Comparative chemical composition and antimicrobial activity of essential oils from leaves and seeds of star apple (Chrysophyllum cainito L.)

Abstract

In this study, the essential oils of seeds and leaves from Star Apple (Chrysophyllum cainito L) were profiled and screened for their antimicrobial potential. The volatile oils were isolated by means of hydro-distillation process using a Clevenger-type apparatus. The oils were analysed by gas chromatography coupled with mass spectrometry (GC–MS). Antimicrobial activity of the isolated essential oils was investigated by agar well diffusion method against seven bacterial isolates and two fungal isolates. The docking studies of the essential oil compounds and the reference drug were carried out using Maestro software. Docking calculation were executed on the Schrodinger suite molecular docking platform. A total of thirty-one chemical constituents were identified in essential oil of leaves with β-ocimene (12.37%), TMTT 1,3,7,11-Tridecatetraene (9.73%), α-farnesene (9.19%) and cyclofenchene (7.16%) as major components. Twenty-eight compounds were characterised in the seeds oil with citral (36.60%), Geranial, (28.50%) and (E)-3,7-Dimethylocta-3,6-dienal (10.82%) as main constituents. The two oils contained beta and alpha ocimene, cyclofenchene and pelargonaldehyde in common. The lowest MIC observed with essential oil of the leaves was 0.781251 mg/mL for Staphylococcus aureus and 1.625 mg/mL for Escherichia coli. The lowest MIC recorded for essential oil of the seeds was 12.5 mg/mL for Staphylococcus aureus, Escherichia coli and Klebsiella pneumoniae. Essential oil obtained from leaves exhibited higher antimicrobial and antifungal activities against all tested organisms than essential oil of seeds. Compound 11 with -3.303 kcal/mol, with the best docking score of all compound from the leaf and seed of all the sitemaps and this is close to that of reference compound on sitemap three with docking score of -5.055 kcal/mol. Also, the anti E. coli activities of both leaf and seed EOs could be associated with inhibitory activities of the compound on the active sites of the protein. In conclusion, the essential oils extracted from the leaves and seeds of Chrysophyllum cainito demonstrated notable antimicrobial potential, with the leaf oil exhibiting superior activity, likely due to its richer chemical profile and stronger interaction of its constituents with microbial target sites, as supported by both in vitro and in silico analyses.

Graphical Abstract

Highlights

➢ This is the first report on the minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC) and the minimal fungicidal concentration (MFC) of C. caimito leaf and seed oils.

➢ This research report for the first time a comparative analysis of the chemical constituents of C. caimito leaf and seed oils.

➢ Also, this research reports the biological significance of each of the dominant constituents of C. caimito leaf and seed oils.

➢ The rationale for the good to moderate antimicrobial results was justified by the weak binding energies as revealed in the docking studies.

Introduction

Chrysophyllum cainito L. (C. cainito) is a tree plant found in tropical regions, it belongs to Sapotaceae family. C. cainito is a fruit-bearing tree grown for its ornamental purpose. The skin is rich in latex; though the skin and the rind are not edible. The seed is flat, hard and light brown in colour [1].

There are three different species of C. cainito depending on its origin such as star apple, cainito, and caimito [2, 3]. There are about 4 types of C. cainito fruit locally grown in Indonesia [4].

The centre of the seed of C. cainito has the appearance of an asterisk when cut transversely, hence it is called “star apple.” C. cainito tree species are commonly found in Argentina, Mexico, Peru, at the Pacific side of Guatemala, and Vietnam, Malaysia, China, India, and other countries with low altitudes [3, 5]. Since ages, C. cainito has been used in trado-medicinal healthcare system, where the plant parts, especially the fruits, has been used by patients as folk medicine [2].

The biological activities of C. cainito has been reported recently, which included antioxidant potential [6], antidiabetic activity [7, 8], anti-inflammatory potential [9], anticancer activities [10], antimicrobial activity and antihypertensive property [11]. Preliminary phytochemical screening of Chrysophyllum cainito leaf extract indicated the presence several secondary metabolites including saponins, sterols, flavonoids, glycosides, tannins [12, 13].

