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. 2021 Mar 5;10(3):488. doi: 10.3390/plants10030488

Antimicrobial Activities of Sesquiterpene-Rich Essential Oils of Two Medicinal Plants, Lannea egregia and Emilia sonchifolia, from Nigeria

Akintayo L Ogundajo 1, Tolulope Ewekeye 2, Olubunmi J Sharaibi 2, Moses S Owolabi 1,*, Noura S Dosoky 3, William N Setzer 3,4,*
Editors: Daniela Rigano, Kwang-Hyun Baek, Filippo Maggi
PMCID: PMC8000775  PMID: 33807551

Abstract

Lannea egregia (Anacardiaceae) and Emilia sonchifolia (Asteraceae) are plants used in traditional medicine in southwestern Nigeria. The essential oils from the leaves of L. egregia and E. sonchifolia were obtained by hydrodistillation and analyzed by gas chromatography–mass spectrometry. Both essential oils were dominated by sesquiterpenoids. The major components in L. egregia leaf essential oil were α-panasinsen (34.90%), (E)-caryophyllene (12.25%), α-copaene (11.39%), and selina-4,11-diene (9.29%), while E. sonchifolia essential oil was rich in γ-himachalene (25.16%), (E)-caryophyllene (15.72%), and γ-gurjunene (8.58%). The essential oils were screened for antimicrobial activity against a panel of bacteria and fungi and displayed minimum inhibitory concentrations ranging from 156 μg/mL to 625 μg/mL. Based on these results, either L. egregia or E. sonchifolia essential oil may be recommended for exploration as complementary antibacterial or antifungal agents.

Keywords: α-panasinsen; γ-himachalene; (E)-caryophyllene; α-copaene; selena-4,11-diene; antibacterial; antifungal

1. Introduction

Medicinal plants are widely used in treatment of diseases, and this has encouraged researchers to investigate plants that are of pharmacological value with potential therapeutic application in the management of human health [1]. Many ethnomedicinal plants have been investigated and reported to possess antiviral [2], anticancer [3], antiprotozoal [4], antibacterial [5], antifungal [6], anti-inflammatory [7], antioxidant [8] and other biocidal activities [9,10,11]; hence, their usefulness in folk medicine for treatment of various diseases has given credence to the application of the ethnopharmacological approaches for drug discovery.

The genus Lannea is in the family Anacardiaceae, which consists of nearly 800 species in 82 genera. There are around 40 Lannea species distributed across the savanna region of the West African tropics from Guinea through Ghana to Nigeria [12,13]. Several important members of Lannea species include L. kerstingii, L. welwitschii, L. schimperii, L. egregia, L. acida, L. microcarpa, and L. fruticosa [14]. Lannea egregia Engl. and K. Krause, locally called “ekudan” in Yoruba in Nigeria, “sambituliga” in Ivory Coast, and “tiuko” in Guinea [15], is a tropical woody perennial plant about 13 m in height with alternate leaves growing in the savanna region and shares the same local name as L. barteri (Oliv.) Engl., in Ivory Coast, Benin, and Guinea [16].

Ethnomedicinally, L. egregia has seen traditional use in treatment of various ailments in humans. The roots and bark are used externally for ulcers, sores, and leprosy [12]. The plant decoction is taken as a remedy for diarrhea, edema, epilepsy, rheumatism, insanity, paralysis, and gastric pains [17,18], as well as to improve the hemoglobin level and as part of vermifuge medicine [16]. The macerated roots have been used to treat wounds [19]. Traditionally, leaves of L. egregia, boiled with fermented corn water, were used for treatment of hemorrhoids [19] and to manage cancer [20]. The leaf, stem bark, and root extracts of L. egregia from Olokemeji Forest, Nigeria, were shown to have antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, and Escherichia coli using a zone-of-inhibition assay, but showed only weak activity with minimum inhibitory concentrations (MIC) ranging from 6.3 to 25 mg/mL [21]. In this work, a phytochemical screening was carried out, but individual components were not identified.

