The research team set out to qualitatively and quantitatively assess the antimicrobial efficacy of penicillin, gentamicin, vancomycin, rifampicin, kanamycin, ampicillin, clotrimazole, and amphotericin-B alongside essential oils from lemongrass (Cymbopogon flexuosus), palmarosa (Cymbopogon martinii), and peppermint (Mentha piperita). The pathogens under scrutiny included the Gram-positive bacteria Staphylococcus aureus, Staphylococcus epidermidis, Mycobacterium smegmatis, and Enterococcus faecalis, together with two fungal pathogens, Malassezia furfur and a clinical isolate of Candida albicans. These microorganisms are not exotic threats but everyday residents of the human skin, gut, and mucous membranes that can turn dangerous when the body’s homeostasis is disturbed by surgery, injury, malnutrition, immunodeficiency, or medication.

Opportunistic pathogens occupy a peculiar niche in medical microbiology. In an immunocompetent person, they live harmlessly as commensals, part of the vast microbial community that coexists in mutualistic association with the human body. But when the immune system is subverted or the normal balance of microflora is upset, these facultative parasites can escalate from mild infections to life-threatening bloodstream invasions. S. aureus, for instance, can circumvent immune defenses to cause carbuncles, folliculitis, cellulitis, abscesses, and bacteremia. S. epidermidis, which dwells predominantly on human skin, is a major culprit in hospital-acquired infections and infections of medical implants. E. faecalis, a gastrointestinal commensal, can trigger oral cavity infections, periodontitis, urinary tract infections, and nosocomial disease. Even M. smegmatis, which is not linked to tuberculosis in humans, has the potential to cause soft tissue and pulmonary infections in vulnerable patients.

On the fungal side, the study examined two dimorphic and polymorphic pathogens. C. albicans, an exclusively opportunistic fungus, is well known for causing oral, cutaneous, and vulvovaginal candidiasis, as well as candidaemia and hospital-acquired infections. M. furfur, isolated in this study from a human scalp, is associated with skin conditions such as dandruff, seborrheic dermatitis, and pityriasis versicolor. Combatting these infections traditionally requires reliance on synthetic or semi-synthetic antimicrobials spanning classes such as β-lactams, aminoglycosides, glycopeptides, quinolones, azoles, and polyenes. Yet the indiscriminate use of these drugs has fueled the emerging phenomenon of antibiotic resistance and can carry harmful effects on essential organs, prompting researchers to explore natural alternatives.

Methodologically, the study employed two complementary assays. The disk diffusion test involved spreading 100 microliters of microbial suspension evenly onto fresh nutrient or Sabouraud agar plates, then placing 5-millimeter filter paper disks loaded with 5 micrograms of standard antimicrobials or 30 microliters of essential oil. After incubation at 37 degrees Celsius for 24 hours for bacteria and 28 degrees Celsius for 48 hours for fungi, clear zones of growth inhibition were measured in millimeters, with each test performed in triplicate. The minimum inhibitory concentration, or MIC, was determined by broth dilution in 96-well microtiter plates, with cultures adjusted to 0.5 McFarland standard turbidity to obtain 1 × 10^8 colony-forming units per milliliter. A redox indicator dye, shifting from purple to pink in the presence of cellular reductases, provided a visual readout of microbial viability. The minimum bactericidal and fungicidal concentrations were deduced by spot inoculation onto fresh agar plates from the last three wells of each dilution series.

The disk diffusion results exposed the patchy nature of synthetic drug efficacy. Penicillin produced the maximum zone of inhibition against S. aureus at 40.0 millimeters and suppressed E. faecalis with a 33.0-millimeter zone, while rifampicin and ampicillin also generated strong zones against S. aureus at 32.0 and 32.3 millimeters respectively. Gentamicin proved the most favorable drug against S. epidermidis and M. smegmatis, each at roughly 19 to 20 millimeters. Yet penicillin and ampicillin were completely inactive against M. smegmatis, and S. epidermidis showed no response to penicillin, vancomycin, or ampicillin. Vancomycin, often considered a drug of last resort for Gram-positive infections, exhibited activity against three of the four tested bacteria but proved ineffective against S. epidermidis. Among the antifungals, clotrimazole was highly effective against both M. furfur and C. albicans, producing zones of 34.0 and 32.0 millimeters respectively, while amphotericin-B showed more modest inhibition at 17.5 and 18.0 millimeters.