Pino et al. [14] and Oranusi et al. [13] reported on the chemical composition of Chrysophyllum cainito extracts, their essential oils (EOs) and their antimicrobial activities on bacteria. However, the comparative studies on the chemical compositions and antimicrobial activities of the essential oils of the leaves and seeds of Chrysophyllum cainito is limited. Therefore, comparative evaluation of the seeds and leaves EOs is necessary. This study therefore was aimed at investigating comparatively the chemical compositions of the EOs of the seeds and leaves of Chrysophyllum cainito, and to screen the oils for their antimicrobial potential against clinical isolates of selected bacteria and fungi in order to validate its ethnomedicinal use for treatment of inflammation caused by pneumonia and laryngitis [15].

Materials and methods

Sample collection extraction of plant materials

The Chrysophyllum cainito leaf and seed samples were collected at Ajayi Crowther University, Oyo, Oyo-State, Nigeria, in February, 2022. The seed and leaf samples were identified at the Herbarium section of Botany Department, University of Ibadan by Mr. Esimehhuai Donatus. The sample was also authenticated at the Herbarium section of Forestry Research Institute of Nigeria (FRIN), Ibadan, with Herbarium number-111375FHI, and a specimen of the plant was deposited at the herbarium section of the Institute, the air-dried samples were then pulverised. Batches of 500 g plant materials were subjected to hydrodistillation process for 3 h using a Clevenger-type apparatus designed according to British Pharmacopeia. The resulting EOs were collected in vial bottles, weighed and refrigerated at 4 °C prior to analysis.

GC–MS analysis of essential oil

The analysis of the EOs to determine the chemical composition of the oils was carried out with the use of GC–MS. The analysis was performed using an 8860 A gas chromatograph coupled to a 5977 C inert mass spectrometer with electron impact source (Agilent Technologies). The stationary phase was HP‐5 capillary coated with 5% of Phenyl Methyl Siloxane (30 m length × 0.32 mm diameter × 0.25 μm film thickness) (Agilent Technologies).

Helium was the carrier gas at a constant flow rate of 1.573 mL/min, an initial nominal pressure of 1.9514 psi and at an average velocity of 46 cm/s. One microliter of the samples was injected with 50:1 Split mode at an injection temperature of 300 °C. Purge flow was 21.5 mL/min at 0.50 min with a total gas flow rate of 23.355 ml/min; gas saver mode was switched on. The oven was initially programmed at 40 °C (1 min), then ramped at 10 °C/min to 270 °C (4 min). Run time was 30.25 min with a 3 min solvent delay.

The mass spectrometer was operated in electron‐impact ionization mode at 70 eV with ion source temperature of 230 °C, quadrupole temperature of 150 °C and transfer line temperature of 280 °C. Scanning of possible compounds was from m/z 50–550 amu at 2.62 s/scan rate and were identified by comparing measured mass spectral data with those in NIST 14 Mass Spectral Library and with the reported data.

Antimicrobial assay

Determination of antimicrobial activity

This was carried out using agar diffusion method [15]. A 0.1 mL of a 1 ratio 100 dilution of an overnight culture of each bacterial and fungal isolate (collected at the University teaching Hospital Ibadan (UTH), deposited at the department of Pharmaceutical Microbiology laboratory, University of Ibadan, Nigeria, and was used to seed the surfaces of sterile well-set Mueller Hinton and Sabouraud Dextrose agar media respectively. The plates were allowed to set in the incubator at 37 °C for about 20 min. A standard cork borer (8 mm diameter) was used to cut equidistance wells on the surfaces of the agar. One grams each of the EOs was re-formed with 50% DMSO to final concentrations of 200 mg/mL, 100 mg/mL, 50 mg/mL, 25 mg/mL, 12.5 mg/mL, 6.25 mg/mL, and 3.125 mg/mL. Gentamycin (7.8 µg/mL) served as the positive control while methanol (50%) was used as the negative control. The agar plates were incubated at 37 °C for 24 h after which the diameter of the zones of inhibition were measured. Results were presented as average of triplicate experiments.

