Diagnostic Flowchart

Gram-positive (purple)

Thick peptidoglycan cell wall retains crystal violet stain.

Gram-negative (pink/red)

Thin peptidoglycan + outer membrane; loses crystal violet, takes up safranin.

Gram-variable / Acid-fast

Some organisms stain inconsistently or require special stains (e.g., Ziehl-Neelsen for Mycobacteria).