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This page describes products and general use; it is not medical advice.
A Gram stain kit is a set of the four reagents used for the Gram stain, the differential staining method that sorts bacteria into Gram-positive (purple) and Gram-negative (pink). Gram staining is done in clinical and public health laboratories (CDC), and the people who buy for those laboratories choose the kits. This guide covers what is in a kit, what each bottle does, and what to read on a kit’s insert and label. It does not repeat the full organism-by-organism walkthrough in Gram stain: procedure, reagents and how to read the result.
What is a Gram stain?
A Gram stain is a differential staining test: a smear of bacteria is stained in sequence so that cells end up purple or pink, which gives a first, rapid grouping of organisms. It belongs to the family of methods described in differential staining, and it is usually an early step when identifying bacteria. A kit is the packaged set of reagents for that test.
What is in a Gram stain kit?
The American Society for Microbiology (ASM) lists the four components of the Gram stain: a primary stain (crystal violet), a mordant (Gram’s iodine), a decolorizer (“alcohol, acetone, or a mixture of alcohol and acetone”) and a counterstain (safranin). A kit packages these four.
| Reagent | Role in the stain | Source |
|---|---|---|
| Crystal violet | Primary stain, applied first to the heat-fixed smear | ASM |
| Gram’s iodine (a stabilized iodine in one manufacturer’s kit) | Mordant, applied after the crystal violet | ASM; Polysciences |
| Decolorizer: alcohol, acetone, or a mixture of alcohol and acetone | Applied rapidly after the iodine; this is the step that separates the two groups | ASM |
| Safranin | Counterstain, applied last | ASM |
Because the decolorizer may be alcohol, acetone or a mixture, two kits that look alike can differ in this one reagent. The kit’s own label and insert say which one it contains.
How does Gram stain work?
All cells take up crystal violet. After the iodine and the decolorizer, Gram-positive cells keep the crystal violet and stay purple; Gram-negative cells lose it and then take up the counterstain, which makes them pink. CDC’s Gram stain job aid gives the same explanation: cells that retain the crystal violet after the procedure are Gram-positive, and those that do not are Gram-negative. It adds that the decolorizer is usually a 50/50 mixture of acetone and alcohol, and that the counterstain can be safranin or carbol fuchsin, so a kit may not match the table exactly.
How to Gram stain bacteria
The order is the same whichever kit is used: crystal violet on the heat-fixed smear, then iodine, then a rapid decolorization, then safranin (ASM). The time for each step is set by the kit insert or the laboratory’s own procedure, not by this page. CDC’s job aid, which demonstrates the procedure based on ASM protocols, says timing and reagents may vary and to follow the manufacturer’s instructions. For a worked example of reading a result, see the S. pyogenes Gram stain and Salmonella Gram stain guides.
What is the purpose of a Gram stain?
The purpose is to stain bacteria from cultures or specimens by the differential Gram method. One manufacturer states the intended use of its set in these terms, for stat procedures and for laboratories that stain manually, and labels it for in vitro diagnostic use.
What color does Gram negative stain?
Gram-negative bacteria stain pink from the safranin counterstain. Gram-positive bacteria stain purple from the retained crystal violet (CDC job aid).
Kit contents in one manufacturer’s set
As one concrete example, the Polysciences Gram Stain Set (Stabilized), product number 25036, contains four reagents of 2 oz each: A) crystal violet, B) stabilized iodine, C) decolorizer, D) safranin. They are supplied in self-contained, foil-sealed, pre-filled multi-well containers, with extra foils for resealing. The manufacturer states that slides are immersed in the reagents in sequence, air dried and read, and that the set is stored at room temperature.
| Item | What the manufacturer states |
|---|---|
| Contents | Crystal violet, stabilized iodine, decolorizer, safranin; 2 oz each |
| Packaging | Foil-sealed, pre-filled multi-well containers; extra foils for resealing |
| Intended use | Differential Gram staining of bacteria from cultures or specimens; in vitro diagnostic use |
| Storage | Room temperature |
| Hazards | Flammable liquid and irritant |
This is one product, not a survey of the market. Other kits can use other packaging, volumes and decolorizers.
What to check on a kit before using it
- Contents and volumes: confirm all four reagents are present and the volumes are stated.
- Decolorizer type: alcohol, acetone or a mixture (ASM; CDC describes a 50/50 acetone and alcohol mixture as usual); it is not the same in every kit.
- Intended use: whether the label says in vitro diagnostic use, as the example above does.
- Packaging format: sealed multi-well containers versus other formats, and how they are resealed.
- Storage and hazard labelling: storage temperature and the flammable or irritant warnings.
- Timings and counterstain: CDC says timing and reagents may vary, so follow the manufacturer’s instructions and the laboratory’s own procedure; check whether the kit uses safranin or carbol fuchsin.
Safe use and handling basics
The manufacturer example above is labelled a flammable liquid and an irritant and calls for gloves and a fume hood. Follow the kit’s safety data sheet and the laboratory’s own procedure. For general bench practice see aseptic technique.
Common mistakes
- Treating one kit as interchangeable with another: the decolorizer, for one, is not the same in every kit (ASM).
- Using the generic order of steps without the timings the kit insert or laboratory procedure gives.
Frequently asked questions
What is a Gram stain test?
A rapid differential staining test that separates bacteria into purple (Gram-positive) and pink (Gram-negative) groups.
What is in a Gram stain kit?
Crystal violet, Gram’s iodine, a decolorizer and safranin (ASM).
Do all kits use the same decolorizer?
No. ASM lists alcohol, acetone or a mixture of alcohol and acetone as options, and CDC notes reagents and timing may vary.
Sources
- American Society for Microbiology, Gram Stain Protocols, asm.org/protocols/gram-stain-protocols, checked 2026-10-29.
- Polysciences, Gram Stain Set (Stabilized), product 25036, polysciences.com/default/neat-stain-gram-stain-kit, checked 2026-10-29.
- CDC REACH, How to Perform a Gram Stain (video job aid transcript), checked 2026-10-29.
