Tests for Bacterial Motility: Methods, How to Read Them, and Where They Fool You

A motility tube comes back cloudy throughout, and it is tempting to call the organism motile and move on. But a mucoid Klebsiella (reliably non-motile) can throw exactly the same haze when growth slips between the agar and the glass. Read the same tube wrong in the other direction and you miss a true positive: a strict aerobe like Pseudomonas may only cloud the top few millimeters, because it will not grow down the anaerobic depth of the stab.

Motility looks like a simple cloudy-or-clear call, and most of the time it is. The skill is knowing the handful of situations where the tube lies to you, and which method to reach for when the answer actually matters. That is what this article is about: the methods available, how to read each one, and where each one fools you.

Principle

The motility test determines whether an organism is motile or non-motile; a characteristic used to separate genera and species. Most motile bacteria are bacilli, though a few motile cocci exist (notably Enterococcus gallinarum and E. casseliflavus). Motility is usually driven by flagella; a few organisms move instead by axial filaments (endoflagella), which cannot be resolved by ordinary light microscopy.

There are two broad approaches. Microscopy on a fresh culture (wet mount or hanging drop) directly visualizes movement and is the most accurate when a young broth culture is available. Semisolid-medium testing infers motility from how growth spreads from a central stab. The rest of this article maps these methods, then shows how to read and troubleshoot each.

Methods for testing motility

Hanging drop / wet mount (microscopy). A drop of young broth culture is suspended under a coverslip and watched directly for movement. Best for fresh cultures and for telling true motility from Brownian movement and passive drift. The full technique, and how to distinguish real motility from artefact, is covered in the Hanging Drop Method article.

Semisolid-stab motility medium. The organism is stabbed into a low-agar tube; motile organisms spread out and cloud the medium, non-motile ones grow only along the stab. This is the routine method for Enterobacteriaceae in most labs, and you can find details about this method in this article below.

SIM medium. A semisolid tube that reads H₂S, indole, and motility together; motility is the "M." For the combined reading, see the SIM medium article.

MIU (Motility, Indole, Urease). A semisolid tube reading motility, indole, and urease; useful in the Proteus/Morganella/Providencia group. Motility is read exactly as in a plain motility tube.

Distilled-water / immobilization test. A quick way to separate Vibrio (immobilized in distilled water, motile in peptone water) from Aeromonas (motile in both), and to give a presumptive cholera identification by abolishing darting motility with O1/O139 antiserum. This Vibrio-specific confirmation is covered in the Vibrio cholerae laboratory diagnosis page.

Procedure (Semisolid-Stab Tube)

1. Use a semisolid agar tube: SIM, plain motility test medium (with or without TTC, triphenyltetrazolium chloride), or motility-nitrate medium.
2. With a straight wire, make a single stab down the center of the tube to about half its depth. Withdraw the wire along the same track.
3. Incubate under conditions favoring motility, generally 37°C. For organisms more motile at room temperature, incubate a parallel tube at 20–28°C (see below).
4. Examine against light after 6 h, and again at 1 and 2 days; timing depends on the organism's generation time. A medium with 1% glucose can be used for anaerobes.

Reading the tube

Hold it to the light and look at the stab line.

• Non-motile: growth confined to the stab, sharp margins, surrounding medium stays clear.
• Motile: diffuse, hazy spreading growth that clouds the medium away from the stab.

TTC tip: at a final concentration of about 0.005%, tetrazolium makes the reading far easier. It is colorless when oxidized but turns red where bacteria have grown and reduced it, so motile spread shows up as a red haze rather than faint turbidity.

Figure: Motile organisms will migrate out from the line of inoculation, causing visible turbidity throughout the tube (corner tubes are motile). Non-motile organisms will grow only along the line of inoculation (observe the tube at the center).

Quality Control

Test each newly prepared or purchased motility test media for sterility and performance using standard bacterial isolates.

Results

The tube read itself is covered under "Reading the tube" in the Procedure above. For which organism shows which motility pattern (tumbling, darting, swarming, corkscrew) and the temperature-dependent exceptions, see Motility Patterns of Bacteria.

