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Martin agar refers to the family of enriched, selective culture media developed by John E. Martin Jr. and James D. Thayer—most notably Thayer-Martin and Martin-Lewis agar—specifically engineered to isolate pathogenic Neisseria species from mixed-flora specimens. Targeted antimicrobial inhibitors in an enriched chocolate agar base suppress commensal bacteria and fungi while recovering fastidious pathogens.
Definition and scope
Martin agar denotes the class of selective microbiological media designed to recover fastidious Neisseria species—chiefly Neisseria gonorrhoeae and Neisseria meningitidis—from anatomical sites colonized by dense normal microflora. In diagnostic practice, swabs from the endocervix, urethra, rectum, or oropharynx contain abundant Gram-positive bacteria, enteric bacilli, saprophytic Neisseria, and yeasts. Without selective suppression, these commensals rapidly overgrow slow-growing gonococci.
The medium is named for microbiologists James D. Thayer and John E. Martin Jr. of the U.S. Public Health Service, who introduced selective chocolate agar formulations in 1964 and 1966. Subsequent refinements produced Modified Thayer-Martin (MTM / MTM II) agar and Martin-Lewis (ML) agar. These media remain international reference standards for gonococcal isolation, surveillance, and antimicrobial resistance testing.
For solid media preparation fundamentals, see our reference guide on what is an agar plate.
How it works or how it is used in practice
Martin agar combines an enriched basal medium with targeted antimicrobial inhibitors. The basal matrix consists of GC agar base supplemented with 1% hemoglobin (providing factor X / hemin) and a defined chemical enrichment such as IsoVitaleX (providing factor V / NAD, vitamins, and dextrose). This replicates chocolate agar, supplying essential nutrients for fastidious pathogens.
Selectivity is achieved through specific antimicrobial additives (historically VCN or VCNT):
- Vancomycin (3.0 mg/L in MTM; 4.0 mg/L in Martin-Lewis): Inhibits Gram-positive cocci and bacilli by blocking cell-wall peptidoglycan synthesis.
- Colistin (7.5 mg/L as colistimethate sodium): Destabilizes the outer membrane of Gram-negative commensal rods while sparing pathogenic Neisseria.
- Nystatin (12,500 units/L in MTM) or Anisomycin (20.0 mg/L in Martin-Lewis): Inhibits yeasts and molds, particularly mucosal Candida albicans.
- Trimethoprim (5.0 mg/L as trimethoprim lactate): Blocks dihydrofolate reductase, preventing swarming of Proteus species across the agar surface.
Inoculated plates must be incubated immediately at 35°C ± 2°C in a humidified atmosphere with 3% to 5% CO₂. Because pathogenic Neisseria are capnophilic and sensitive to chilling and drying, delays in incubation markedly reduce recovery rates. Technicians examine plates at 18 to 24 hours and re-incubate negative cultures through 48 hours. Colonies of Neisseria gonorrhoeae appear small (1–2 mm), translucent to grayish-white, raised, and smooth. Presumptive colonies are confirmed via Gram stain, oxidase testing, and definitive MALDI-TOF or biochemical assays.
For supportive non-selective media, review our guide on what is a TSA plate, and operational parameters for anaerobic bacteria culture.
Types or related concepts
Several formulations in the Martin lineage balance microbial suppression against the recovery of delicate gonococcal strains. The table below outlines these variations based on manufacturer technical data and peer-reviewed literature.
| Medium Type | Basal Matrix | Selective Agents | Target Microorganisms | Suppressed Organisms |
|---|---|---|---|---|
| Chocolate Agar | GC base + Hemoglobin + IsoVitaleX | None (non-selective) | Fastidious pathogens (Neisseria, Haemophilus) | None (supports both flora and pathogens) |
| Thayer-Martin (1966) | GC base + Hemoglobin + IsoVitaleX | Vancomycin, Colistin, Nystatin (VCN) | N. gonorrhoeae, N. meningitidis | Gram-positives, Gram-negative rods, yeasts |
| Modified Thayer-Martin (MTM II) | GC base + Hemoglobin + IsoVitaleX | Vancomycin, Colistin, Nystatin, Trimethoprim (VCNT) | N. gonorrhoeae, N. meningitidis | Gram-positives, Gram-negative rods, yeasts, swarming Proteus |
| Martin-Lewis (ML) Agar | GC base + Hemoglobin + IsoVitaleX | Vancomycin (4 mg/L), Colistin, Anisomycin, Trimethoprim (VCAT) | N. gonorrhoeae, N. meningitidis | Gram-positives, Gram-negative rods, Candida, swarming Proteus |
| New York City (NYC) Medium | Corn starch agar + Lysed horse blood + Plasma | Vancomycin (2 mg/L), Colistin, Amphotericin B, Trimethoprim | N. gonorrhoeae, N. meningitidis, genital mycoplasmas | Gram-positives, Gram-negative rods, yeasts, swarming Proteus |
Where it matters for labs, clinics or supply decisions
Media selection and handling directly affect diagnostic sensitivity and surveillance workflows:
- Antimicrobial Resistance (AMR) Surveillance: While nucleic acid amplification tests (NAATs) are standard for screening, they do not produce viable isolates. Martin agar culture remains essential for phenotypic antimicrobial susceptibility testing (AST) to track resistant strains in programs like CDC’s Gonococcal Isolate Surveillance Project (GISP).
