Lysine iron Agar (LIA), a differential medium, is used to test organisms’ ability to deaminate or decarboxylate Lysine. Lysine deamination, an aerobic process, occurs in the media. Lysine decarboxylation, an anaerobic process occurring in the media’s butt, is also known as Lysine decarboxylation. Edwards and Fife created LIA in 1961 in order to presumptively determine Salmonella species. This includes Salmonella arizonae that is lactose fermenting, which has been linked to foodborne gastroenteritis outbreaks.
Principle of Lysine Iron Agar (LIA)
Lysine iron Agar is made up of peptones and lysine. It also contains ferric ammonium citrate, sodium thiosulfate, and a small amount lysine. The medium has an anaerobic butt and an aerobic slant. The medium is stabbed at the bottom of the butt, and streaked with slant. The medium turns yellow when glucose ferments.
Cadaverine is produced when an organism produces lysine-decarboxylase. Cadaverine neutralizes organic acids created by glucose fermentation. The medium’s butt reverts back to an alkaline state (purple). The butt will remain acidic (yellow) if the decarboxylase has not been produced.
When oxidative deamination occurs of lysine, a compound forms that, when ferric ammonium citrate is present and a coenzyme flavin mononucleotide is present, forms a burgundy-colored color on the slant. The LIA slant will remain purple if deamination is not observed. Bromocresol purple is the pH indicator. It is yellow below pH 5.2, and purple above pH 6.8.
Essential nutrients are provided by yeast extract and peptone. Dextrose is an excellent source of fermentable carbohydrate. H2S formation is indicated by sodium thiosulate and ferric ammonium citrate. Blackening occurs when hydrogen sulfide is produced in cultures.
Composition of Lysine Iron Agar (LIA)
| Ingredients | Gms/liter |
| Peptone | 5.000 |
| Yeast extract | 3.000 |
| Dextrose (Glucose) | 1.000 |
| L-Lysine | 10.00 |
| Ferric ammonium citrate | 0.500 |
| Sodium thiosulphate | 0.040 |
| Bromocresol purple | 0.020 |
| Agar | 15.000 |
Final pH (at 25°C): 6.7±0.2
Preparation and Method of Use of Lysine Iron Agar (LIA)
- Take 34.56 grams and mix it with 1000 ml of distilled water.
- To dissolve the medium completely, heat to boiling
- Place in tubes. For 15 minutes, sterilize with autoclave at 15 lbs pressure (121degC).
- To form slants with deep pits, cool the tubes in a slanted position.
- Use a straight inoculating needle to inoculate LIA. Do this by twice stabbing the middle of the medium into the tube, then streaking the slant.
- Cover the tube tightly, and let it incubate in ambient air at 35-37°C for 18-24 hours.
- For growth and color changes in tube and slant, as well as blackening at the top of slant, examine at 18-24 and 24 hours.
Result Interpretation on Lysine Iron Agar (LIA)
| Organisms | Growth |
| Citrobacter freundii | Luxuriant growth; butt: acidic reaction, yellowing of the medium; slant: alkaline reaction, purple or no color change; positive reaction for H2S, blackening of medium |
| Escherichia coli | Luxuriant growth; butt: alkaline reaction, purple or no color change; slant: alkaline reaction, purple or no color change; negative reaction for H2S |
| Proteus mirabilis | Luxuriant growth; butt: acidic reaction, yellowing of the medium; slant: deep red, lysine deamination; positive reaction for H2S, blackening of medium |
| Salmonella Arizonae | Luxuriant growth; butt: alkaline reaction, purple or no color change; slant: alkaline reaction, purple or no color change; positive reaction for H2S, blackening of medium |
| Salmonella Enteritidis | Luxuriant growth; butt: alkaline reaction, purple or no color change; slant: alkaline reaction, purple or no color change; positive reaction for H2S, blackening of medium |
| Salmonella Typhimurium | Luxuriant growth; butt: alkaline reaction, purple or no color change; slant: alkaline reaction, purple or no color change; positive reaction for H2S, blackening of medium |
| Shigella flexneri | Luxuriant growth; butt: acidic reaction, yellowing of the medium; slant: alkaline reaction, purple or no color change; negative reaction for H2S |
Uses of Lysine Iron Agar (LIA)
- It is recommended to differentiate enteric organisms on the basis of their ability to decarboxylate, deaminately lysine or to form hydrogen sulfuride (H2S).
- This medium can be used to detect lactose fermentation and non-fermenting Salmonella.
- This agar can be used to distinguish different Gram-negative bacteriailli, especially among the Enterobacteriaceae.
- Proteus species and Providencia species can be presumed to be identified using this medium because they produce a red-slant and an acid butt.
Limitations of Lysine Iron Agar (LIA)
- For complete identification, it is important to perform biochemical, immunological and molecular testing on colonies grown in pure culture.
- It is crucial to make sure that the medium’s butt is stabbed. This test is invalidated if you fail to stab your butt.
- LIA is less sensitive than other iron-containing mediums like Sulfide Indole Motility Medium (SIM) Medium or TSIA in detecting hydrogen sulfuride.
- Proteus sp. That produces hydrogen sulfide won’t blacken the medium
- Some species and strains might have delayed reactions or fail to ferment the carbohydrate as required.
- Citrobacter species may cause gas production to be irregular, or even suppressed.
