Flagella Staining: Principle, Ryu and Leifson Methods, Procedure, and Results

Why flagella need a special stain (principle)

Most motile bacteria have flagella, and the number, position, and arrangement of those flagella help tell genera and species apart. But flagella are a problem to see: they are only about 20 nanometers thick, far below the resolving power of the light microscope, so an ordinary stain leaves them invisible.

Flagella stains solve this by building material up on the flagellum until it is thick enough to see. A mordant (a chemical that helps dye bind and layer) coats the flagellar surface with dye or a metal such as silver, thickening it many times over. This is why flagella staining differs from ordinary staining: it needs unusually careful handling of the slides, the stain, and the cells, because the flagella are fragile and shear off easily.

For what the flagella themselves are and how they are arranged, see the article on bacterial flagella structure.

Two methods for staining flagella

There are two established methods, and the difference is simply whether the cells are stained wet or dried first:

1. Ryu method: a wet-mount procedure. Simpler and well suited to routine use.
2. Leifson method: a dried-smear procedure. More demanding, historically the reference method.

Both work by the same principle (a mordant thickens the flagella so dye can make them visible). The number and arrangement of flagella are what you are trying to reveal, because they are critical for identifying motile bacteria.

Ryu method (wet-mount technique)

The wet-mount (Ryu) technique works well when a stable stain and ordinary slides and coverslips are used. It is simple enough for routine use. The preparations are not permanent, because the stain precipitates as the wet mount dries.

Procedure

1. Grow the organism at room temperature on blood agar for 16 to 24 hours.
2. Add a small drop of water to a microscope slide.
3. Dip a sterile inoculating loop into sterile water.
4. Touch the loopful of water to the margin of a colony briefly (this lets motile cells swim up into the droplet).
5. Touch the loopful of motile cells to the drop of water on the slide. Do not agitate the loop in the droplet: stirring shears the flagella off the cells.
6. Cover the faintly turbid drop with a coverslip. A good wet mount has barely enough liquid to fill the space under the coverslip; small air spaces around the edge are actually helpful.
7. Examine immediately at 40x to 50x for motile cells. If you see no motile cells, do not go on to stain.
8. If motile cells are present, leave the slide at room temperature for 5 to 10 minutes so the cells settle and adhere to the glass slide or the coverslip.
9. Apply two drops of Ryu flagella stain gently to the edge of the coverslip. The stain flows in by capillary action and mixes with the cell suspension; the tiny air pockets at the edge help this along.
10. After 5 to 10 minutes at room temperature, examine for flagella.
11. Look at 100x (oil) in the zone of best stain concentration, about halfway between the edge of the coverslip and the center of the mount.
12. Focus on the cells stuck to the coverslip rather than those on the slide: the stain residue collects mostly on the slide, so the coverslip cells are cleaner to read.

Ryu stain composition (for reference). The Ryu stain has two parts. Solution I (the mordant) contains 10 mL of 5% aqueous phenol, 2 g of tannic acid, and 10 mL of saturated aqueous aluminum potassium sulfate (potassium alum). Solution II (the stain) is a saturated ethanolic crystal violet solution (12 g crystal violet in 100 mL of 95% ethanol). The working stain is made by mixing 1 part Solution II with 10 parts Solution I, then filtering to remove coarse precipitate.

Leifson method (dried-smear technique)

The Leifson method stains flagella on a smear that has been air-dried first, rather than in a wet mount. It is more technically demanding than the Ryu method and is sensitive to timing and to the cleanliness of the slides, which is why the simpler Ryu method is often preferred for routine work. But the Leifson method is the classic reference technique and is still used, especially where a permanent stained preparation is wanted.

Principle. As in the Ryu method, a mordant (tannic acid with a metal salt) plus a dye (basic fuchsin or a fuchsin/crystal-violet mix, depending on the formulation) coats and thickens the flagella. The key practical difference is that the cells are fixed to the slide by air-drying before the stain is applied, so timing the stain correctly is what makes or breaks the result.

Outline of the method

1. Grow a young, actively motile culture (an old culture loses flagella).
2. Prepare a clean, grease-free slide. Slide cleanliness is critical for the Leifson method; any residue causes uneven staining.
3. Place a drop of the bacterial suspension at one end and let it run down the slide, then air-dry without heat fixation (heat destroys flagella).
4. Flood the dried smear with Leifson stain and time it carefully; the stain must be removed at the right moment (watch for a fine precipitate forming across the smear, which signals the endpoint).
5. Rinse gently with water, air-dry, and examine at 100x (oil).

