Lemongrass essential oil may target bacteria even more effectively than conventional antibiotics, research suggests

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Lemongrass essential oil can be even more effective than traditional antibiotics against some Streptococcus bacteria under certain conditions, new research to be presented at MLSFF26 has found.

Anna K

The research findings by Anna Kocel, a University of the West of England undergraduate, could help to reduce reliance on conventional antibiotics during a growing antimicrobial resistance crisis.

Her work will be presented at the Minoritised Life Scientists Future Forum (MLSFF26) at Edinburgh International Conference Centre from March 23 to 25. MLSFF26 is the only major conference in Europe dedicated to supporting and showcasing the contributions of marginalised and underrepresented communities in the life sciences.

The science conference is expected to attract hundreds of scientists to Edinburgh. With the city’s remarkable history of scientific invention and discovery, it’s the ideal location for young scientists to share their own breakthroughs.

“My research investigated the anti-microbial potential of Cymbopogon flexuosus (lemongrass essential oil) against Streptococcus pyogenes and Streptococcus mutans and found the lemongrass essential oil to be even more effective than the EUCAST standard antibiotic against both species, Benzylpenicillin, in some conditions,” Anna said.

Tooth enamel

“Streptococcus mutans colonises and invades tooth enamel, metabolising dietary sugars into lactic acid and forming biofilms- causing tooth caries (more commonly known as cavities) and the gold standard treatment is Chlorhexidine.

“Streptococcus pyogenes colonises and invades human pharyngeal epithelial cells, causing Streptococcal pharyngitis which is treated with oral penicillin.

“Resistance against both treatments is continually increasing, highlighting the need for research into alternative treatments to combat this. The World Health Organisation has revealed 1 in 6 common bacterial infections are now resistant to penicillin treatments. Therefore, more disease cases are able to be treated when alternative treatments are available to avoid human resistance.”

Lemongrass essential oils

Anna tested different Cymbopogon flexuosus Lemongrass essential oils (LEOs) against S. mutans and S. pyogenes. Two were from the brand Absolute Aromas - one being organic and one being inorganic. The third LEO was from the brand Amphora Aromatics.

She performed HS-GC-MS (Headspace Gas Chromatography Mass-Spectrometry) to produce chromatograms identifying the ten most abundant volatile organic compounds, direct disc diffusion assays to measure inhibition zones and susceptibility in direct contact, volatilisation assay to measure zones of inhibition and susceptibility in indirect contact and broth micro-dilution assay to determine minimum inhibitory concentrations.

The chromatograms revealed the ten most abundant volatile compounds. All three oils contained a similar amount of citral and citral isomers geranial and neral, all comprising 56-59% of total composition. Each oil demonstrated concentration-dependent inhibition of S. pyogenes and S. mutans in direct disc diffusion assays, consistent with their high citral contents. Stronger inhibition was observed against S. pyogenes compared to S. mutans.

Zones of inhibition

At 100% concentration, all three oils produced large zones of inhibition against S. pyogenes at means of 23-24cm. Noticeably, under many conditions larger zones of inhibition were produced at 100% concentrations of the oils than the zones of inhibition produced by the gold standard antibiotic used as the positive control, Benzylpenicillin.

Antimicrobial activity against S. mutans was much lower, particularly the Absolute Aromas organic oil which exhibited a mean zone of inhibition of just 10.85cm at 100% concentration.

Despite similar levels of citral between the oils, their vapour-phase activity differed significantly. Almost all oils exhibited strong inhibition at 100% concentration against both bacteria. However, only the Absolute Aromas Inorganic oil and the Amphora oil inhibited S. mutans. The Absolute Aromas organic oil showed minimal vapour activity against S. mutans with no zone of inhibition observed at all.

Surprising findings

Strangely, the largest zones of inhibition produced in the volatilisation assay were by the Absolute Aromas organic oil against S. pyogenes with a mean of 24.74cm at 100% concentration. Although disc diffusion assays demonstrated greater inhibition of S. pyogenes, broth micro-dilution analysis revealed lower optical density readings for S. mutans at inhibitory concentrations than S. pyogenes, suggesting species-specific susceptibility differences influenced by the assay format’s level of oil dispersion.

“The findings suggest that while citral content likely contributes to contact-dependent antibacterial activity, seen in the direct disc diffusion assay, vapour phase efficacy may be influenced by compositional differences and volatility rather than just citral percentage alone,” Anna said.

“Factors such as the geranial to neral ratio and abundance of trace compounds may have a more significant influence on results than assumed. Surprisingly, S. pyogenes presented as more susceptible under agar diffusion conditions whilst S. mutans presented as more susceptible under liquid conditions.”

Natural antimicrobial strategies

The study directly supports the development of natural, accessible antimicrobial strategies, reducing reliance on conventional antibiotics during the growing antibiotic resistance crisis. “Lemongrass essential oil can be found easily in many countries around the world, the results deeming LEO as an effective antibiotic can make treatments cheaper and more accessible. In countries where people pay for healthcare, this research provides them with alternative treatment options,” Anna said.

“Additionally, the results revealing the species may be more susceptible under different conditions can support different forms of treatment. During development of solid treatments such as antibiotic tablets or liquid treatments such as sprays or creams we may be able to base the volatile makeup needed in HS-GC-MS based on direct disc diffusion assay, volatilisation assay or broth micro-dilution assay results to make the treatment as effective as possible.

“Future investigations are needed to determine the individual contributions of geranial and neral as well as investigate how trace compounds may influence the vapour activity.”

This study was a final year dissertation study, undertaken in the University of the West of England laboratories. The study was supported by the University of the West of England and led by supervisors Emmanuel Adukwu, an AMI Trustee, and Oluwadamilola Okeyoyin.

Europe’s only conference for minoritised life scientists, the Minoritised Life Scientists Future Forum (MLSFF) will be held at Edinburgh International Conference Centre (EICC) from March 23 to 25. Find out more about MLS Future Forum HERE.