Determination of Minimum Inhibitory Concentration (MIC)

The in vitro minimum inhibitory concentrations (MIC) of the oil samples against the selected microorganisms were determined by the agar dilution method as described by [16] with slight modifications. To exactly 19 mL of a sterile molten Mueller Hinton agar which was maintained at 45 °C was added a 1 mL of the EOs already diluted to give concentrations of 200 mgmL⁻¹,100 mgmL⁻¹, 50 mgmL⁻¹, 25 mgmL⁻¹, 12.5 mgmL⁻¹, 6.25 mgmL⁻¹ and 3.125 mgmL⁻¹ using the reported method by [17].

Determination of minimum bactericidal/fungicidal concentration (MBC/MFC)

The in vitro minimum bactericidal concentrations (MBC) of the plant EOs were determined by the modified method of [18]. Exactly 0.5 mL of oil samples at different concentrations as used in the MIC assay that showed no visible growth on the agar plates was added to 0.5 mL suspension of each of the test organisms in micro titre plate wells. These organisms were incubated at 37 °C for 24 to 48 h. Samples were streaked out from the wells on to Mueller Hinton/blood agar and Sabouraud dextrose agar to determine the minimum bactericidal concentration of the EOs required to kill the bacterial and fungal isolates using the reported method by [15].

Docking studies

Preparation of ligand for docking

Compounds from the essential oil were drawn using a 2-D sketcher in the Schrodinger suite and imported into the Maestro suite workspace in 3-D form. All the conformers generated were used for the docking process. The reference drug, Fluoroquinolone was obtained from pubchem.ncbi.nih.gov as SDF format. The Ligprep wizard of the Schrodinger suite was then used to prepare all the ligands by assigning bond length, bond angle, generating possible ionization state at pH 7 using Epik and finally optimized using OPLS4 force field [19, 20].

Protein preparation

The protein structures of Escherichia coli DPS with PDB 6GCM, with resolution 1.59 A which provides protection for the cells during exposure to severe environmental assault (including oxidative stress and nutritional deprivation), was downloaded from a Research Collaboratory for Structural Bioinformatics (RCSB) Protein Data Bank (PDB). The retrieved crystal coordinates were prepared in the protein preparation wizard of the Schrodinger suite. With default parameters of assigning bond order, optimizing, and minimizing using OPLS4 force field. The sitemaps were generated using the sitemap wizard. This gives the site volume, the site score, size, Dscore, philic and phobic values of the sitemap. This generates possible pockets for binding in the protein molecules. The Receptor Grid Generation was carried out on each sitemap to know the residual numbers that are involved in the binding process. This was actualized at 1 A with a partial charge cut off 0.25 A [19, 20].

Molecular docking

The docking studies of the compounds from the EOs and the reference drug with DPS (6GCM) were carried out in Maestro (version 13.2) released 2023-1. The binding strength of the docked complexes were analyzed through docking score, glide e-model and ligand efficiency. These energies measure the degree of stability of the binding between the protein molecules and the ligand molecules. Also, an interaction with amino acid residue were taken note of via hydrogen bonding interaction, pi-pi stacking, metal coordination, salt bridge and solvent exposure. Docking calculation were executed in the Extra Precision (XP) mode of the Glide module in the Schrodinger suite molecular docking platform. The complex with the highest negative docking scores has better binding towards the respective proteins. Dorking calculation of compounds and conformer from leaf and seed EOs against the DPS (with PDB 6GCM) isoforms will provide a selectivity profile that may be interesting for the development of a novel anti-E. coli agent with a limited side effect. The 6GCM is an excellent target for the design of novel therapeutic strategies [19–21].

Statistical analysis

The experiments were repeated at least three times. The result values were expressed as mean ± SD. The comparisons between means were determined using One-way Analysis of Variance (ANOVA) followed by Least Significant Difference (LSD) test, with Microsoft Excel. The data were considered significantly different statistically, if p < 0.05.

Results and discussion

Chemical Constituents of the C. cainito leaf Essential oils

Chemical composition

The percentage yield of EOs from the two samples were calculated by the equation:

$$Percentageyieldofoil=\frac{\mathit{volumeofoilsample}}{\mathit{weightofdriedplantsample}}\times 100$$

The percentage yield of the seed and leaf oils were found to be 1.04 and 1.22% (v/w), respectively.

