AAB Medical Technologist (MT) – Microbiology Practice Exam 2025 - Free Microbiology Practice Questions and Study Guide

Gram-positive bacteria stain purple during the Gram staining procedure. This characteristic is due to the structural differences in their cell walls compared to gram-negative bacteria. Gram-positive bacteria possess a thick peptidoglycan layer, which retains the crystal violet dye used in the initial stage of the Gram staining process. After applying iodine as a mordant, the bacteria are washed with alcohol or acetone, which dehydrates the thick cell wall of gram-positive bacteria and makes it even less permeable. Consequently, they do not lose the crystal violet stain during this decolorization step.

As a result, when the final counterstain, usually safranin (which is red), is applied, it does not affect the already purple-stained gram-positive bacteria, leading to their retained purple color even after the entire staining process is completed. This distinction is crucial for microbiologists and helps in the identification and classification of bacterial species based on their cell wall properties.