The most common reason the Leifson method fails is mistiming the stain or using a slide that is not perfectly clean.

Expected results

Read the stained mount and note:

1. Whether flagella are present or absent.
2. The number of flagella per cell.
3. The position of the flagella on the cell (one end, both ends, or all around).

What these tell you about the organism (the arrangement types and which bacteria show them) is covered in the bacterial flagella structure article; on this page, the arrangement is simply what you record from the stain.

Quality control

Run known control organisms alongside your test to confirm the stain is working:

Control	Organism	Expected result
Peritrichous (flagella all around)	Escherichia coli	Flagella over the whole cell surface
Polar (flagella at one end)	Pseudomonas aeruginosa	Flagella at one pole
Negative (no flagella)	Klebsiella pneumoniae	No flagella stained

These control strains verify both a positive stain (the two flagellated controls) and that the stain is not producing false positives (the non-flagellated control).

Troubleshooting

Problem	Likely cause	Fix
No flagella seen, but cells were motile	Flagella sheared off during handling	Do not agitate the loop; transfer cells gently; use a young culture
No flagella and no motile cells	Culture too old, or grown too warm	Use a fresh 16 to 24 hour culture grown at room temperature
Heavy background precipitate	Stain not filtered, or left too long	Filter the working stain; keep to the 5 to 10 minute window
Uneven or patchy staining (Leifson)	Greasy or unclean slide	Use scrupulously clean, grease-free slides
Faint or no staining	Mordant weak or stain too dilute	Prepare fresh stain; check the mordant

How to Remember

Flagella are too thin to see, so the stain fattens them up. A flagellum is about 20 nm across, well below what a light microscope can resolve. The mordant layers dye or metal onto it until it is thick enough to appear. Every flagella stain is really a thickening trick.

Ryu is wet and simple; Leifson is dried and fussy. Ryu stains a wet mount and is the routine workhorse. Leifson stains an air-dried smear, gives a permanent slide, but is sensitive to timing and slide cleanliness. Wet = Ryu = routine; dried = Leifson = reference.

The enemy is shearing. Flagella snap off with rough handling, so the whole procedure is about being gentle: do not stir the loop, do not heat-fix, use a young culture. If motile cells show no flagella, you probably sheared them.

Read the coverslip, not the slide (Ryu). The stain residue settles on the slide, so the cells on the coverslip are the clean ones to focus on.

Key exam facts

Feature	Detail
Purpose	Reveal the number and arrangement of flagella for identifying motile bacteria
Why special	Flagella (~20 nm) are below light-microscope resolution; a mordant thickens them so dye can show them
Ryu method	Wet-mount; simple, routine; not permanent (stain precipitates as it dries)
Leifson method	Dried-smear; reference method; permanent slide; sensitive to timing and slide cleanliness
Key reagent role	Mordant (tannic acid + metal salt) layers dye/metal onto the flagellum
Main pitfall	Shearing flagella off (rough handling, old culture, heat fixation)
Magnification	Check motility at 40-50x; read flagella at 100x (oil)
QC controls	Peritrichous (E. coli), polar (P. aeruginosa), negative (K. pneumoniae)
What you record	Presence/absence, number, and position of flagella

Where Students Get Confused

• "Why can't you just use an ordinary stain for flagella?" Because flagella are far too thin (about 20 nm) to see by light microscopy at all. Flagella stains work by depositing a mordant and dye onto the flagellum to thicken it many times over. Without that build-up, the flagellum is invisible no matter which ordinary dye you use.
• "Ryu and Leifson are different stains." They are different methods more than different chemistries. Both use a mordant to thicken the flagella so a dye can show them. The real difference is that Ryu stains a wet mount (simple, routine, not permanent) while Leifson stains an air-dried smear (a permanent slide, but fussier about timing and slide cleanliness).
• "The cells were motile, so why did I see no flagella?" Almost always shearing. Flagella detach easily with rough handling, an old culture, or heat fixation. Motility tells you flagella were there; seeing none after staining usually means they were knocked off during preparation, not that the organism lacks them.

References

• Madigan MT, Bender KS, Buckley DH, Sattley WM, Stahl DA. Brock Biology of Microorganisms. 16th ed. Pearson; 2021.
• Procop GW, Church DL, Hall GS, Janda WM. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Wolters Kluwer; 2017.
• Breakwell DP, Moyes RB, Reynolds J. Differential staining of bacteria: flagella stain. Current Protocols in Microbiology. 2009;Appendix 3:A.3G. DOI: 10.1002/9780471729259.mca03gs15