Gram-positive bacteria

Why they stain this way

Gram-positive cell walls carry a thick peptidoglycan layer that traps the crystal violet-iodine complex through the alcohol decolourisation step, so the cells stay purple and never take up the safranin counterstain.

That same thick wall is why the classic Gram-positive drugs work the way they do, and why arrangement - clusters versus chains, cocci versus rods - is usually the next branch after the stain itself.

PetriKey lists 23 Gram-positive bacteria. Only bacteria appear here - lab methods and antimicrobials carry gram associations rather than a stain of their own, so they are filed under lab & diagnostics and antimicrobials.

Browse by shape

Shape is the next branch after the stain. Each Gram-positive shape group has its own study page with the identifying lab clues for every member.