Chapter 3 · Chapter 4

The Gram stain

Four reagents, about three minutes, and one answer: thick wall or thin. It is the first thing done to a clinical isolate and the single most useful stain in the discipline.

Two walls, one difference

Drawn at the same scale. Everything the stain reads comes down to how thick that peptidoglycan layer is.

Fig. 1Cross-sections through a Gram-positive and a Gram-negative envelope, outside the cell at the top and cytoplasm at the bottom.

The four steps

Both cell types look identical until the alcohol goes on. Everything before step three is setup; everything after it is bookkeeping.

Fig. 2What each cell type looks like after each reagent. Step three is where the two types separate.

Step	Reagent	Role	Time	What it does
1	Crystal violet	Primary stain	≈ 1 minute	Everything takes up the primary stain. At this point the two types look identical.
2	Gram's iodine	Mordant	≈ 1 minute	Iodine binds the crystal violet into a large complex that cannot slip out through thick peptidoglycan.
3	Alcohol or acetone	Decolouriser	10–20 seconds — the critical step	Alcohol dissolves the Gram-negative outer membrane and washes the complex out through its thin peptidoglycan. Thick Gram-positive peptidoglycan dehydrates and traps the complex inside.
4	Safranin	Counterstain	≈ 1 minute	Safranin stains anything that lost the crystal violet. Gram-positive cells are already too dark for it to show.

What the result means

The colour is a proxy. What you are actually reading is how thick the peptidoglycan is, and that changes how the organism is treated.

Gram-positive

Purple

Peptidoglycan: Thick layer
Membranes: One membrane, beneath the peptidoglycan
Also in the wall: Teichoic acids — unique poly-alcohols running through the wall
You will meet: Staphylococcus aureus, Streptococcus pneumoniae, Bacillus subtilis, Clostridium difficile

Gram-negative

Pink

Peptidoglycan: Thin layer
Membranes: Two membranes — one below and one above the peptidoglycan
Also in the wall: The outer membrane carries lipopolysaccharide (LPS), whose lipid A acts as an endotoxin
You will meet: Escherichia coli, Neisseria gonorrhoeae, Pseudomonas aeruginosa, Salmonella enterica

Why positive stays purple

Iodine locks the crystal violet into a complex too large to pass through a thick, dehydrated peptidoglycan mesh. The alcohol shrinks the mesh further, which closes the exit.

Why negative turns pink

The alcohol dissolves the lipid outer membrane outright. With only a thin peptidoglycan layer left, the complex washes straight out, and safranin colours the empty cell.

Where it goes wrong

Almost every failed Gram stain fails at step three. If a result looks wrong, suspect the alcohol or the age of the culture before anything else.

Mistake	You get	Because
Over-decolourising	Gram-positive cells read as negative	Leave the alcohol on too long and even thick peptidoglycan gives up the crystal violet. This is the most common error by far.
Under-decolourising	Gram-negative cells read as positive	Too little alcohol and the complex never washes out of the thin wall.
Smear too thick	Patchy or falsely positive	Alcohol cannot reach the cells in the middle of a heavy smear.
Culture too old	Gram-positive cells read as negative	Ageing cells lose wall integrity, so old cultures stain unreliably. Use a culture 18–24 hours old.
Skipping the iodine	Everything reads as negative	Without the mordant the crystal violet is small enough to wash straight out of either wall.

Organisms the stain cannot help with

MycobacteriumWaxy mycolic acid in the wall repels the dyes. Use the acid-fast stain.
MycoplasmaNo cell wall at all, so there is nothing for the stain to read.
SpirochetesToo thin to see reliably by light microscopy even when stained; darkfield or silver stains are used.