Gram stain
The Gram stain is a differential staining method separating many bacteria into two broad patterns according to how their cell envelope retains the primary stain during decolorization.
Open interactive lesson & self-check ↗Gram stain: two cell envelope patterns
Original conceptual SVG · scalableIdealized bacterial envelope cross sections. Gram staining can be atypical or unreliable in certain organisms.
Trace the sequence
A fixed smear is exposed to crystal violet, iodine, decolorizer and a counterstain. Technique and timing influence the result.
Interpret broadly
Gram-positive bacteria commonly appear purple; Gram-negative bacteria commonly appear pink or red after counterstaining.
Recognize limits
The result does not by itself identify species or determine antibiotic susceptibility. Some bacteria stain atypically, poorly or not at all.
Reference and next reading
Explore the original curriculum and publisher-hosted resources for full-depth reading; this note is an original schematic introduction, not an exhaustive chapter.