A total of thirty (30) compounds were identified in leaf representing 87.47% of EOs constituent; containing β-ocimene (12.37%) as highest component followed by geranyllinalool (9.73%), α-farnesene (7.99%), (E)-Hex-3-enyl butyrate (7.20%), cyclofenchene (7.16%) and perhydrosqualene (6.53%). All other components were detected at quantities below 6.0%. The chemical classes of the leaf oil were sesquiterpene hydrocarbon (27.36%), oxygenated monoterpene (23.18%), monoterpene hydrocarbons (21.22%) and oxygenated sesquiterpene (4.43%) as presented in Table 1. However, twenty-eight (28) constituents were detected in EOs of the seed containing majorly oxygenated monoterpene (89.67%); Citral (36.60%),

Table 1.

Chemical Constituents of Chrysophyllum cainitos leaf essential oil

S/N	Retention time (mins)	Compound name*	Kovats Retention Index (RI)	Percentage area (%)	Formula
1	3.728	Caproaldehyde	870	2.10	C6H12O
2	5.044	1-Hexanol	958	2.15	C6H12O
3	5.353	Methylcyclopentane	806	1.01	C6H12
4	8.615	3,7-Dimethylocta-1,3,7-triene	1368	1.06	C10H16
5	8.798	β-ocimene	1318	12.37	C10H16
6	9.696	Cyclofenchene	958	7.16	C10H16
7	9.742	Pelargonaldehyde	1446	2.65	C9H18O
8	9.931	Methacrylic acid	1068	1.72	C9H14O4
9	10.137	Alloocimene	1426	0.63	C10H16
10	11.030	Hex-3-enyl butyrate	1446	7.20	C10H18O2
11	11.110	Isobutyric acid	874	1.65	C4H8O2
12	11.155	trans-2-Hexenyl butyrate	1188	2.11	C10H18O2
13	11.705	Pentanoic acid-3-hexen-1-yl ester	1218	1.33	C11H20O
14	12.775	undecyl aldehyde	1578	1.35	C11H22O
15	13.770	cis-3-Hexenyl hexanoate	1236	1.73	C12H22O2
16	13.868	2,6,6-trimethyl-3-cyclohexane	1178	0.76	C13H20O
17	14.137	Lauraldehyde	1456	1.96	C12H24O
18	14.365	β-Caryophyllene	1512	0.70	C16H24
19	15.172	(2,6,6-Trimethyl-1-cyclohexen-1-yl)−3-buten-2-one	1590	1.10	C13H20O
20	15.395	α-farnesene	1568	9.19	C15H24
21	15.487	2,4-di-tert-butylphenol	1486	1.98	C14H22O
22	16.242	Geranyllinalool	1654	9.73	C16H26
23	17.066	Cedrelanol	1814	0.65	C15H26O
24	17.450	α-bulnesene	1586	0.65	C15H24
25	19.143	Hexahydrofarnesyl acetone	1766	0.73	C18H36O
26	19.727	methyl ester 7-hexadecenoic acid	1718	0.69	C17H32O2
27	19.950	Methyl palmitate	1828	1.23	C17H34O2
28	21.472	Oleic acid	1732	1.13	C18H34O
29	24.968	n-Octadecane	2876	0.56	C18H38
30	27.744	Perhydrosqualene	2868	6.53	C₃₀H₅₀
		Monoterpene hydrocarbons		21.22
		Oxygenated monoterpenes		23.18
		Sesquiterpene hydrocarbons		27.37
		Oxygenated sesquiterpenes		4.43
		Non-terpene derivatives		11.28
		TOTAL		87.47

Geranial, (28.50%), isocitral (10.82%), exo-isocitral (3.13%), and Nerol; (2.68%). The remaining classes are as follows: monoterpene hydrocarbons (3.52%), oxygenated sesquiterpene (1.04%) and sesquiterpene hydrocarbons (0.23%). The dominant class of compounds in the EOs of the leaf is sesquiterpene hydrocarbon (27.36%) while in the seed EOs, it is oxygenated monoterpene (89.67%) as reported in Table 1.