Uses

In the laboratory, motility testing using a semi-solid medium is commonly used for the identification of gram-negative bacteria of the Enterobacteriaceae family. Motility testing is done in conjunction with other biochemical testing using special biochemical media.

1. Sulfide Indole Motility (SIM) Medium: It is a semisolid agar used to determine hydrogen sulfide (H₂S) production, indole formation, and motility.
2. Motility Indole urease (MIU) test: It is used to determine motility, indole formation, and urease production.

A motility test is also used for the species differentiation of gram-positive cocci, and enterococci.  Enterococcus faecium and E. faecalis are non-motile, whereas E. gallinarum and E. casseliflavus/E. flavescens generally are motile.

Limitations

1. Mucoid Klebsiella strains may give a false-positive motile reaction in tube media; this is due to mucoid strains spilling between the medium and the tube, giving a cloudy appearance which is often confused with motility. False-positive results can be avoided by the use of media with adequate tube depth and careful reading with attention to the density of growth in the central stab.
2. A large number of E. casseliflavus and E. gallinarum have been reported as nonmotile using some tube motility medium.
3. False-negative reactions may occur if bacterial flagella are damaged due to heating, shaking, or other trauma. Such environmental shock will render the organism nonmotile.
4. Some microorganisms do not produce flagellar proteins at 35 to 37°C but do so at 22°C.
5. Triphenyltetrazolium chloride may be inhibitory to certain fastidious bacteria.
6. Strict aerobes can read false-negative in a deep stab. Organisms like Pseudomonas and Vibrio will not grow down the oxygen-poor depth of the tube, so spread may be limited to the top few millimeters even when the organism is motile. Read the surface growth, not just the deep stab, and prefer microscopy (hanging drop) for these.

Distilled-water / immobilization test (Vibrio vs Aeromonas).

Vibrio species are immobilized in distilled water but stay motile in peptone water; Aeromonas stays motile in both. Adding O1/O139 antiserum to a darting-motile preparation and watching the movement stop give a presumptive V. cholerae O1/O139 identification, a fast, low-resource call during a suspected outbreak.. The full method and interpretation live on the Vibrio cholerae laboratory diagnosis page.

How to Remember

Motile clouds, non-motile stays home. In a stab tube, motile organisms leave the line and cloud the medium; non-motile growth stays on the stab with the surrounding agar clear. TTC turns that spread red so you are reading color, not squinting at haze.

Two liars to watch: mucoid up, aerobe down. Mucoid Klebsiella fakes motility by seeping up the glass (false positive); strict aerobes like Pseudomonas and Vibrio hide motility down in the anaerobic depth (false negative, spread only at the top). Different directions, opposite errors.

Cold makes some bugs swim: 25 not 37. Listeria and Yersinia enterocolitica are motile at 20–25°C but not at 35–37°C. If motility matters for these, incubate cool.

Where Students Get Confused

Calling any cloudy tube "motile." Turbidity throughout is the usual sign, but mucoid strains and even contamination can cloud a tube. Look specifically at whether growth has left the stab line, and use adequate tube depth and TTC to read it cleanly.

Trusting a clear deep stab for a strict aerobe. Pseudomonas and Vibrio may only grow and spread near the surface. A clear depth does not mean non-motile for these; confirm by microscopy.

Testing at the wrong temperature. Listeria and Y. enterocolitica can read non-motile at 37°C purely because they do not build flagella at that temperature. This is a temperature artifact, not a true negative.

Confusing true motility with Brownian movement on a wet mount. Small particles and non-motile cells jiggle in place (Brownian) or drift together with the fluid (passive drift). True motility is directional and independent of neighbors. The Hanging Drop article covers this distinction in full.

Reading motility as an identification by itself. Motility narrows possibilities (and flags exceptions like non-motile Bacillus anthracis among motile Bacillus), but it is interpreted alongside the rest of the panel, never alone.

References and further readings

1. Leber AL, editor. Clinical Microbiology Procedures Handbook. 4th ed. Washington, DC: ASM Press; 2016. doi:10.1128/9781683670438.CMPH
2. Procop GW, Church DL, Hall GS, Janda WM, Koneman EW, Schreckenberger PC, Woods GL. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Philadelphia: Wolters Kluwer; 2017.
3. Tille PM. Bailey and Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.