Aromatogram: Essential Oil Sensitivity Testing

Overview

The aromatogram is a microbiological laboratory technique that evaluates in vitro the antimicrobial activity of essential oils against a pathogenic germ isolated from a patient. Its principle is directly transposed from the antibiogram, the fundamental diagnostic tool of medical bacteriology, which tests a bacterium's sensitivity to antibiotics. The aromatogram simply replaces antibiotic discs with discs impregnated with essential oils, thus determining which essential oils are most effective against the specific germ responsible for the patient's infection.

The concept was formalized in the 1970s by Dr. Marcel Girault, a French physician and researcher, then developed and popularized by Drs. Paul Belaiche and Jean Valnet. The aromatogram embodies the philosophy of personalized medicine: rather than prescribing broad-spectrum essential oils empirically, it precisely targets the most active essential oils against the specific germ responsible in a given patient. This individualized approach is particularly relevant in the current context of rising antibiotic resistance, as essential oils present multiple and complex mechanisms of action that make resistance development much less likely than with conventional antibiotics.

The aromatogram is primarily used for recurrent infections (repeated cystitis, chronic vaginosis, recurrent sinusitis, chronic serous otitis, chronic skin infections), infections resistant to conventional antibiotics, and situations where the patient desires a natural alternative to antibiotic therapy. It is performed by specialized medical biology laboratories, on prescription from a physician or health professional trained in clinical aromatherapy.

Core Principles

• Analogy with the antibiogram: the classic antibiogram uses the agar diffusion method (Kirby-Bauer): paper discs impregnated with antibiotics are placed on agar seeded with the patient's bacterium. After 18-24 hours of incubation at 37°C, the inhibition zone around each disc is measured. The larger the zone, the more sensitive the bacterium. The aromatogram reproduces exactly this principle, replacing antibiotic discs with essential oil-impregnated discs
• Antimicrobial mechanisms of essential oils: essential oils exert antimicrobial activity through multiple simultaneous mechanisms: bacterial cell membrane disruption by lipophilic molecule insertion into the lipid bilayer, ATP synthesis inhibition through oxidative phosphorylation uncoupling, interference with bacterial enzymatic systems, modification of bacterial quorum sensing, and anti-biofilm effect (ability to penetrate and destroy bacterial biofilms — a major property since biofilms are responsible for 65% of chronic infections and are 1000 times more resistant to antibiotics than planktonic bacteria)
• Aromatic MIC and MBC concepts: by analogy with antibiotic science, clinical aromatherapy uses Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC). The MBC/MIC ratio determines whether the essential oil is bacteriostatic (ratio greater than 4) or bactericidal (ratio less than or equal to 4)
• Aromatic synergy: the aromatogram tests each essential oil individually, but clinically they are often prescribed in synergistic combinations. Some laboratories offer synergistic aromatograms testing predefined blends. The FIC (Fractional Inhibitory Concentration) index quantifies synergy: FIC below 0.5 indicates synergy, 0.5-1 additive effect, above 1 antagonism
• In vitro specificity and limitations: like any in vitro test, in vitro antimicrobial activity does not always translate to equal in vivo efficacy. Bioavailability factors, patient immune status, infection location, and in vivo biofilm presence modulate actual therapeutic efficacy. The aromatogram should be interpreted as a valuable guide, not an absolute guarantee of clinical effectiveness

Technical Methodology

Pre-analytical phase: sampling and germ identification

Sampling follows standard microbiological protocols: ECBU for urinary infections, vaginal swab for vaginosis, throat swab for tonsillitis, skin swab for dermatological infections, stool culture for digestive infections. Germ identification uses classical bacteriology methods (Gram staining, selective media culture, API biochemical galleries) or modern methods (MALDI-TOF mass spectrometry, PCR).

Analytical phase: performing the aromatogram

The reference method is agar disc diffusion: Mueller-Hinton agar is poured in 90 mm Petri dishes. The agar surface is uniformly seeded with a calibrated suspension of the isolated germ (0.5 McFarland, approximately 10^8 CFU/ml). Sterile filter paper discs (6 mm, Whatman no. 1) are individually impregnated with 10-15 microliters of pure essential oil. Discs are placed 24 mm apart on the seeded agar, 5-7 per dish. Dishes are incubated at 37°C for 18-24 hours. After incubation, the diameter of each inhibition zone is measured in millimeters.

Result interpretation

Results are classified in three categories: Sensitive (S) — diameter 15 mm or greater: the germ is highly sensitive, first choice for prescription. Intermediate (I) — 8-14 mm: moderate sensitivity, usable at higher dose or in synergistic combination. Resistant (R) — below 8 mm: the germ is resistant, this oil will not be prescribed. Some laboratories use a finer 5-level classification with "very sensitive" (above 20 mm) and "very resistant" (below 5 mm).

Standard essential oil panel

A standard aromatogram tests 12-20 essential oils selected based on germ type and infection site. For urinary E. coli infections, the typical panel includes: compact oregano, mountain savory, Ceylon cinnamon bark, tea tree, thyme CT thymol, thyme CT linalool, palmarosa, rose geranium, niaouli, ravintsara, clove, tropical basil, and peppermint. For fungal infections (Candida), the panel emphasizes antifungals: tea tree, palmarosa, geranium, bay laurel, lemongrass, cinnamon, clove.

