Martin Lewis Agar is a specialized microbiological culture medium widely used in clinical and research laboratories to isolate and identify pathogenic Neisseria species, such as Neisseria gonorrhoeae (causing gonorrhea) and Neisseria meningitidis (causing meningitis). This selective medium is critical for accurate diagnosis and effective management of these serious bacterial infections, making it an essential tool for microbiology students and professionals.
Historical Contextof Martin Lewis Agar
Martin Lewis Agar is an advanced version of Thayer-Martin Agar, developed to improve the isolation of Neisseria gonorrhoeae from clinical specimens with mixed microbial flora. By refining the antibiotic composition and concentrations, Martin Lewis Agar enhances selectivity and recovery rates, making it more effective for isolating both Neisseria gonorrhoeaeand Neisseria meningitidis. Understanding its development helps microbiology students appreciate the evolution of selective media in clinical diagnostics.
Principle of Martin Lewis Agar
Martin Lewis Agar operates as a selective medium designed to promote the growth of pathogenic Neisseria species while suppressing unwanted bacteria and fungi. It builds on the foundation of Thayer-Martin Agar with improved antibiotic formulations for better specificity.
How It Works
- Selective Inhibition: The medium contains antibiotics such as:
Vancomycin: Inhibits Gram-positive bacteria. Colistin: Suppresses Gram-negative bacteria, except Neisseria species. Anisomycin: Prevents fungal growth. Trimethoprim: Inhibits Proteus species.
- Nutrient-Rich Base: The medium uses chocolate agar enriched with hemoglobin to provide essential nutrients for Neisseria growth.
- Growth Supplements: IsoVitaleX (or equivalent VX supplement) supplies vital growth factors like glucose, L-cysteine, and NAD to support Neisseria species.
Composition of Martin Lewis Agar
Martin Lewis Agar is a modified Thayer-Martin Agar with increased vancomycin levels to enhance selectivity. Its base is chocolate agar, with a final pH of 7.2 ± 0.2 at 25°C. Below is the detailed composition:
| Components | Amount |
|---|---|
| Pancreatic digest of casein | 7.5 g |
| Hemoglobin (Hb) | 10.0 g |
| Selected meat peptone | 7.5 g |
| NaCl (sodium chloride) | 5 g |
| K 2 HPO 4 (Dipotassium phosphate) | 4 g |
| Corn starch | 1 g |
| KH 2 PO 4 (Monopotassium phosphate) | 1 g |
| Supplement solution | 10 ml |
| VCAT inhibitor | 10 ml |
| Agar | 12 g |
| Final pH 7.2 ± 0.22 at 25°C |
Composition per liter:
Source: Martin-Lewis agar is available as a prepared medium from BD Diagnostic Systems.
Composition of Supplement Solution per liter:
Source: The supplement solution IsoVitaleX® enrichment is available from BD Diagnostic Systems. This enrichment may be replaced by supplement VX from BD Diagnostic Systems.
Composition of VCAT inhibitor per 10.0mL
Preparation of Martin Lewis Agar
- Add all components (except supplement solution and VCAT inhibitor) to 980 mL of distilled/deionized water.
- Gently heat and stir until boiling.
- Autoclave at 121°C (15 psi) for 15 minutes.
- Cool to 45–50°C.
- Aseptically add sterile supplement solution and VCAT inhibitor, mixing thoroughly.
- Pour into sterile Petri dishes.
Storage and Shelf Life
Dehydrated Media: Store in a sealed container in the dark at temperatures below 30°C.
Prepared Media: Refrigerate at 2–8°C and use within 60 days.
Caution: Avoid using media showing signs of deterioration (e.g., shrinking, cracking, or discoloration) or contamination, as this may lead to inaccurate results.
Uses of Martin Lewis Agar
Martin Lewis Agar is primarily used for:
- Isolating Neisseria gonorrhoeae and Neisseria meningitidis from clinical specimens (e.g., urethral, cervical, or cerebrospinal fluid samples) with mixed microbial flora.
- Supporting the growth of penicillinase-producing Neisseria strains when enriched with Penicillin G.
- Facilitating accurate diagnosis of gonorrhea and meningitis in clinical settings.
Its selective antibiotics (vancomycin, colistin, anisomycin, and trimethoprim) effectively suppress Gram-positive bacteria, non-Neisseria Gram-negative bacteria, fungi, and Proteus species, ensuring reliable isolation of target pathogens.
Limitation of Martin Lewis Agar
- Nutritional Variability: Some Neisseria strains may exhibit poor growth due to nutritional differences.
- Antibiotic Sensitivity: Certain Neisseria gonorrhoeae strains may be inhibited by the antibiotics in the medium.
- Need for Non-Selective Media: A parallel non-selective medium (e.g., chocolate agar) is recommended to ensure comprehensive isolation.
- Confirmation Required: Additional biochemical or serological tests are necessary to confirm Neisseria species identity.
References
- Martin, J. E., Armstrong, J. H., & Smith, P. B. (1974). New system for cultivation of Neisseria gonorrhoeae. Applied microbiology, 27(4), 802–805. https://doi.org/10.1128/am.27.4.802-805.1974
- Granato, P. A., Paepke, J. L., & Weiner, L. B. (1980). Comparison of modified New York City medium with Martin-Lewis Medium for recovery of Neisseria gonorrhoeae from clinical specimens. Journal of clinical microbiology, 12(6), 748–752. https://doi.org/10.1128/jcm.12.6.748-752.1980
- Atlas, R.M., & Snyder, J.W. (2013). Handbook of Media for Clinical and Public Health Microbiology (1st ed.). CRC Press. https://doi.org/10.1201/b15973
- Thayer J. and Martin J.E. Jr., 1966, Public Health Rep., 81:559