Emilia sonchifolia (L.) DC. (Asteraceae) is a bushy annual herb distributed mainly in Asian countries, but naturalized throughout the tropics [13]. It has been traditionally used as an important medicinal plant in most tropical and subtropical countries [22], including in the South-South region of Akwa Ibom State, Nigeria [23,24]. The plant has been used to treat diarrhea, night blindness, sore throat, chest pain, liver disease, eye inflammation, stomach tumor, rashes, measles, earache, inflammation, convulsions, fever, muscular aches, and asthma [24,25,26]. There have been several studies reported in the literature on the biological activities and phytochemical screening of extracts of E. sonchifolia (Table 1). The plant extracts have shown anti-inflammatory, antioxidant, cytotoxic, analgesic, wound-healing, antimetastatic, immunomodulatory, and antiangiogenic activities [27].

Table 1.

Biological activities of Emilia sonchifolia extracts.

Emilia sonchifolia Extract (Geographical Source) Phytochemicals Identified Biological Activity Ref.
Methanol plant extract (Kerala, India) None identified In vitro cytotoxicity (L-929 murine lung fibroblast, IC50 = 15 μg/mL) [28]
Aqueous leaf extract (Nsukka, Nigeria) None identified Anti-inflammatory (mouse paw edema assay, ED50 = 780 mg/kg) [23]
Ethanol plant extract (Kerala, India) None identified Inhibition of perchlorate oxidative stress (rat model) [29]
Methanol leaf extract (Ibiono, Nigeria) None identified Analgesic (acetic acid writing, formalin hind paw, and hot plate assays, mouse model) [25]
CH3OH/CH2Cl2 (1:1) extract of aerial parts (Nsukka, Nigeria) Quercetrin, chlorophyll, caffeic acid derivative Anti-inflammatory (inhibition of pro-inflammatory cytokines, mouse model) [30]
Ethanol extract of aerial parts (Liuzhou, China) Emiline (pyrrolidine alkaloid) Neuroprotective (in vitro PC12 cells) [31]
Aqueous HCl (0.5 N) plant extract (Taiwan) Pyrrolizidine alkaloids: senecionine, seneciphylline, integerrimine, senkirkine, otosenine, neosenkirkine, petasitenine, acetylsenkirkine, desacetyldoronine, acetylpetasitenine, and doronine None carried out, but pyrrolizidine alkaloids known to be hepatotoxic. [32]
Aqueous plant extract (Kerala, India) None identified Wound-healing activity (rat model) [33]
Ethanol leaf extract (Abraka, Nigeria) None identified Antifungal activity (Curvularia lunatus, MIC = 72 mg/mL) [34]
Methanol leaf extract (Uyo, Nigeria) None identified Antioxidant (FRAP and DPPH assays) [26]
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IC50 = Median inhibitory concentration. ED50 = Median effective dose. FRAP = Ferric ion Reducing Antioxidant Power. DPPH = 2,2-diphenyl-1-picrylhydrazyl.

We report herein our investigation into the collection of L. egregia leaf essential oil and the essential oil from the leaves of E. sonchifolia from southwest Nigeria, the analysis of the essential oil compositions, and antimicrobial screening of the essential oils. This investigation is part of our ongoing research aimed at the characterization of the bioactivity and the compositions of the essential oils from Nigerian medicinal plants for potential exploitation in pharmaceutical applications.

2. Results and Discussion

2.1. Essential Oil Compositions

2.1.1. Lannea egregia

Hydrodistillation of the leaves of L. egregia collected from Agbegi-Odofin Village, Ikire, Osun State, Nigeria, yielded a pale-yellow essential oil with an average yield of 0.68 ± 0.2% on a weight-to-weight basis. The essential oil was analyzed by gas chromatography—mass spectrometry (GC-MS) (Table 2, Figure 1). The essential oil showed monoterpene hydrocarbons (1.53%), oxygenated monoterpenoids (2.86%), sesquiterpene hydrocarbons (86.43%), oxygenated sesquiterpenoids (1.15%), and non-terpenoids (6.50%). The predominant sesquiterpene hydrocarbons include α-panasinsen (34.90%), (E)-caryophyllene (12.25%), α-copaene (11.39%), and selina-4,11-diene (9.29%). The major oxygenated monoterpenoid was linalool (1.12%).

Table 2.

The chemical constituents of Lannea egregia leaf essential oil.