- Specimen Site Protocols: Swabs from non-sterile mucosal sites require selective media (MTM or Martin-Lewis). Conversely, specimens from sterile sites (such as synovial fluid or CSF) must be inoculated onto non-selective chocolate agar in parallel, preventing false negatives in vancomycin-susceptible isolates.
- Cold Shock and Storage: Media must be stored at 2°C to 8°C inverted and shielded from light. Plates must warm to room temperature (20°C–25°C) before inoculation, as cold media causes thermal shock and reduces recovery.
- Quality Control and Shelf Life: Because enrichment supplements and antibiotics degrade over time, laboratory quality management requires discarding plates showing dehydration or past expiration dates.
For microscopic identification protocols used alongside culture, see our overview of acid-fast staining.
Common misunderstandings
Several common misconceptions lead to diagnostic and handling errors:
- Assuming all gonococci grow on selective agar: Between 3% and 10% of N. gonorrhoeae strains are inhibited by vancomycin in selective media. When testing sterile body fluids, parallel plating on non-selective chocolate agar is necessary.
- Confusing chocolate agar with Thayer-Martin agar: While both appear brown from lysed blood, non-selective chocolate agar lacks antibiotics and will be rapidly overgrown if inoculated with non-sterile clinical swabs.
- Omitting capnophilic incubation: Pathogenic Neisseria require 3% to 5% CO₂ for reliable growth; incubation in ambient air frequently yields false negatives.
- Attempting culture on MacConkey agar: MacConkey agar contains bile salts and crystal violet that prevent Neisseria growth, and lacks factor X and factor V.
Frequently asked questions
Can Neisseria grow on MacConkey agar?
No, pathogenic Neisseria cannot grow on MacConkey agar. MacConkey agar contains bile salts and crystal violet that inhibit fastidious organisms, selecting exclusively for hardy enteric Gram-negative bacilli. Additionally, it lacks the required hemin and NAD growth factors.
What does Thayer-Martin agar grow?
Thayer-Martin agar selectively grows pathogenic Neisseria species, specifically Neisseria gonorrhoeae and Neisseria meningitidis, while suppressing contaminating bacteria and fungi from clinical specimens.
What is a chocolate agar?
Chocolate agar is an enriched, non-selective growth medium made by adding blood to molten nutrient agar at high temperatures (56°C–80°C). This lyses erythrocytes, releasing hemin (factor X) and NAD (factor V) and giving the agar its characteristic brown color.
What is the difference between chocolate agar and Thayer-Martin?
Chocolate agar is non-selective and supports both fastidious pathogens and ordinary commensal flora. Thayer-Martin agar uses chocolate agar as its base but adds selective antibiotics (vancomycin, colistin, nystatin, and trimethoprim) to inhibit contaminants while isolating Neisseria.
Sources
- Thayer JD, Martin JE Jr. A selective medium for the cultivation of N. gonorrhoeae and N. meningitidis. Public Health Reports. 1964;79(1):49–57. PMC1915461
- Thayer JD, Martin JE Jr. Improved medium selective for cultivation of N. gonorrhoeae and N. meningitidis. Public Health Reports. 1966;81(6):559–562. PMC1919807
- Centers for Disease Control and Prevention. Recommendations for the Laboratory-Based Detection of Chlamydia trachomatis and Neisseria gonorrhoeae — 2014. MMWR Recommendations and Reports. 2014;63(RR-02):1–19. CDC MMWR RR-02
- Becton, Dickinson and Company. BD BBL™ Modified Thayer-Martin (MTM II) Agar Instructions for Use. BD Diagnostic Systems Technical Data Sheet. BD BBL IFU
- Hardy Diagnostics. Thayer Martin, Modified: Instructions for Use (Cat. no. E30 / G45). Hardy Diagnostics Technical Documents. Hardy Diagnostics IFU