Some major differences were noticed in the quantitative and qualitative composition of the two essential oils but it was concluded from the data that both oil samples were rich in monoterpenes. In EOs of the seed, the main class was formed by oxygenated monoterpene: Citral, Geranial, isocitral, exo-isocitral and Nerol. However, none of these oxygenated monoterpenes were detected in essential oil of the leaf. Both leaf and seeds EOs contained beta and alpha ocimene, cyclofenchene and pelargonaldehyde.

The dominant antimicrobial constituents of Chrysophyllum cainitos leaf EOs is shown in Fig. 1, shows. These constituents may be responsible in part or in synergy for the observed antimicrobial activities showed by the oil sample.

Fig. 1.

Some dominant antimicrobial constituents of Chrysophyllum cainitos leaf Essential oil

Chemical constituents of the C. cainito seed essential oils

The seed EOs of C. cainito was found to contain twenty-six (26) oil components (Tables 1 and 2). The seed EOs of C. cainito consist majorly of Citral (36.60%), Geranial, (28.50%), (E)−3,7-Dimethylocta-3,6-dienal (10.82%), exo-isocitral (3.13%), and nerol; (2.68%). The remaining constituents are as follows: oxygenated monoterpene (89.67%), non-terpene derivatives (4.43%), monoterpene hydrocarbons (3.52%), oxygenated sesquiterpene (1.04%), and sesquiterpene hydrocarbons (0.23%).

Table 2.

Chemical constituents of Chrysophyllum cainitos seed essential oil

S/N	Retention time (mins)	Compound name*	Kovats Retention Index (RI)	Percentage area (%)	Formula
1	5.347	α-Ocimene	1298	0.26	C10H16
2	5.800	ẟ-Terpinene	1376	0.24	C10H16
3	7.596	Sabinene	1266	0.76	C10H16
4	7.739	Rosefuran	1414	0.20	C10H14O
5	7.848	Cyclofenchene	980	1.51	C10H16
6	7.997	Pelargonaldehyde	1162	1.68	C9H18O
7	8.941	Dipropylacetylene	1176	0.28	C9H16
8	9.193	Exo-isocitral	1574	3.13	C10H16O
9	9.410	Terpineol	1318	0.32	C10H18O
10	9.708	3,6-Octadienal	1258	8.42	C10H16O
11	9.959	Rose furan oxide	1462	0.66	C10H14O2
12	10.131	3,7-Dimethylocta-3,6-dienal	1556	10.82	C10H16O
13	10.228	Propanoic acid	688	0.25	C10H18O2
14	11.095	Rhodinal	1488	0.47	C10H20O
15	11.344	Geranial	1264	28.56	C10H16O
16	11.590	Nerol	1286	2.68	C10H18O
17	11.899	Citral	1272	34.63	C10H16O
18	13.627	3-Carene	1348	1.32	C10H16
19	15.424	2,4-bis(1,1-dimethylethyl)- Phenol	1558	0.19	C14H22O
20	16.351	β-Caryophyllene	1518	0.19	C13H22
21	16.711	Xylene-2-carboxaldehyde	1484	0.19	C9H10O
22	16.780	Viridiflorol	1746	0.23	C15H24
23	17.209	Rosefuran	1668	1.17	C9H10O
24	17.770	Palmitaldehyde	1984	0.19	C16H32O
25	19.956	methyl palmitate	1826	0.30	C17H34O2
26	20.620	Ethyl palmitate	1816	0.30	C18H36O2
		Monoterpene hydrocarbons		3.52
		Oxygenated monoterpenes		89.67
		Sesquiterpene hydrocarbons		0.23
		Oxygenated sesquiterpenes		1.04
		Non-terpene derivatives		4.43
		TOTAL		98.89

The dominant class of compounds in the leaf EOs of C. cainito is sesquiterpene hydrocarbon (27.36%) while in the seed EOs, it is oxygenated monoterpene (89.67%). Research have shown that seasonal variations and geographical locations are the major factors that affect the composition of essential oil in samples [22]. Both leaf and seed EOs contained β- and α- ocimene, cyclofenchene, pelargonaldehyde. β-Caryophyllene; a minor constituent was found in both EOs. (Tables 1 and 2). Pino et al. [14] reported the characterisation of one-hundred and four compounds in the aroma concentrates of Chrysophyllum cainito. (E)−2-hexenal, 1-hexanol, limonene, linalool, copaene and hexadecanoic acid were found to be the major constituents. Similarly, geranyllinalool and 1-hexanol, were also identified as minor and dominant constituents in the leaf EOs.