Sr. No. RT RI(calc) RI(db) Compound Ave % St Dev
1 15.838 977 978 β-Pinene 0.21 0.06
2 16.152 983 986 6-Methylhept-5-en-2-one 0.38 0.04
3 16.494 989 991 2-Pentylfuran 0.25 0.02
4 18.725 1024 1025 p-Cymene 0.34 0.03
5 19.044 1028 1030 Limonene 0.24 0.02
6 19.149 1030 1031 β-Phellandrene 0.11 0.01
7 20.961 1057 1058 γ-Terpinene 0.22 0.01
8 21.811 1070 1069 cis-Linalool oxide 0.12 0.02
9 22.81 1085 1086 Terpinolene 0.08 0.02
10 23.099 1089 1091 p-Cymenene 0.32 0.04
11 23.752 1099 1099 Linalool 1.12 0.01
12 23.979 1102 1104 Hotrienol 0.13 0.02
13 24.116 1104 1104 Nonanal 0.30 0.02
14 24.759 1113 1112 (E)-2,4-Dimethylhepta-2,4-dienal 0.26 0.04
15 26.577 1139 1139 (Z)-3-Ethylidene-1-methyl-1,4-cycloheptadiene 0.22 0.02
16 27.252 1149 --- Unidentified 0.44 0.02
17 28.064 1160 1169 p-Dimethoxybenzene 0.54 0.06
18 29.26 1177 1172 Lavandulol 0.12 0.02
19 29.416 1180 1180 Terpinen-4-ol 0.44 0.05
20 30.132 1190 1192 Methyl salicylate 0.30 0.00
21 30.415 1194 1195 α-Terpineol 0.26 0.05
22 32.05 1218 1219 β-Cyclocitral 0.42 0.03
23 32.783 1228 --- Unidentified 0.47 0.05
24 34.591 1255 1257 Carvenone 0.25 0.01
25 40.81 1347 1349 α-Cubebene 0.20 0.02
26 42.245 1369 1367 Cyclosativene 0.61 0.06
27 42.704 1376 1375 α-Copaene 11.39 0.22
28 43.231 1384 1382 β-Bourbonene 1.25 0.06
29 44.584 1404 --- Unidentified 0.61 0.05
30 45.524 1419 1417 (E)-Caryophyllene 12.25 0.10
31 46.156 1429 1430 β-Copaene 0.57 0.02
32 47.186 1446 1447 Geranyl acetone 0.62 0.06
33 47.774 1455 1454 α-Humulene 1.85 0.03
34 48.052 1460 1457 allo-Aromadendrene 0.65 0.04
35 48.978 1474 1478 γ-Muurolene 0.57 0.03
36 49.235 1478 1479 α-Amorphene 0.41 0.06
37 49.533 1483 1476 Selina-4,11-diene 9.29 0.03
38 49.853 1488 1492 β-Selinene 4.26 0.04
39 50.065 1492 1492 Valencene 3.86 0.09
40 50.295 1495 1497 α-Selinene 1.24 0.08
41 50.425 1497 1497 α-Muurolene 1.35 0.06
42 51.313 1512 1512 γ-Cadinene 0.88 0.05
43 51.704 1519 1521 α-Panasinsen 34.90 0.25
44 52.716 1536 1538 α-Cadinene 0.37 0.05
45 52.96 1540 1541 α-Calacorene 0.54 0.02
46 55.41 1581 1577 Caryophyllene oxide 0.60 0.05
47 61.503 1688 1694 Acorenone B 0.55 0.06
48 62.926 1714 1715 Pentadecanal 1.82 0.12
49 69.609 1840 1841 Phytone 1.81 0.06
Total identified 98.48
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RT = retention time (min); RI(calc) = retention index determined with respect to a homologous series of n-alkanes on a ZB-5 ms column; RI(db) = retention indices from the databases. Monoterpene hydrocarbons (Sr. Nos. 1, 4–7, 9, 10), 1.53%; oxygenated monoterpenoids (Sr. Nos. 8, 11, 12, 18, 19, 21, 22, 24), 2.86%; sesquiterpene hydrocarbons (Sr. Nos. 25–28, 30, 31, 33–45), 86.43%; oxygenated sesquiterpenoids (Sr. Nos. 46, 47), 1.15%; others (Sr. Nos. 2, 3, 13–15, 17, 20, 32, 48, 49), 6.50%. Sr. No. 16 MS(EI): 152(7%), 137(37%), 119(27%), 109(100%), 93(16%), 91(28%), 81(33%), 79(26%), 77(21%), 67(91%), 55(23%), 43(71%), 41(30%). Sr. No. 23 MS(EI): 151(2%), 136(26%), 121(72%), 108(43%), 93(100%), 91(32%), 79(32%), 77(25%), 43(39%), 41(23%). Sr. No. 29 MS(EI): 225(2%), 210(44%), 195(100%), 182(12%), 167(66%), 152(20%), 137(18%), 125(29%), 82(15%), 70(18%), 69(13%), 56(16%), 55(20%), 54(26%), 43(19%), 42(17%), 41(21%).

