghei ANKA strain, while Govindarajan [124] reported its activity against Protist blastocystis.

Sripisut and Laphookhieo [125] reported the isolation of a new carbazole alkaloid, sansoakamine (8), together with 11 known compounds from the stems of C. excavata. Only compound 7 was found to show moderate anti-malarial efficacy against P. falciparum with an MIC value of 6.74 μg/mL. However, compound 7 which exhibits moderate activity has a similar characteristic to compound 3 with their only difference been the position of the CHO group which is at the R2 for compound 7 and R3 for compound 3. In addition, the activities of compounds isolated from the acetone root extract of Clausena guillauminii [126] against Plasmodium falciparum were all reported. Clausena species have been reported for their antiprotozoal activities, with C. anisata having the highest report, as shown in Figure 21.

Figure 21

Antiprotozoal activity screening of Clausena plants.

Overall, little research has been done regarding the antiprotozoal activities of Clausena species, with more attention being given to malaria, an infectious disease caused by protozoan parasites of the genus Plasmodium as shown in Figure 22. This is to draw our attention to the fact that more research on other Clausena plants with different protozoan parasite species is needed, as this will aid in the discovery of new Clausena species with diverse antiprotozoal properties.

Figure 22

Protozoal species tested against Clausena plants.

As shown in Table 3, only O-methyl-mukonal (17) from the stem of C. excavata and mukonal (10), methoxymukonal (18), and clauraila D (19) from the root of C. guillauminii were the only active compounds against P. falciparum among the 30 compounds isolated from the root and stem of C. excavata and C. guillauminii, respectively (Figures 23–25).

Figure 23

Antibacterial Clausena species isolated compound structures.

Figure 24

Antifungal Clausena species isolated compound structures.

Figure 25

Antiprotozoal Clausena species isolated compound structures.