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Short answer
Lysine iron agar (LIA) is a solid slant medium that tells apart bacteria by three readouts in one tube: lysine decarboxylation (purple butt), lysine deamination (red slant) and hydrogen sulfide production (black precipitate). Read the butt and the slant separately, after 18-24 hours of incubation, and record the three results together. It is a laboratory-use medium and not a diagnostic by itself.
This page is general laboratory reference information, not medical advice and not a procedure for any individual patient.
Definition and scope
The Remel instructions for use (IFU 453772) describe LIA as a “solid medium recommended for use in qualitative procedures for differentiation of microorganisms based on the production of lysine decarboxylase and hydrogen sulfide.” The medium was developed in 1961 to detect lactose-positive Arizona strains implicated in outbreaks of food-borne disease. In 1966 Johnson and colleagues recommended it as an aid in identifying Salmonella, and Ewing later recommended using it with triple sugar iron (TSI) agar for enteric pathogens in routine stool examination.
HiMedia’s technical data sheet M377 gives the same purpose, differentiating enteric organisms, especially Salmonella Arizonae, by lysine decarboxylation and deamination and by H₂S formation, and states that the composition is in accordance with FDA Bacteriological Analytical Manual (BAM) and APHA methods for biochemical testing of Salmonella from food samples.
How the medium works
Each ingredient has one job, per the Remel IFU:
- Gelatin peptone and yeast extract supply nutrients.
- Dextrose is the fermentable carbohydrate. Fermentation makes acid, which turns the indicator yellow.
- Bromcresol purple is the pH indicator: purple when alkaline, yellow when acidic.
- L-lysine is the substrate for both lysine decarboxylase and lysine deaminase.
- Sodium thiosulfate and ferric ammonium citrate form a black precipitate in the butt when H₂S is produced.
Organisms that decarboxylate lysine (the IFU names Salmonella) produce cadaverine, which keeps the butt purple (alkaline). Organisms that deaminate lysine (the IFU names Proteus) produce an alpha-ketocarboxylic acid and the slant turns red. HiMedia adds that the alpha-ketocarboxylic acid reacts with the iron salt near the surface, under oxygen, to give a reddish-brown compound.
How to read slant and butt reactions
Interpretation from the Remel IFU:
| Reaction | Positive | Negative |
|---|---|---|
| Lysine decarboxylation (butt) | Purple slant and purple butt | Purple slant and yellow butt (acid from glucose fermentation only) |
| Lysine deamination (slant) | Red slant | Slant stays purple |
| H₂S production | Black precipitate | No blackening |
The IFU adds that the butt reaction may be masked by H₂S production, so a black butt can hide the purple or yellow underneath.
Quality-control organisms and expected results
The IFU lists these control strains (incubation under ambient conditions for 18-24 hours at 33-37 °C). They double as a lot-acceptance checklist for a new batch of tubes:
| Control strain | Slant / butt | H₂S |
|---|---|---|
| Escherichia coli ATCC 25922 | Purple / purple | Negative |
| Proteus mirabilis ATCC 12453 | Red / yellow | Negative |
| Salmonella Typhimurium ATCC 14028 | Purple / purple | Positive |
HiMedia adds that the medium should be validated for any organism that is not on its certificate of analysis.
LIA, TSI and decarboxylase media
LIA is meant to be run alongside other media, not instead of them. The Remel IFU states that “LIA is not a substitute for TSI or Moeller Decarboxylase media.” HiMedia explains one reason to pair it with TSI: many lactose-fermenting Salmonella ferment lactose rapidly, which suppresses H₂S on TSI, so they can be overlooked; using LIA and TSI together gives greater discrimination between coliforms such as E. coli and Shigella. For how to read the TSI side of the pair, see triple sugar iron agar results. A selective plate such as XLD agar is a different tool again, and a sugar-and-indicator tube such as phenol red broth answers a different question (carbohydrate fermentation) from LIA’s lysine reactions.
