University of Central Florida (UCF) Biomedical Exit Practice Exam

The defining feature is the cell wall structure: Gram-positive bacteria have a thick peptidoglycan layer and lack an outer membrane, while Gram-negative bacteria have a thinner peptidoglycan layer plus an outer membrane. This thick peptidoglycan traps the crystal violet–iodine complex during staining, and without an outer membrane to release it, the stain is retained after the decolorization step. That makes them appear purple under the microscope. In contrast, Gram-negative bacteria have the outer membrane and a thinner peptidoglycan layer, so they decolorize and take up the counterstain, appearing pink. So the description—lacking an outer membrane and having a thick peptidoglycan layer—best fits Gram-positive. The other options either describe Gram-negative features or are redundant labels that don’t add a distinct characteristic.