Certified Ophthalmic Medical Technologist (COMT) Practice Exam

Which test is utilized to classify bacteria into two main categories?

1. Acid-fast stain

2. Gram stain

3. Crystal violet stain

4. Schaeffer-Fulton stain

The correct answer is: Gram stain

The Gram stain is a fundamental laboratory technique used to classify bacteria into two primary categories: Gram-positive and Gram-negative. This classification is based on the structural differences in the bacterial cell wall. In the procedure, crystal violet dye is applied to the bacterial sample, followed by iodine, which helps fix the dye. After that, the sample is washed with alcohol or acetone, which acts as a decolorizer. In Gram-positive bacteria, the thick peptidoglycan layer in the cell wall retains the crystal violet dye, appearing purple under a microscope. Conversely, Gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane, do not retain the crystal violet after decolorization and take up a counterstain, typically safranin, appearing pink. This distinction is crucial in microbiology and helps guide treatment decisions for bacterial infections. Other stains mentioned, such as the acid-fast stain, are used for specific bacteria like Mycobacterium tuberculosis but do not categorize bacteria broadly like the Gram stain. Crystal violet and Schaeffer-Fulton stains are also specific and do not serve the general classification purpose that the Gram stain does. Thus, the Gram stain is the correct and pivotal test for classifying bacteria into