Certified in Public Health (CPH) Practice Exam

The Gram-staining technique is used to classify which type of micro-organism?

1. Viruses

2. Protozoa

3. Bacteria

4. Prions

The correct answer is: Bacteria

The Gram-staining technique is specifically designed to classify and differentiate bacteria based on their cell wall composition. This method involves staining bacterial cells with a crystal violet dye followed by a mordant like iodine, and then decolorizing with alcohol or acetone, followed by a counterstain, usually safranin. As a result of this process, bacteria are categorized as either Gram-positive or Gram-negative. Gram-positive bacteria retain the violet stain due to their thick peptidoglycan layer, appearing purple under a microscope, while Gram-negative bacteria, which have a thinner layer and an outer membrane, do not retain the violet stain and take on the counterstain, appearing pink. Viruses, protozoa, and prions do not possess the cell wall structures necessary for this staining technique. Viruses are acellular and do not have cell walls. Protozoa are single-celled organisms that are classified differently and do not respond to Gram staining in the same way as bacteria. Prions, which are infectious proteins, also lack cellular structures entirely and therefore cannot be classified using this method. Thus, the Gram-staining technique is uniquely applicable to bacteria.