The dominant antimicrobial constituents of Chrysophyllum cainitos seed EOs is shown in Fig. 2. These constituents may be responsible in part or in synergy for the observed antimicrobial activities shown by the oil sample.

Fig. 2.

Some antimicrobial constituents of Chrysophyllum cainitos seed Essential oil

Biological significance of the dominant constituents of C. caimito leaf and seed oils

Citral has gained much attention from researchers due to which plenty of its bioactivities such as, antimicrobial activity [23, 24], antiviral activity [25], anti-inflammatory activity [26, 27], anti-leishmanial activity [28, 29], inhibitory activity against cytokines effect [30], chemopreventive activity [31], allelopathic activity [32], aldose reductase inhibitor activity [33], spasmolytic activity [34], antiadipogenic activity [35], insect repellent effect [35], anti-parasitic activity [36], Insecticidal effects [37].

Geranial; an oxygenated monoterpene has been reported for its antimicrobial activity [38], antifungal activity [39], as a plant insect repellent [40], anti-inflammatory agent [41] and anthelmintic property [42]. Also, Nerol; a major constituent of C. cainitos have been reported previously for its antimicrobial activity [43] and anticancer properties [43]. β-ocimene in EOs is associated with anticonvulsant activity, antifungal activity, antitumor activity, and pest resistance [44–46]. Ocimene is also a volatile pheromone important for the social regulation of honeybee colonies [47]. α-fernesene has been reported for its insecticidal activity [48].

Antimicrobial Analyses of seed and leaf essential oils of Chrysophyllum caimito

Previously, the pulp and seed samples of Chrysophyllum cainito was analyzed for its antimicrobial potentials against some clinical isolates of bacteria and fungi, the microbial counts of the fruits ranged from 1.0 × 10⁹ cfu/mL to 2.4 × 10¹⁰ cfu/mL for total aerobic plate counts, 1.0 × 10⁷ to 2.0 × 10⁷ (cfu/mL) for fungal counts, and 1.0 × 10⁸ to 1.2 × 10⁹ (cfu/mL) for coliform counts, the current work justifies this finding [13].

Currently, the EOs from the seed and leaf of Chrysophyllum cainito were screened against 7 bacterial and 2 fungal clinical isolates. The results of the antimicrobial susceptibility test are represented in Table 3, Figs. 3, and 4. The MICs of the seed EOs against all tested micro-organisms ranged between 12.5 and 50 mg/mL, whereas those of the leaf EOs were between 0.781251 and 50 mg/mL. In general, the leaf EOs exhibited better antimicrobial activity when compared to the seed EOs. MICs of the leaf oil were lower than those of the seed oil. The lesser the MICs the better the activity of the essential oils. Salmonella typhi, Candida albicans, Tricophytum rubrum, Pseudomonas aeruginosa and Bacillus subtilis all displayed very low sensitivity (resistance) to the antimicrobial activity of the leaf and seed EOs as compared to other test organisms. Escherichia coli (ATCC 25922), Klebsiella pneumoniae and Staphylococcus aureus (ATCC 25723) showed high level of susceptibility to the activity of the EOs. The Minimal Bactericidal Concentration is the least concentration of the oil sample capable of causing death of the microbes. Essentially, the leaf and seed oils were mostly potent against Staphylococcus aureus (MBC: 12.5 mg/mL each for leaf and seed) and E. coli (MBC: 12.5 mg/mL for leaf and 25 mg/mL for seed), has the best MBC potential of 12.5 µg/mL for the seeds and leaf oils respectively. The seeds and leaf oils also displayed a moderate mortality concentration against Escherichia coli (ATCC 25922), while the remaining organisms recorded low MBC contractions of 50 to > 50 μg/mL for the oil samples. Both EOs displayed strong and broad-spectrum antimicrobial activity against Candida albicans (ATCC 18804) and Tricophytum rubrum which were the fungi used in this study (Table 3, Figs. 3, 4).

Table 3.

