Glossary
Gram stain
A Gram stain is a four-reagent differential stain that sorts bacteria by cell wall: a thick wall holds the purple dye (Gram-positive), a thin wall loses it and takes the pink counterstain (Gram-negative).
A Gram stain is a differential stain that sorts bacteria into two groups by how their cell wall behaves against a dye. Four reagents go on in order, and the wall's thickness decides the result.
Crystal violet stains every cell purple. Iodine acts as a mordant, forming a large crystal-violet–iodine complex inside the cell. Alcohol or acetone is the decolorizer: it washes the complex out of a thin wall but not out of a thick one. Safranin is the counterstain, turning the now-colorless cells pink or red.
A Gram-positive cell has a thick peptidoglycan wall that traps the complex, so it stays purple. A Gram-negative cell has only a thin peptidoglycan layer, under an outer membrane; the alcohol removes the complex, and the safranin colors it pink or red.
The result carries clinical weight because the two walls respond differently to antibiotics — a Gram-negative cell's outer membrane keeps many drugs out — so a stain from a sputum sample narrows the field before a culture answers. It is a stain, not a culture: it describes the wall and the shape (pink bacilli, purple cocci), never a species. The word is capitalized, being named for Hans Christian Gram.
Taught in
Related terms
- prokaryote — A prokaryote is a cell with no nucleus and no membrane-bound organelles — its DNA sits loose in the cytoplasm, as it does in bacteria.
- chain of infection — The chain of infection is the six links an infection needs — agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host — and breaking any one of them prevents it.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)