Lysine Iron Agar (LIA)- Composition, Principle, Preparation, Results, Uses

Lysine iron agar (LIA) is a type of culture medium that is used to differentiate various gram-negative bacteria based on their ability to perform certain biochemical reactions involving the amino acid lysine. LIA can help identify members of the Enterobacteriaceae family and other gram-negative bacilli.

LIA contains the following ingredients:

• Lysine: an amino acid that can be decarboxylated or deaminated by some bacteria.
• Peptones: a mixture of proteins and peptides that provide nitrogen and carbon sources for bacterial growth.
• Glucose: a fermentable carbohydrate that can lower the pH of the medium when metabolized by bacteria.
• Ferric ammonium citrate: a source of iron that reacts with hydrogen sulfide (H2S) to form a black precipitate.
• Sodium thiosulfate: a source of sulfur that can be reduced to H2S by some bacteria.
• Bromocresol purple: a pH indicator that changes color from yellow (acidic) to purple (alkaline).
• Agar: a solidifying agent that forms a slant and a butt in the tube.

LIA differentiates enteric bacteria based on their ability to decarboxylate or deaminate lysine and to form hydrogen sulfide. Lysine decarboxylation occurs in the anaerobic butt of the medium, while lysine deamination occurs on the aerobic slant. The pH indicator and other indicators show the results of these reactions.

To prepare LIA:

1. Suspend 34.56 grams of the dehydrated medium in 1000 ml of distilled water.
2. Heat to boiling with agitation to completely dissolve the medium.
3. Dispense into tubes and sterilize by autoclaving at 121°C for 15 minutes.
4. Cool the tubes in a slanted position to form slants with deep butts.

To use LIA:

1. Inoculate LIA by twice stabbing through the center of the medium to the bottom of the tube and then streaking the slant.
2. Cap the tube tightly and incubate at 35°-37°C in ambient air for 18 to 24 hours.
3. Examine at 18 – 24 and 40 – 48 hours for growth and color changes in tube butt and slant, and for blackening at the apex of slant.

The results of LIA are interpreted by observing the color changes and the presence or absence of blackening in the slant and the butt of the medium. The color changes are due to the pH indicator, while the blackening is due to the production of hydrogen sulfide (H2S) by some organisms.

LIA is used to identify and distinguish various Gram-negative bacilli, especially among the Enterobacteriaceae family. It can differentiate organisms based on their ability to decarboxylate or deaminate lysine and to form hydrogen sulfide. LIA is particularly useful for distinguishing different Gram-negative bacilli and for the detection of lactose fermenting and non-fermenting Salmonella.

LIA is not a definitive test for the identification of enteric organisms and requires careful inoculation and interpretation. It may not differentiate some closely related species or strains of enteric organisms and can give false-positive or false-negative results due to environmental factors or inoculum size. Other iron-containing mediums may be more sensitive in detecting hydrogen sulfide production.

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