The essential oils told a different story. Lemongrass oil, applied at 3 percent volume per volume, displayed prominent zones of inhibition against every pathogen examined, including the strains resistant to conventional antibiotics. Its zones ranged from 12.7 millimeters against S. aureus to 30.3 millimeters against M. smegmatis, with particularly strong activity against the fungal pathogens at 27.0 and 28.3 millimeters for M. furfur and C. albicans respectively. Peppermint oil was most effective against M. smegmatis at 31.7 millimeters, while palmarosa oil showed its highest inhibition against M. furfur at 16.7 millimeters. Notably, M. smegmatis, which was resistant to penicillin and ampicillin, was prominently inhibited by both lemongrass and peppermint oils. A similar pattern emerged against S. epidermidis, though peppermint oil showed very low inhibition in that case.

The MIC data reinforced these findings with quantitative precision. Penicillin and rifampicin inhibited S. aureus and E. faecalis at an MIC of 0.10 micrograms per milliliter, while gentamicin suppressed S. epidermidis and M. smegmatis at the same concentration. Kanamycin required a higher MIC of 0.39 micrograms per milliliter to restrict S. epidermidis, and vancomycin was effective at slightly higher concentrations for all pathogens except E. faecalis. Clotrimazole showed antifungal activity at an MIC of 0.10 micrograms per milliliter for both fungal pathogens, whereas amphotericin-B required a higher concentration of 3.12 micrograms per milliliter. For the essential oils, palmarosa yielded the most favorable results against the bacterial pathogens, with MIC values ranging from 0.07 to 0.09 percent volume per volume. Peppermint oil was particularly effective against S. aureus at 0.07 percent, while lemongrass required slightly higher concentrations at 0.19 percent. Against the fungal pathogens, palmarosa and peppermint oils inhibited both at 0.3 percent, while lemongrass achieved inhibition at just 0.12 percent. Crucially, the minimum bactericidal concentrations aligned with the MIC values, indicating that the oils were not merely suppressing growth but killing the pathogens outright.

The mechanistic explanation for essential oil activity lies in their complex bioactive composition. These natural extracts contain a wide variety of terpenes and phenolic compounds whose antimicrobial mode of action relies mainly on cell membrane disruption, interference with enzymatic activity, inhibition of cellular respiration, and disruption of DNA and RNA synthesis. Because these mechanisms target multiple cellular structures simultaneously rather than a single molecular target, pathogens pose minimal risk of developing resistance through the routes that plague conventional antibiotics, such as target alteration or the development of efflux pumps to expel the drug. Combined with their low toxicity, economic viability, and biodegradability, essential oils may be better suited as medications in contrast to synthetic antimicrobials, particularly in an era when multidrug-resistant pathogens have become a serious threat to existing antibacterial treatment.

The authors conclude that although the antimicrobial potency of essential oils cannot surpass standard antibiotics in absolute terms, their efficacy managed to align with synthetic drugs even when differing concentrations were taken into account. All three essential oils inhibited the growth of all tested pathogens to some degree, contrary to several antibiotics that failed to suppress the proliferation of specific microorganisms. Lemongrass oil depicted the most favorable results, giving distinct zones of inhibition between 12.7 and 30.3 millimeters for the majority of tested pathogens while remaining efficient toward resistant strains. The researchers caution that additional research under in vivo conditions is needed to thoroughly assess the full therapeutic potential of these oils and provide valuable insights into potential treatments as substitutes against relevant diseases. Yet the message is clear: nature’s pharmacy, refined over millions of years of plant evolution, may hold answers that synthetic chemistry alone cannot provide.

