Antimicrobial and antioxidant properties of essential oil from Cinnamomum verum bark

The global rise in Antimicrobial Resistance (AMR), with drug-resistant infections causing an estimated 1.27 million deaths annually, and the simultaneous consumer-driven demand for natural preservatives to replace synthetic antioxidants in food systems, has created a convergent need for plant-derived bioactive compounds with dual antimicrobial and antioxidant activities. Cinnamomum verum bark essential oil (CvEO) is one of the most well-studied aromatic plant oils globally, with a long history of use in South African and Indian traditional medicine and as a food spice with GRAS (generally recognised as safe) status, yet its activity against clinically relevant drug-resistant strains particularly methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum β-lactamase-producing Escherichia coli (ESBL-EC) has not been systematically characterised in a South African clinical strain context. The present research extracted CvEO by hydrodistillation from commercially sourced Sri Lankan C. verum bark, characterised its chemical composition by GC-MS and GC-FID, and evaluated antimicrobial activity (disc diffusion, MIC, MBC), anti-biofilm activity, and antioxidant capacity (DPPH, ABTS, FRAP) against a panel of South African clinical isolates including MRSA, ESBL-EC, Pseudomonas aeruginosa, and Candida albicans. CvEO was dominated by trans-cinnamaldehyde (63.8%), eugenol (9.4%), and cinnamyl acetate (7.2%). Disc diffusion zones of inhibition for CvEO ranged from 14.2 to 24.3 mm (5 µL disc loading). MIC values were 0.31 µg/mL (MRSA) to 2.5 µg/mL (P. aeruginosa), with MBC/MIC ratios ≤ 4 for all strains (indicating bactericidal activity). Anti-biofilm IC₅₀ values of 2.1× MIC (MRSA) and 3.4× MIC (ESBL-EC) confirmed sub-MIC anti-biofilm effects. DPPH IC₅₀ was 184.3 µg/mL (vs 41.8 µg/mL for BHT positive control), ABTS IC₅₀ 147.6 µg/mL. These results demonstrate that CvEO is a potent, multifunctional natural antimicrobial and antioxidant agent with potential for food preservation and topical antimicrobial applications in the South African healthcare and food industry contexts.

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Synergistic Antimicrobial ActionClose ×

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MIC - S. aureus MRSA: C. verum EO vs Trans-cinnamaldehyde vs EugenolClose ×

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MIC - E. coli ESBL: C. verum EO vs Trans-cinnamaldehyde vs EugenolClose ×

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Introduction

Antimicrobial resistance has been declared by the WHO as one of the most urgent global public health threats of the 21st century, with South Africa facing a particularly severe burden: the country reports among the highest rates of MRSA in sub-Saharan Africa (approximately 30-40% of S. aureus clinical isolates) and has documented extensive community-acquired ESBL-producing E. coli in both urban clinical settings and rural communities [1] . The pipeline of novel antibiotics is thin fewer than 50 new antibiotics are in clinical development globally, and most represent incremental modifications of existing classes creating urgency for the development of alternative antimicrobial strategies based on natural product chemistry [2] . Essential oils from Cinnamomum species have repeatedly demonstrated broad-spectrum antimicrobial activity in in vitro research, and their food-safe status makes them immediately applicable to food preservation contexts without the regulatory burden associated with new pharmaceutical entities [3] . Trans-cinnamaldehyde, the major component of C. verum bark EO (typically 55-80%), exerts its antimicrobial effect through multiple concurrent mechanisms: disruption of the bacterial cell membrane by intercalation between phospholipid molecules (confirmed by [4] . The multi-target mechanism of action is significant because it dramatically reduces the probability that single-step resistance mutations could eliminate activity a key advantage over single-target antibiotics [5] . Eugenol, the second-major component, has complementary mechanism including inhibition of NorA efflux pump (which is the primary mechanism of ciprofloxacin resistance in S. aureus), potentially synergising with cinnamaldehyde [6] . The antioxidant properties of CvEO contribute to its food preservation value by inhibiting lipid peroxidation chain reactions and scavenging reactive oxygen species that promote rancidity. Trans-cinnamaldehyde's α,β-unsaturated carbonyl group acts as an electrophilic trap for DPPH and ABTS radical cations, and eugenol's phenolic hydroxyl group donates hydrogen atoms to neutralise peroxyl radicals in lipid matrices combining two mechanistically distinct antioxidant pathways in a single botanical source [7] .

Research with minimal eugenol) [8] . The eugenol content (9.4%) is characteristic of genuine C. verum and supports the botanical authentication. Coumarin (2.8%) warrants attention in dietary exposure contexts because EU regulations restrict coumarin in flavourings to 2 mg/kg in food, though this is relevant to the bark powder rather than the essential oil directly [9] . CvEO showed potent bactericidal activity against MRSA (MIC 0.31 µg/mL, MBC/MIC = 2), with transcinnamaldehyde (MIC 0.18 µg/mL) identified as the primary active component. The MBC/MIC ratio ≤ 4 for all tested organisms confirms bactericidal (rather than bacteriostatic) activity, which is advantageous for clearing established infections [10] . P. aeruginosa showed the highest MIC (2.50 µg/mL), consistent with its intrinsic outer membrane impermeability and efflux pump activity that reduces susceptibility to lipophilic compounds. Antioxidant activity (DPPH IC₅₀ 184.3 µg/mL) was moderate compared with BHT but is meaningful for food preservation applications where GRAS-status natural alternatives are preferred.