Figure 1.

Figure 1

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Gas chromatogram of Lannea egregia leaf essential oil. Major compounds are indicated by Sr. Nos. from Table 2.

As far as we are aware, there have been no published reports on essential oils from Lannea species, so essential oil compositional comparisons at the genus level are not possible. Both α-copaene and (E)-caryophyllene are common essential oil components, including the Anacardiaceae (see, for example [35,36]). Selin-4,11-diene, on the other hand, is relatively uncommon in the family, but has been observed in Sclerocarya birrea leaf essential oil [37] and Haematostaphis barteri leaf essential oil [38]. Likewise, α-panasinsen is a rare volatile component in the Anacardiaceae, but detected as an aroma component of Mangifera indica cv. Alphonso [39] and Sclerocarya birrea subsp. caffra [40] fruits.

2.1.2. Emilia sonchifolia

Hydrodistillation of the leaves of E. sonchifolia yielded a pale-yellow essential oil (0.46%). A total of 62 constituents, 97.60% of E. sonchifolia volatile oil, were identified by GC-MS. The volatile oil composition is displayed in Table 3 and visualized in Figure 2. The leaf oil was dominated by sesquiterpenoids: γ-himachalene (25.16%), (E)-caryophyllene (15.72%), γ-gurjunene (8.58%), (E)-β-farnesene (3.96%), germacrene D (3.53%), and caryophyllene oxide (3.05%), in addition to the fatty acid palmitic acid (5.24%), and the monoterpene β-pinene (4.87%).

Table 3.

Chemical composition of Emilia sonchifolia leaf essential oil.