Subject of Research: Comparative antimicrobial efficacy of synthetic antibiotics and plant essential oils against human pathogenic bacteria and fungi

Article Title: Comparative antimicrobial evaluation of synthetic antibiotics and essential oils against human pathogenic bacteria and fungi

Article References: Khaliq, A., Mishra, A. K., & Rabbani, A. (2024). Comparative antimicrobial evaluation of synthetic antibiotics and essential oils against human pathogenic bacteria and fungi. Innovations in Agriculture, 7, 1-7. https://doi.org/10.3897/ia.2024.124222

Image Credits: AI Generated

DOI: 10.3897/ia.2024.124222

Keywords: antimicrobial resistance, essential oils, lemongrass, antibiotics, Candida albicans, Staphylococcus aureus, natural remedies, disk diffusion assay, Comparative, antimicrobial, evaluation, synthetic

The review, led by Eliakira Kisetu Nassary of Sokoine University of Agriculture in Tanzania together with colleagues, followed the PRISMA-ScR framework, the recognised reporting standard for scoping reviews. The team searched Scopus, Web of Science, PubMed, ScienceDirect and Google Scholar for publications from January 2010 through February 2026, screening 286 records and ultimately including 95 peer-reviewed studies in the evidence synthesis. Rather than simply summarising the literature, the authors assigned each study to a specific segment of the citral biosynthetic pathway and scored the depth of experimental validation using a purpose-built tool they call the Evidence Maturity Index, or EMI. The index ranks evidence hierarchically: transcriptomic prediction scores lowest at 1.0, integrated transcriptome-metabolome studies score 1.5, recombinant enzyme characterisation scores 2.0, and in planta functional validation, the strongest form of proof, scores 2.5.

The biosynthetic story begins in the chloroplast. The plastidial methylerythritol phosphate, or MEP, pathway supplies the isoprenoid building blocks that are assembled into geranyl diphosphate, the immediate precursor of all monoterpenes. Transcriptome assemblies and comparative expression analyses have repeatedly flagged genes encoding 1-deoxy-D-xylulose-5-phosphate synthase (DXS), its reductoisomerase partner DXR, and geranyl diphosphate synthase (GPPS) as central players in precursor production. In the review’s evidence map, the MEP pathway was the single most heavily studied segment, accounting for 24 of the 95 included studies, with GPPS-related work adding another 16. But here lies the review’s central warning: sheer volume of publications did not translate into mechanistic certainty, because most of these investigations relied on gene expression patterns rather than direct measurements of enzyme activity or metabolic flux.

The best-resolved step in the entire pathway turns out to be the conversion of geranyl diphosphate into geraniol. Despite being represented by only 13 studies, geraniol synthase achieved one of the highest validation scores in the EMI analysis. Recent work combining recombinant enzyme assays, subcellular localisation and functional analyses revealed something unexpected: a dual-localised geraniol synthase operating in both plastids and cytosol, working alongside a previously uncharacterised cytosolic geranyl pyrophosphatase. This cooperation between cellular compartments refines the older picture of monoterpenes being built exclusively inside chloroplasts and demonstrates that citral formation depends on metabolic exchange across cellular boundaries. Comparable compartmental coordination has since been documented in camphor trees, suggesting the pattern may be widespread among aromatic plants.

The final oxidation step, in which geraniol is converted into the aldehydes geranial and neral, has historically been the murkiest part of the pathway. For years, alcohol dehydrogenases were implicated purely on the basis of transcript abundance and correlations with citral content. That changed when recent functional studies in lemongrass deployed recombinant enzyme assays, virus-induced gene silencing and transient overexpression to demonstrate a direct enzymatic contribution to citral accumulation. The review notes that these findings require an update to previous classifications that treated ADH-mediated oxidation as merely predictive. Even so, comparative enzyme kinetics among ADH isoforms, their substrate preferences and their relative contributions to geranial versus neral formation remain unexplored, leaving the terminal chemistry only partially resolved.