Main Indications

• Recurrent cystitis: the premier indication. Defined as at least 3 episodes yearly, affecting 20-30% of women after a first episode. The urinary aromatogram identifies essential oils specifically active against the patient's E. coli strain (responsible for 80% of cystitis). Post-aromatogram protocol includes curative phase (5-7 days of selected oral EOs), consolidation phase (2-3 weeks at reduced dose), and prevention phase (one week monthly for 3-6 months)
• Recurrent bacterial vaginosis and vaginal candidiasis: the vaginal aromatogram targets essential oils active against Gardnerella vaginalis or specific Candida strains. Personalized vaginal ovules are prepared in pharmacy with the 2-3 most active essential oils at 2-5%
• Chronic and recurrent ENT infections: chronic sinusitis, serous otitis, recurrent tonsillitis. Nasal or pharyngeal sampling identifies the responsible germ. The aromatogram is particularly valuable in chronic Staphylococcus aureus sinusitis, where biofilm formation creates antibiotic resistance but essential oils may penetrate biofilms
• Chronic skin infections: severe acne, recurrent folliculitis, chronic impetigo, infected chronic wounds. Cutaneous aromatogram identifies active essential oils for personalized topical formulations
• Antibiotic-resistant infections: the aromatogram's growing importance amid rising antibiotic resistance. In vitro studies demonstrate certain essential oil activity against multiresistant strains (MRSA, ESBL-producing Enterobacteriaceae, multiresistant Pseudomonas aeruginosa)

Patient Journey

1. Initial consultation and sampling prescription (30 min): the practitioner evaluates the clinical situation, collects complete infectious history, current treatments, essential oil contraindications, and overall immune status. They prescribe the microbiological sampling with "aromatogram" notation alongside the standard antibiogram
2. Sample collection: the patient visits a medical biology laboratory. Some specialized labs accept postal shipment of samples in appropriate transport media
3. Result reception and interpretation (7-14 days post-sampling): the report includes germ identification, colony count, standard antibiogram results, and aromatogram results with inhibition zone diameters and S/I/R classification for each tested essential oil. The practitioner interprets results considering clinical context
4. Personalized prescription (20 min): the practitioner develops a personalized aromatic formula selecting 2-4 essential oils among the most active (classified S), favoring those with the best tolerance profile. The formula is written as a compounding preparation
5. Pharmacy preparation: the pharmacist prepares the formulation with pharmaceutical-quality essential oils in prescribed galenic forms
6. Therapeutic follow-up: a control sample verifies germ eradication after treatment. For recurrent cystitis, a semi-annual control aromatogram monitors strain sensitivity evolution

Technical Variants and Evolutions

Broth micro-dilution aromatogram: alternative to agar diffusion, precisely determining MIC and MBC. 96-well microplates with decreasing essential oil concentrations. More quantitative and reproducible but costlier.

Biofilm aromatogram: major innovation testing activity on in vitro-formed biofilms rather than planktonic bacteria. Much more representative of clinical reality, as most chronic infections involve biofilm bacteria. Results show certain essential oils (oregano, cinnamon, tea tree) retain significant biofilm activity while many antibiotics lose efficacy.

Vapor-phase aromatogram: tests antimicrobial activity of essential oil vapors rather than direct contact. An impregnated disc is placed on the inverted Petri dish lid without agar contact. Particularly relevant for respiratory infections where essential oils are administered by inhalation.

Combined aromatogram (synergy): tests 2-3 essential oil combinations using the checkerboard method to determine FIC index and identify synergistic, additive, or antagonistic combinations.

Myco-aromatogram: specialized variant for fungal infections. Methodology adapted to fungal culture requirements: Sabouraud medium, incubation at 30°C for 48-72 hours for Candida and 7-14 days for dermatophytes.

Limitations and Precautions

• In vitro/in vivo correlation: the main limitation. A highly active in vitro essential oil may prove less effective in vivo due to insufficient bioavailability at the infection site, in vivo biofilm presence, patient immune status, and commensal flora interactions. Conversely, modest in vitro activity may translate to clinical efficacy through indirect mechanisms (immunostimulation, anti-biofilm effect, quorum sensing modulation)
• Methodological standardization: unlike the antibiogram with strict international standards (EUCAST, CLSI), the aromatogram lacks a fully standardized international protocol. Methodological parameters vary between laboratories, making results difficult to compare. Standardization efforts are underway through AFNOR and ISO
• Essential oil volatility: the most volatile molecules partially evaporate during 18-24 hour incubation, reducing effective concentration and potentially underestimating activity. Some laboratories use hermetically sealed Petri dishes
• Cost and accessibility: not reimbursed by French Social Security (nor most complementary insurance). Cost: 50-150 euros depending on oils tested and laboratory. Only a few laboratories offer this test routinely, limiting use to justified recurrent or resistant infections
• Result delay: 7-14 days between sampling and results, unsuitable for acute infections. Practitioners prescribe empirical broad-spectrum aromatic treatment (oregano + tea tree + ravintsara) while awaiting results, then adjust based on the aromatogram
• Interpretation by trained professional: results require interpretation by a professional with dual microbiology and clinical aromatherapy competence. A "Sensitive" oil must not be prescribed automatically if contraindicated for the patient. The professional must cross-reference aromatogram data with patient profile, contraindications, and drug interactions
• Commensal flora precautions: targeted aromatic treatment remains active on beneficial commensal flora. Commensal flora restoration (adapted probiotics, prebiotics) is an essential complement to any aromatic anti-infective treatment