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