One formula, two manufacturers
The “classic” LIA formula is not identical between suppliers. Per litre, from the two manufacturer data sheets:
| Component | Remel (IFU 453772) | HiMedia (M377) |
|---|---|---|
| L-lysine | 10.0 g | 10.0 g |
| Peptone | 5.0 g (gelatin peptone) | 5.0 g (peptone) |
| Yeast extract | 3.0 g | 3.0 g |
| Dextrose | 1.0 g | 1.0 g |
| Ferric ammonium citrate | 0.5 g | 0.5 g |
| Sodium thiosulfate | 0.04 g | 0.04 g |
| Bromcresol purple | 0.02 g | 0.02 g |
| Agar | 13.5 g | 15.0 g |
| Prepared medium | 33 g/L | 34.56 g/L |
| Final pH | 6.7 ± 0.2 | 6.7 ± 0.2 |
The active ingredients match; the agar content and the weight of dehydrated medium per litre differ. Follow the instructions for use that come with the lot in hand rather than a formula copied from a textbook or another supplier.
Where it matters for labs and supply decisions
- Intended use is labelled. The Remel IFU states “For Laboratory Use only. It is not intended for use in the diagnosis of disease or other conditions.”
- Regulatory class. Differential culture media are classified under 21 CFR 866.2320: biological materials “intended for medical purposes to cultivate and identify different types of pathogenic microorganisms” by a visible biochemical change such as colour. The regulation sets them as Class I (general controls), exempt from premarket notification subject to the limitations in 866.9.
- Lot checks. Because manufacturers differ in formula, a lab’s QC record should name the supplier, the lot and the control strains in the table above.
- Appearance before use. The IFU notes that a slight precipitate on the slant before inoculation does not affect performance.
Common misunderstandings
A yellow butt is not a failed tube
A purple slant over a yellow butt is the expected pattern for an organism that ferments glucose but does not decarboxylate lysine, such as the IFU control strain Proteus mirabilis (red slant, yellow butt). The yellow butt reports acid from glucose only.
Black does not always show
The IFU, citing MacFaddin (1985), cautions that H₂S may not appear with non-lysine-decarboxylating organisms such as Proteus because acid in the butt may suppress it. Conversely, heavy H₂S can mask the butt colour.
Not every Salmonella reads the same
The IFU notes that Salmonella Paratyphi A does not produce lysine decarboxylase, so its butt will not follow the purple pattern of S. Typhimurium.
Other known variability
- Morganella morganii may give a variable slant at 24 hours.
- Gas production is irregular or suppressed, except in Citrobacter.
FAQ
What does a purple butt mean on lysine iron agar?
A purple (alkaline) butt means the organism decarboxylated lysine, producing cadaverine. A yellow butt means no lysine decarboxylation and only glucose fermentation (Remel IFU 453772). If the butt is black, H₂S may be hiding the underlying colour.
Is LIA a substitute for TSI?
No. The Remel IFU states LIA is not a substitute for TSI or Moeller decarboxylase media. HiMedia describes using LIA and TSI together for better discrimination, for example when lactose-fermenting Salmonella suppress H₂S on TSI.
How does LIA relate to decarboxylase broth?
They are different media. Remel names Moeller decarboxylase medium as a separate medium that LIA does not replace. Preparation and reading of decarboxylase broth follow its own supplier instructions.
What is the red slant on LIA?
A red slant indicates lysine deamination, as seen with Proteus; the alpha-ketocarboxylic acid formed is the basis of the colour change.
Sources
- Thermo Fisher Scientific (Remel), Lysine Iron Agar instructions for use, IFU 453772, rev. 1 Dec 2011. documents.thermofisher.com. Checked 2026-10-31.
- HiMedia Laboratories, Lysine Iron Agar technical data M377. himedialabs.com. Checked 2026-10-31.
- U.S. Code of Federal Regulations, 21 CFR 866.2320, Differential culture medium. ecfr.gov. Checked 2026-10-31.
Related reading: urease test results and MacConkey agar.