The weakest link, according to the EMI, is transcriptional regulation. Only nine studies examined regulatory factors, and nearly all relied on co-expression networks and candidate gene prediction. Transcriptomic analyses have associated terpene accumulation with transcription factors of the AP2/ERF and MYB families, but direct evidence of promoter binding or transcriptional activation in Cymbopogon is essentially absent. A compelling comparator exists: in the laurel family, a MYB44 transcription factor was experimentally shown to regulate alcohol dehydrogenase expression, providing a mechanistic framework that lemongrass researchers could replicate using chromatin immunoprecipitation, yeast one-hybrid assays and promoter-reporter systems. Until such experiments are performed in Cymbopogon itself, the regulatory hierarchy controlling citral production remains an inference rather than a demonstrated fact.

Beyond the pathway itself, the review highlights how environmental conditions, agronomic practices and post-harvest handling shape citral accumulation in ways that are still poorly connected to molecular mechanisms. Drought studies have revealed significant genotype-by-environment interactions for essential oil yield, and transcriptomic evidence suggests water stress alters expression of upstream isoprenoid genes, including GPPS-related components. Harvest interval, cutting frequency and developmental stage all modify oil yield and composition, likely through changes in glandular structure, precursor availability and enzymatic activity. Crucially, the authors urge a clear separation between biosynthetic regulation and post-harvest chemistry: drying conditions, storage duration and distillation procedures can shift the geranial-to-neral ratio through volatilisation, oxidation or chemical conversion, meaning that two samples of the same cultivar may differ chemically without any difference in genetic capacity.

The practical implications reach into breeding and biotechnology. Current cultivar improvement relies heavily on phenotypic evaluation, oil yield and chemical profiling, with molecular markers directly tied to citral biosynthesis still scarce. The review argues that genome-wide association studies and quantitative trait locus mapping across diverse Cymbopogon germplasm, combined with multi-environment phenotyping, could identify genomic regions controlling citral concentration, geranial-neral ratios and environmental stability. Emerging technologies could accelerate this transition: long-read sequencing promises better genome annotation and resolution of terpene gene families, single-cell transcriptomics could reveal which specialised cells actually manufacture citral, spatial metabolomics can map where intermediates accumulate within tissues, and artificial intelligence-assisted multi-omics integration may predict pathway behaviour under contrasting production environments.

The overarching lesson of the evidence map is that research volume and mechanistic resolution are not the same thing. Pathway segments drowning in transcriptomic datasets scored lower on the Evidence Maturity Index than segments supported by fewer but functionally decisive experiments, a pattern the authors say should redirect future effort toward enzyme kinetics, metabolic flux analysis, targeted gene perturbation using RNA interference or CRISPR/Cas editing, and integrated studies that simultaneously measure gene expression, protein activity, metabolite flux and final oil composition. As global demand for naturally derived bioactive compounds pushes lemongrass cultivation into new regions and production systems, the gap between knowing which genes are present and knowing which genes control the chemistry becomes the decisive bottleneck. Closing it, the review concludes, will determine whether citral-rich lemongrass cultivars of the future are selected by trial and error or engineered by design.

Subject of Research: Molecular pathways and validation status of citral biosynthesis in Cymbopogon species

Article Title: Evidence mapping of molecular pathways regulating citral biosynthesis and validation status in Cymbopogon spp.

Article References: Nassary, E. K., Magubika, A. J., Mhagama, E. A., Salimu, A. I., Kilasi, N. L., Tryphone, G. M., & Shitindi, M. J. (2026). Evidence mapping of molecular pathways regulating citral biosynthesis and validation status in Cymbopogon spp.. Discover Plants, 3(1), Article 378. https://doi.org/10.1007/s44372-026-00852-1

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00852-1

Keywords: citral, Cymbopogon, lemongrass, monoterpene biosynthesis, MEP pathway, geraniol synthase, alcohol dehydrogenase, evidence mapping, transcriptomics, metabolomics, essential oil, plant breeding