OSMT (Ontario Society of Medical Technologists) Practice Exam

How is a rod shaped microorganism that stains red on Gram staining described?

1. Gram-positive bacillus

2. Gram-negative bacillus

3. Gram-negative cocci

4. Gram-positive spirillum

The correct answer is: Gram-negative bacillus

A rod-shaped microorganism that stains red on Gram staining is identified as a Gram-negative bacillus. This classification is based on the composition of the bacterium's cell wall. Gram-negative bacteria have a thinner peptidoglycan layer located between their inner and outer membranes, which does not retain the crystal violet dye used in the Gram staining procedure. Instead, they take up the counterstain, typically safranin, which results in a red coloration. In contrast, Gram-positive bacteria possess a thicker peptidoglycan layer, allowing them to retain the crystal violet dye and appear blue or purple after staining. The term "bacillus" specifically denotes a rod shape, distinguishing these organisms from cocci, which are spherical bacteria. Therefore, understanding the structure of a microorganism's cell wall is crucial for identifying it correctly after the Gram staining process.