Antimicrobial Activity of Chrysophyllum canito leaf and Seed Essential oils

S/N	ORGANISM	C.ASOIL		C.ALOLI		BROTH ONLY	BROTH and OIL	BROTH and ISOLATES	GENTAMICIN		KETOCONAZOLE
		MIC (mg/mL)	MBC (mg/mL)	MIC (mg/mL)	MBC (mg/mL)				MIC ($\mathrm{\mu }$ g/mL)	MBC ($\mathrm{\mu }$ g/mL)	MIC (mg/mL)	MFC (mg/mL)
1	S.a	12.5	12.5	0.781251	12.5	_	_	+	5	5	NA	NA
2	B.s	> 50	> 50	50	50	_	_	+	5	10	NA	NA
3	E.c	12.5	25	1.625	12.5	_	_	+	10	10	NA	NA
4	P.a	50	> 50	50	> 50	_	_	+	> 10	> 10	NA	NA
5	K.p	12.5	50	3.125	25	_	_	+	10	10	NA	NA
6	S.typ	50	> 50	50	50	_	_	+	> 10	> 10	NA	NA
7	C.a	> 50	> 50	> 50	> 50	_	_	+	NA	NA	1	1
8	T.r	50	50	> 50	> 50	_	_	+	NA	NA	0.25	0.5

Key: S.a Staphylococcus aureus, B.s Bacillus subtilis, E.c Escherichia coli, S.typ Salmonella typhi, PS.a Pseudomonas aeruginosa, K.p Klebsiella pneumonae, C.a Candida albicans and T.r Tricophytum rubrum

Fig. 3.

Antimicrobial (MIC) Results of C. cainito seed and leaf oils

The study highlighted the antimicrobial potential of both leaf and seed oils, with the leaf oils exhibiting stronger antimicrobial activity, particularly against Staphylococcus aureus and Escherichia coli.

Molecular docking studies

The docking score of compound 11 from seed EOs with the docking score (−3.303) showed the docking score of all compound from the both leaf and seed of all the sitemaps and close to that of reference compound on sitemap three with docking score of −5.055. The other compounds with close docking score include; −3.175 kcal/mol (1), −2.781 kcal/mol (2), −2.726 kcal/mol (12), −2.620 kcal/mol (10), −2.443 kcal/mol and −2.440 kcal/mol (3). The anti E. coli activities of both leaf and seed EOs could be associated with inhibitory activities of the compound on the active sites of the protein.

Compound 1, 2, 3, 4 and 7 have no interaction with any of the five sitemaps. Compound 5 interacted only with sitemap 3 through hydrogen bond with ASN99; 2.16. Compound 6 interacted with sitemaps 3 and 5 through hydrogen bonding with LYS101; 2.67, THR16; 1.76, and LYS157; 2.14. Compound 8 interacted with sitemaps 3 and 5 through hydrogen bonding with TYR162; 2.01 and LYS157; protein residues, 1.83.

Compound 9 interacted only with sitemap 3 through hydrogen bonding with LYS101; 2.13 and TYR16; 2.00. Compound 10 interacted with sitemaps 2, 3 and 5 through hydrogen bonding with ASP146; 2.10, TYR16; 1.98 and LYS157; 1.90.

Compound 11 interacted with sitemaps 1, 3, 4 and 5 through hydrogen bonding with ASP99; 2.29, SER100; 1.79, GLU163; 1.76, ASP146; 1.56 and GLU64; 1.76.

Compound 12 interacted with sitemaps 3 and 5 through hydrogen bonding with various protein residues ASN96_; 1.75 and LYS157_; 1.74.

The reference drug (13 and 14) with the five sitemaps through hydrogen bonding and pi-pi stacking. The hydrogen bonded amino acids include; THR149_; 2.06, THR149; 1.93, ASP142; 1.78, GLU64; 1.72, GLU64; 1.68, TYR16; 2.31, TYR16; 1.85, GLU163; 1.94 and SER100; 1.96. The pi-pi stacking amino acids include; PHE161; 5.36 and PHE69; 5.39.

