Motility Testing
A bench visual for reading semisolid motility medium deeps containing TTC indicator, used to determine whether bacterial species are motile.
Motility tubes A-B
Diffuse red growth radiating from the stab line indicates motility. Growth restricted to the stab line indicates non-motility.
Diffuse, cloudy red growth spreading outward from the central stab line throughout the medium.
Sharp, well-defined red growth restricted strictly along the path of the stab inoculation, with surrounding medium remaining clear.
Diffuse, cloudy red growth spreading outward from the central stab line throughout the medium.
Sharp, well-defined red growth restricted strictly along the path of the stab inoculation, with surrounding medium remaining clear.
Visual cues & outcomes
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Diffuse red cloudiness throughout | Organism has flagella and migrated away from the inoculation path, reducing TTC to red formazan along the way. | Positive — Motile (e.g., E. coli) |
| Sharp red growth line only | Organism lacks motility; growth and TTC reduction occur strictly where the needle penetrated the agar. | Negative — Non-motile (e.g., S. aureus) |
| No red color but cloudy | TTC indicator might be absent or not reduced. Check growth or check under microscope if in doubt. | Inconclusive — repeat test |
Clinical and taxonomic significance
| Pattern | What to call it | How to think about it |
|---|---|---|
| E. coli / Enterobacterales | Usually motile (except Shigella and Klebsiella) | Helps differentiate Klebsiella (non-motile) from Enterobacter (motile). |
| Bacillus species | B. anthracis (non-motile) vs. B. cereus (motile) | Crucial biodefense and diagnostic distinction. |
| Listeria monocytogenes | Umbrella-like motility at 25°C, non-motile at 35°C | Temperature-dependent motility is highly diagnostic. |
- Motility testing requires a semisolid medium (usually 0.4% agar) to allow flagellated cells to swim.
- TTC (triphenyltetrazolium chloride) is often added as an electron acceptor; reduced TTC forms insoluble red formazan, highlighting growth.
- Non-motile bacteria only grow along the stab line; motile bacteria render the entire tube cloudy.
Best mental model: If the red color spreads out like a cloud, the bacteria can swim; if it is a sharp pencil line, they are stuck in place.