Lysine Iron Agar Reaction Guide
An original Learn Microbes bench visual for reading LIA tube patterns without relying on textbook photos.
LIA tubes A-E
Read the slant first, then the butt, then H2S blackening.
No glucose fermentation pattern; lysine readout is not supported without acid butt first.
Hydrogen sulfide blackening can mask the butt reaction.
Glucose fermented, lysine decarboxylase negative.
Red slant suggests lysine deamination.
Use the baseline color to judge true reaction shifts.
No glucose fermentation pattern; lysine readout is not supported without acid butt first.
Hydrogen sulfide blackening can mask the butt reaction.
Glucose fermented, lysine decarboxylase negative.
Red slant suggests lysine deamination.
Use the baseline color to judge true reaction shifts.
Color-to-result anchors
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Purple slant | Alkaline reaction | Usually written as K |
| Yellow butt | Acid from glucose fermentation | Usually written as A |
| Purple butt after early acid | Lysine decarboxylase positive | Supports LDC-positive interpretation |
| Red slant | Lysine deaminase activity | Think Proteeae-style deamination logic |
| Blackening | H2S production | May obscure the butt color |
LIA study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| K/A | Lysine decarboxylase negative | Glucose fermented, butt stays acid/yellow. |
| K/K | Lysine decarboxylase positive when glucose was fermented first | Butt returns alkaline/purple after decarboxylation. |
| K/K + H2S | H2S positive | Black precipitate can cover the butt color; report H2S separately. |
| R/A | Lysine deaminase positive | Red slant with acid butt; separates deaminating organisms. |
| Control | Baseline medium | Do not interpret as a patient isolate reaction. |
- Always separate slant, butt, and H2S.
- LIA is especially useful in Enterobacterales branch logic.
- Use LIA with TSI/KIA, indole, urease, motility, and organism context.
Original visuals should teach the concept, not recreate a textbook figure.