Compounds 1, 2, 3, 4, 5, 6 and 7 found in leaf EOs and one conformer for cyclofenchene, compounds 8, 9, 10, 11 and 12 which are present in seed EO and compounds 13 and 14 (Flouroquinolone and Flouroquinolone isomer) are known standard anti E. coli drugs and a conformer used as reference compound (Fig. 4 and Tables 4, 5a, 5b, 6, 7).

Fig. 4.

Antimicrobial (MBC/MFC) Results of C. cainito seed and leaf oils

Table 4.

Binding energies of the compound with the five sitemaps from 6GCM

Compounds	Binding Affinity (∆G) Kcal/mol
	Sitemap 1	Sitemap 2	Sitemap 3	Sitemap 4	Sitemap 5
1	−3.175	1.171	−0.369	−1.523	−0.663
2	−2.048	−1.040	−2.781	−2.095	−1.309
3	−2.066	−1.421	−2.443	−2.440	−1.354
4	−1.919	−1.895	−0.444	−1.834	0.840
5	−1.881	−1.837	−2.146	−1.731	−0.835
6	−1.001	−1.433	−0.918	−0.886	0.828
7	0.676	0.494	1.032	−1.897	−0.801
8	−2.311	−2.205	−2.234	−1.801	−1.595
9	−2.187	−1.277	−1.811	−1.827	−0.630
10	−2.045	−1.972	−2.620	−1.335	−0.979
11	−1.531	−1.802	−3.303	−1.676	−0.596
12	−1.185	−1.016	−2.726	−1.030	−1.083
13	−1.933	−2.393	−5.055	−2.100	−2.322
14	−1.471	−0.045	−3.031	−1.338	−0.355

Table 5a.

3-D view of ligand–protein complex

Table 6.

3-D view of leaf EOs compounds, seed EOs compounds and reference drug in different sitemaps

Table 7.

Names of the docked dominant constituents of C. cainitos oils with antimicrobial standards

Compound number	Docked compound
1	1,3,7,11-trideca-tetraene-5,9-triyne
2	Hex-3-enylbutyrate
3	α-fernesene
4	Perhydrosqualene
5	β-Ocimene
6	Cyclofenchene
7	Citral
8	3,7-dimethyl-1,2-Octadienal
9	3,7-dimethyl-octa-3,6-dienal
10	3,6-Octadienal
11	2,7-dimethyl-2,6-octadiene-1-ol
12	7-methyl-3-methyldiene-6-octanal
13	β-Fluoroquinoline
14	Fluoroquinoline

Conclusion

Based on the results obtained in the current report, Chrysophyllum cainito L. leaf EOs contains more oil constituents than the seed EOs. β-ocimene has highest percentage composition in the leaf oil while citral was most abundant in the seed oil. The leaf and seed EOs of C. cainito contained majorly sesquiterpene hydrocarbon (27.36%) while the seed EOs was dominated by oxygenated monoterpenes (89.67%). These compounds in part or in synergy may be responsible for the antimicrobial activity of C. cainito. The leaf and seeds EOs contain β- and α- ocimene as well as cyclofenchene were also found as part of the chemical constituents of the EOs. The leaf EOs MIC and MBC results of the antimicrobial screening showed that the leaf oil had a better activity when compared to the seed EOs. Both EOs displayed strong and broad-spectrum antimicrobial activity towards Staphylococcus aureus, E. coli, Candida albicans and Tricophytum rubrum. The antimicrobial constituents of C. cainitos showed weak binding interactions. The rationale for the good antimicrobial results was justified by the moderate binding energies as revealed in the docking studies.

Further studies are hereby advised on the antimicrobial activity of C. cainito using a different approach and methodology, the drugability and toxicity of the oil samples are required. These findings underscore the diverse therapeutic potential of Chrysophyllum cainito essential oils and their components.

Authors'contributions

O.B: Conceived the research ideas and directed the research, collection, identification of plant samples and in the writing of the manuscript. E.Y: Assisted in gathering literature data and in the extraction of the volatile oil samples in the laboratory. S.O: Conceived the research ideas and edited the final manuscript. A.K: Assisted in gathering literature data and in the extraction of the volatile oil samples in the laboratory. J.O: Edited the final manuscript. I.M: Did the molecular Docking studies.

Funding

Not applicable.

Data Availability

Data is provided within the manuscript files

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interest

The authors declare no competing interests.