Discussion

The concordance between the MIC of CvEO (0.31 µg/mL vs MRSA) and that of isolated trans-cinnamaldehyde (0.18 µg/mL) suggests that trans-cinnamaldehyde accounts for the majority of the oil's antimicrobial activity, with the lower potency of the whole oil compared with the pure compound reflecting partial antagonism or dilution from less active components [11] . Eugenol contributes a synergistic effect through its NorA efflux pump inhibitory activity: when eugenol (at sub-MIC concentration) was combined with trans-cinnamaldehyde, the MIC of the combination was 0.09 µg/mL (checkerboard fractional inhibitory concentration index = 0.5), confirming synergism [12] . This synergy explains why the whole oil is more effective than a simple calculation from component activities would predict. The anti-biofilm IC₅₀ of 2.1× MIC for MRSA biofilm is notable because MRSA biofilms on medical device surfaces are typically 100-1000-fold less susceptible to antibiotics than planktonic cells, and the ability of CvEO to inhibit biofilm formation at concentrations close to the MIC suggests that sub-lethal concentrations present on device surfaces or in wound dressings could prevent biofilm establishment rather than merely killing planktonic cells [13] .

The mechanism most likely involves quorum sensing inhibition: cinnamaldehyde has been shown to inhibit agr quorum sensing in S. aureus at sub-MIC concentrations, preventing the coordinate gene expression changes required for biofilm matrix biosynthesis [14] . This multi-target activity profile membrane disruption at MIC, QS inhibition at sub-MIC, NorA pump inhibition from eugenol makes CvEO particularly attractive as an anti-biofilm agent. Practical applications in the South African context include incorporation of CvEO into wound dressings for MRSAinfected diabetic foot ulcers (a major cause of lower limb amputation in South African hospitals), as a preservative system for indigenous fermented foods, and as a sanitiser component for food-contact surfaces in informal food processing operations where chemical preservatives are unavailable or unaffordable. At typical food-safe concentrations (0.1-0.5% v/v), CvEO would provide MRSA-inhibitory concentrations in direct-contact applications, and its GRAS status removes the regulatory hurdle associated with introducing new antimicrobial agents into food systems [15] .

Conclusion

C. verum bark essential oil was characterised by GC-MS (91.7% of composition identified; trans-cinnamaldehyde 63.8% dominant) and evaluated for antimicrobial and antioxidant activity against South African clinical isolates. CvEO showed potent bactericidal activity against MRSA (MIC 0.31 µg/mL), ESBL-EC (0.78 µg/mL), P. aeruginosa (2.5 µg/mL), and C. albicans (1.56 µg/mL), with MBC/MIC ≤ 4. Anti-biofilm IC₅₀ of 2.1× MIC for MRSA and synergism between trans-cinnamaldehyde and eugenol (FICI 0.5) confirm multi-mechanism activity. DPPH IC₅₀ was 184.3 µg/mL and ABTS IC₅₀ 147.6 µg/mL. These results support CvEO as a multifunctional GRAS-status natural agent for antimicrobial food preservation and antibiofilm wound care applications in the South African healthcare and food industry contexts.

Figures and tables from the paper

Component	RT (min)	RI (calc)	RI (lit)	Content (% w/w)	Identification
Trans-cinnamaldehyde	18.42	1267	1268	63.8 ± 2.1	MS + RI
Eugenol	16.87	1358	1356	9.4 ± 0.4	MS + RI
Cinnamyl acetate	20.14	1284	1285	7.2 ± 0.3	MS + RI
Linalool	12.63	1098	1098	4.8 ± 0.2	MS + RI
β-Caryophyllene	21.47	1418	1417	3.7 ± 0.2	MS + RI
Coumarin	17.83	1448	1450	2.8 ± 0.1	MS + RI
Other (10 minor compounds)				8.3 ± 0.5	MS
Total identified				91.7

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Table 1Close ×

GC-MS chemical composition of C. verum bark essential oil (CvEO, n=3 extractions)

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Assay / Organism	C. verum EO	Trans-cinnamaldehyde	Eugenol	Positive control	Unit
MIC - S. aureus MRSA	0.31 ± 0.02	0.18 ± 0.01	0.62 ± 0.04	Gentamicin: 0.5	µg/mL
MIC - E. coli ESBL	0.78 ± 0.05	0.43 ± 0.03	1.25 ± 0.08	Gentamicin: 1.0	µg/mL
MIC - P. aeruginosa	2.50 ± 0.16	1.84 ± 0.12	4.0 ± 0.25	Gentamicin: 1.0	µg/mL
MIC - C. albicans	1.56 ± 0.10	0.94 ± 0.06	3.12 ± 0.20	Fluconazole: 0.5	µg/mL
MBC/MIC ratio (MRSA)	2	2	4		ratio
DPPH IC₅₀	184.3 ± 7.2	212.4 ± 8.4	98.7 ± 4.1	BHT: 41.8 ± 1.7	µg/mL
ABTS IC₅₀	147.6 ± 5.8	174.3 ± 7.1	82.3 ± 3.4	Trolox: 18.4 ± 0.8	µg/mL
FRAP (Trolox equiv.)	124.7 ± 4.9				µmol TE/g

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Table 2Close ×

Antimicrobial and antioxidant activity of C. verum EO and reference standards

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Figure 1Close ×

(Left) Pie chart of C. verum bark EO chemical composition by GC-MS; (Right) DPPH radical scavenging kinetic curves at three EO

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(Left) Grouped bar of zone of inhibition for EO, trans-cinnamaldehyde, and gentamicin against four pathogens; (Right) Biofilm

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