Sr. No. RT RI(calc) RI(db) Compound Ave % St Dev
1 13.703 924 921 Tricyclene 1.08 0.16
2 16.285 971 974 β-Pinene 4.87 0.04
3 19.510 1027 1024 Limonene 0.25 0.01
4 41.005 1343 1345 7-epi-Silphiperfol-5-ene 0.07 0.00
5 41.195 1346 1345 α-Cubebene 0.11 0.00
6 41.445 1350 1350 α-Longipinene 0.04 0.00
7 42.700 1369 1369 Cyclosativene 1.19 0.02
8 43.105 1375 1374 α-Copaene 1.46 0.01
9 43.305 1378 1374 Isoledene 0.18 0.00
10 43.640 1384 1387 β-Bourbonene 0.50 0.01
11 43.865 1387 1385 α-Bourbonene 0.08 0.00
12 43.985 1389 1389 β-Elemene 2.38 0.05
13 44.170 1392 1390 Sativene 0.04 0.01
14 44.895 1403 1398 Cyperene 0.06 0.00
15 45.140 1407 1409 α-Gurjunene 0.20 0.01
16 45.960 1420 1417 (E)-Caryophyllene 15.72 0.35
17 46.585 1430 1430 β-Copaene 0.23 0.01
18 46.725 1432 1432 trans-α-Bergamotene 0.10 0.01
19 47.485 1444 1447 Isogermacrene D 0.05 0.00
20 47.580 1446 1445 Myltayl-4(12)-ene 0.14 0.00
21 47.700 1448 1444 6,9-Guaiadiene 0.09 0.00
22 47.925 1452 1454 (E)-β-Farnesene 3.96 0.03
23 48.205 1456 1452 α-Humulene 2.97 0.03
24 48.490 1461 1464 9-epi-(E)-Caryophyllene 0.42 0.05
25 48.620 1463 1465 cis-Muurola-4(14),5-diene 0.04 0.02
26 49.285 1474 1476 Selina-4,11-diene 0.49 0.03
27 49.410 1476 1479 γ-Muurolene 0.41 0.01
28 49.540 1478 1475 γ-Gurjunene 8.58 0.36
29 49.605 1479 1483 α-Amorphene 2.81 0.57
30 49.820 1482 1484 Germacrene D 3.53 0.10
31 49.945 1484 1481 γ-Himachalene 25.16 0.78
32 50.040 1486 1487 Aristolochene 0.61 0.13
33 50.140 1488 1492 δ-Selinene 0.52 0.03
34 50.305 1490 1489 β-Selinene 0.81 0.02
35 50.455 1493 1496 Valencene 0.97 0.03
36 50.745 1498 1498 α-Selinene 1.19 0.03
37 50.855 1499 1500 α-Muurolene 2.11 0.04
38 50.965 1501 1505 α-Cuprenene 0.16 0.03
39 51.120 1504 1505 (E,E)-α-Farnesene 0.38 0.05
40 51.375 1508 1505 β-Bisabolene 0.13 0.04
41 51.590 1512 1509 Tridecanal 0.06 0.01
42 51.755 1514 1513 γ-Cadinene 0.35 0.01
43 51.925 1517 1514 Cubebol 0.08 0.01
44 52.030 1519 1522 δ-Cadinene 1.29 0.04
45 52.140 1521 1520 7-epi-α-Selinene 0.41 0.03
46 52.255 1523 1521 trans-Calamenene 0.05 0.01
47 52.345 1524 1521 β-Sesquiphellandrene 0.16 0.02
48 53.165 1538 1537 α-Cadinene 0.10 0.01
49 53.445 1543 1544 α-Calacorene 0.18 0.01
50 53.680 1544 1545 trans-Cadinene ether 0.11 0.00
51 53.845 1547 1548 α-Elemol 0.04 0.00
52 54.680 1561 1564 β-Calacorene 0.05 0.01
53 55.580 1576 1577 Spathulenol 0.32 0.02
54 55.890 1581 1582 Caryophyllene oxide 3.05 0.03
55 57.510 1609 1608 Humulene epoxide II 0.39 0.01
56 59.300 1641 1638 τ-Cadinol 0.21 0.01
57 59.430 1643 1640 τ-Muurolol 0.13 0.00
59 60.055 1654 1652 Himachalol 0.49 0.05
59 59.980 1653 --- Unidentified 0.86 0.05
60 60.255 1658 1658 neo-Intermedeol 0.32 0.00
61 69.900 1839 1841 Phytone 0.27 0.01
62 75.845 1960 1959 Palmitic acid 5.24 1.25
63 80.780 2066 2071 Dibenzyl disulfide 0.23 0.00
Total identified 97.60
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RT = retention time (min); RI(calc) = retention index determined with respect to a homologous series of n-alkanes on a ZB-5 ms column; RI(db) = retention indices from the databases. Monoterpene hydrocarbons (Sr. Nos. 1–3), 6.20%; sesquiterpene hydrocarbons (Sr. Nos. 4–40, 42, 44–49, 52), 80.47%; oxygenated sesquiterpenoids (Sr. Nos. 43, 50, 51, 53–57, 59, 60), 5.13%; others (Sr. Nos. 41, 61, 62, 63), 5.80%. Sr. No. 59 MS(EI): 206(4%), 191(5%), 173(3%), 163(7%), 149(9%), 136(14%), 135(13%), 124(22%), 123(32%), 121(19%), 109(100%), 95(39%), 93(28%), 81(33%), 79(28%), 69(24%), 67(46%), 55(30%), 53(18%), 43(18%), 41(47%).

Figure 2.

Figure 2

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Gas chromatogram of Emilia sonchifolia leaf essential oil. Major compounds are indicated by Sr. Nos. from Table 3.

The essential oil of the aerial parts of E. sonchifolia from Belagavi, Karnataka, India, has been reported [41]. The essential oil from India was also dominated by sesquiterpene hydrocarbons (67.6%), but with a remarkably different composition. The major components in the essential oil from India were γ-muurolene (32.1%) and (E)-caryophyllene (22.7%). γ-Muurolene was not observed in the essential oil from Nigeria, while γ-himachalene, γ-gurjunene, and germacrene D were not reported in the essential oil from India. Both caryophyllene oxide and palmitic acid were found in the essential oil from India (1.1% and 1.2%, respectively). Apparently, the geographical separation of these two samples has a profound effect on the phytochemistry.

Both L. egregia and E. sonchifolia essential oils were dominated by sesquiterpene hydrocarbons, with (E)-caryophyllene abundant in both oils. α-Copaene, abundant in L. egregia essential oil, was found to be only 1.5% in E. sonchifolia oil. Selina-4,11-diene and α-panasinsen were major components in L. egregia essential oil but were not detected in the essential oil of E. sonchifolia. Likewise, γ-himachalene, abundant in E. sonchifolia essential oil, was not detected in the essential oil of L. egregia.

