Lysine iron agar (LIA) is a selective and differential solid medium employed in clinical and food microbiology for the presumptive identification of members of the Enterobacteriaceae family and other Gram‑negative rods. The medium is formulated to detect three principal biochemical activities: lysine decarboxylation, carbohydrate fermentation (typically glucose), and hydrogen sulfide (H₂S) production. Results are interpreted by observing color changes in the agar slant and butt, as well as the presence of black precipitates.
Composition
Typical formulations of LIA contain the following constituents (per liter of distilled water):
| Component | Approximate concentration |
|---|---|
| Peptone (protein source) | 10 g |
| Lysine (amino acid substrate) | 5 g |
| Glucose (fermentable carbohydrate) | 1 g |
| Ferric ammonium citrate (iron source) | 1 g |
| Sodium thiosulfate (H₂S substrate) | 2 g |
| Agar | 15 g |
| Sodium chloride | 5 g |
| Dipotassium phosphate (buffer) | 5 g |
| pH adjusted to 6.8 ± 0.2 (final) | — |
The iron and thiosulfate components enable detection of H₂S, which reacts with ferric ions to form black ferrous sulfide precipitates.
Principles of operation
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Lysine decarboxylation – Lysine decarboxylase hydrolyzes lysine to cadaverine, an alkaline product. An alkaline reaction raises the pH, turning the medium purple (alkaline) due to the phenol red indicator. Decarboxylation is most evident in the butt of the tube, where the organism’s metabolic products accumulate.
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Glucose fermentation – Fermentation of glucose produces acid, lowering the pH and turning the slant yellow (acidic). The butt may subsequently become alkaline if the organism also decarboxylates lysine after the initial acidification.
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Hydrogen sulfide production – Certain organisms reduce thiosulfate to H₂S, which reacts with ferric ions to generate black ferrous sulfide precipitates, visible as blackening of the medium, usually in the butt region.
Interpretation of results
| Reaction pattern | Typical result | Interpretation |
|---|---|---|
| Slant yellow, butt red (alkaline) | – | Glucose fermented (acid slant), lysine decarboxylated (alkaline butt) – e.g., Salmonella spp. |
| Slant yellow, butt yellow | – | Glucose fermented, lysine not decarboxylated – e.g., Escherichia coli |
| Slant red (alkaline), butt red, black precipitate | – | No glucose fermentation, lysine not decarboxylated, H₂S produced – e.g., Proteus spp. |
| Slant red, butt red, no black | – | No glucose fermentation, no lysine decarboxylation, no H₂S – e.g., Shigella spp. |
Procedure
- A sterile inoculating needle or loop is used to streak the surface of the medium (slant) and then the butt.
- Tubes are incubated aerobically at 35–37 °C for 24 hours; an additional incubation up to 48 hours may be required for slow‑reacting organisms.
- After incubation, the color of the slant and butt, as well as any blackening, are recorded according to the criteria above.
Applications
- Clinical diagnostics: Differentiation of enteric pathogens (e.g., Salmonella, Shigella, Escherichia coli, Proteus).
- Food safety testing: Screening of food samples for potential contamination by pathogenic Gram‑negative rods.
- Research: Phenotypic profiling of bacterial isolates in taxonomic and epidemiologic studies.
Limitations
- Some organisms may exhibit atypical or weak reactions, leading to ambiguous results.
- The medium does not differentiate between all Enterobacteriaceae; supplementary biochemical tests are often required for definitive identification.
- H₂S production may be inhibited by high oxygen levels; proper aerobic incubation is essential.
Historical note
Lysine iron agar was introduced in the mid‑20th century as an improvement over earlier media such as triple sugar iron (TSI) agar, providing a more specific assessment of lysine decarboxylase activity while retaining the ability to detect H₂S. Its development contributed to streamlined workflows in clinical microbiology laboratories.