2.2. Antimicrobial Activity

The leaf essential oils of L. egregia and E. sonchifolia were screened for antibacterial and antifungal activity against a panel of microorganisms (Table 4). It has been suggested that essential oils having MIC values < 100 μg/mL show very strong activity, those with MIC of 101–500 μg/mL show strong activity, 500 μg/mL < MIC < 1000 μg/mL are moderately active, and above 1000 μg/mL are inactive [42,43]. Thus, the essential oils in this study can be considered strongly active. It is not readily apparent which essential oil components are responsible for the activities; most sesquiterpenes have not been individually screened for antimicrobial activity. However, three of the major components, β-pinene, linalool, and (E)-caryophyllene, were also screened in this work and these compounds showed activities similar to the essential oils themselves. The observed antimicrobial activities are consistent with some of the ethnobotanical uses of these two plants, and based on these results, either L. egregia or E. sonchifolia essential oil may be recommended for exploration as antibacterial or antifungal agents.

Table 4.

Antibacterial and antifungal activities (MIC, μg/mL) of Lannea egregia and Emilia sonchifolia essential oils from southwest Nigeria.

Organism Lannea egregia
EO		 Emilia sonchifolia
EO		 (–)-β-Pinene (±)-Linalool (E)-Caryophyllene Caryophyllene Oxide Positive Control a
Bacteria
Bacillus cereus 312.5 625 312.5 312.5 312.5 312.5 1.22
Staphylococcus aureus 312.5 1250 256.3 312.5 312.5 78.1 0.61
Staphylococcus epidermidis 312.5 156.3 312.5 312.5 312.5 312.5 <19.5
Streptococcus pyogenes 625 312.5 625 312.5 312.5 625 <19.5
Molds
Aspergillus fumigatus 156.3 156.3 156.3 156.3 156.3 156.3 <19.5
Aspergillus niger 156.3 156.3 78.1 1250 1250 156.3 1.56
Cryptococcus neoformans 312.5 625 312.5 312.5 312.5 312.5 0.78
Microsporum canis 312.5 312.5 312.5 312.5 312.5 312.5 <19.5
Microsporum gypseum 312.5 312.5 312.5 312.5 312.5 156.3 <19.5
Trichophyton mentagrophytes 156.3 312.5 156.3 625 625 156.3 <19.5
Trichophyton rubrum 312.5 312.5 312.5 312.5 312.5 312.5 <19.5
Yeast
Candida albicans 156.3 312.5 156.3 156.3 156.3 312.5 1.56
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MIC = minimum inhibitory concentration (μg/mL). a Gentamicin was the positive control for bacteria, amphotericin B was the positive control for fungi.

3. Materials and Methods

3.1. Plant Materials

Leaves of Lannea egregia and Emila sonchifolia were collected directly from source plants in two locations in southwestern states in Nigeria in the month of August, 2019. Lannea egregia was collected from Agbegi-Odofin Village, Ikire (Osun State, 7°22′20.68″ N, 4°11′14.60″ E), and E. sonchifolia was obtained from the campus of Lagos State University, Ojo (Lagos State, 6°28′1.20″ N, 3°10′58.80″ E). Botanical identification of the two plants was done by Mr. S. A. Odewo at the Herbarium, Forest Research Institute of Nigeria (FRIN), Jericho, Ibadan, Nigeria, where their voucher specimens (Voucher Numbers FHI 112544 and FHI 112546, respectively) have been deposited. The leaves L. egregia and E. sonchifolia were manually removed, chopped, air-dried in the laboratory for 7–10 days, pulverized using an electric blender, and stored in polyethene containers until ready for use.

3.2. Isolation of Essential Oils

A sample (450 g each) of L. egregia leaves and E. sonchifolia leaves was subjected to hydrodistillation thrice in an all-glass Clevenger-type apparatus. Each sample of L. egregia and E. sonchifolia, respectively, was mixed with water in a ratio of 2:6. The mixture was hydrodistilled for 3–4 h with constant stirring until no additional oil was observed to be distilled. For each plant species, the essential oils were combined, dried over anhydrous sodium sulfate to eliminate traces of water, and stored in a sealed vial under refrigeration (4 °C) prior to analysis.

3.3. Gas Chromatography–Mass Spectrometry

The leaf essential oils of L. egregia and E. sonchifolia were analyzed using gas chromatography–mass spectrometry (GC-MS) as previously described by us [38]: Shimadzu GCMS-QP2010 Ultra, ZB-5 ms GC column, GC oven temperature 50 °C–260 °C (2 °C/min), 1-μL injection of 5% solution of each essential oil dissolved in CH2Cl2 (split mode, 30:1). Each essential oil sample was injected three times. Retention indices (RI) were calculated in comparison with a homologous series of n-alkanes. Compounds were identified by comparison of the MS fragmentation and retention indices with those in the databases [44,45,46,47] and with matching factors >90%. Quantification was done by external standard method. Calibration curves of representative compounds from each class were drawn and used for quantification.

3.4. Antibacterial and Antifungal Screening

The essential oils were screened for antimicrobial activity against a panel of bacteria (Bacillus cereus (ATCC No. 14579), Staphylococcus aureus (ATCC No. 29213), and Staphylococcus epidermidis (ATCC No. 12228), Streptococcus pyogenes (ATCC No. 19615), and fungi (Aspergillus fumigatus (ATCC No. 96918), Aspergillus niger (ATCC No. 16888), Cryptococcus neoformans (ATCC No. 32045), Microsporum canis (ATCC No. 11621), Microsporum gypseum (ATCC No. 24102), Trichophyton mentagrophytes (ATCC No. 18748), Trichophyton rubrum (ATCC No. 28188), and Candida albicans (ATCC No. 18804)) using the microbroth dilution technique [48,49] as previously reported by us [38]. Serial dilutions of the essential oils (2500, 1250, 625, 312.5, 156.3, 78.1, 39.1, and 19.5 μg/mL) in appropriate media (cation–adjusted Mueller Hinton broth for bacteria and yeast-nitrogen base growth medium for fungi) were carried out in 96-well microtiter plates. Microorganisms (1.5 × 108 CFU/mL for bacteria and 7.5 × 107 CFU/mL for fungi) were added to the 96-well plates, which were incubated for 24 h at 37 °C for bacteria and 35 °C for fungi. Minimum inhibitory concentrations (MIC) were determined to be the lowest concentrations without turbidity. Gentamicin (Sigma-Aldrich, St. Louis, MO) was the positive antibacterial control, amphotericin B (Sigma-Aldrich, St. Louis, MO) was the positive antifungal control, and dimethylsulfoxide (DMSO) was used as the negative control (50 μL DMSO diluted in 50 μL broth medium, and then serially diluted as above). (–)-β-Pinene, (±)-linalool, (E)-caryophyllene, and caryophyllene oxide (Sigma-Aldrich, St. Louis, MO) were also individually screened for activity.

4. Conclusions

The essential oils of Lannea egregia and Emilia sonchifolia, medicinal plants collected from southwestern Nigeria, were found to be rich in sesquiterpenoids. Both essential oils exhibited antibacterial and antifungal activities that are consistent with traditional uses of the plants. While sesquiterpene hydrocarbons were the predominant chemical class in both essential oils, it is not apparent which individual components may be responsible for the antimicrobial activity. It is likely, however, that synergistic effects are also responsible for the activities of the components. Nevertheless, the essential oils may be recommended for further exploration as complementary antimicrobial agents.

Acknowledgments

N.S.D. and W.N.S. participated in this work as part of the activities of the Aromatic Plant Research Center (APRC, https://aromaticplant.org/, accessed on 3 February 2021).

Abbreviations

ATCC: American Type Culture Collection; CFU, colony forming units; DPPH, 2,2-diphenyl-1-picrylhydrazyl; ED50, median effective dose; EI, electron impact; FRAP, ferric reducing antioxidant power; GC, gas chromatography; IC50, median inhibitory concentration; MIC, minimum inhibitory concentration; MS, mass spectrometry; RI, retention index/retention indices; RT, retention time.

Author Contributions

Conceptualization, M.S.O. and W.N.S.; methodology, M.S.O. and N.S.D.; validation, W.N.S.; formal analysis, N.S.D. and W.N.S.; investigation, A.L.O., T.E., O.J.S., N.S.D. and W.N.S.; data curation, W.N.S.; writing—original draft preparation, M.S.O. and W.N.S.; writing—review and editing, M.S.O., N.S.D. and W.N.S.; supervision, M.S.O.; project administration, M.S.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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