How do gram-negative bacilli differ from gram-positive bacilli?

The distinction between gram-negative and gram-positive bacilli is fundamentally based on their cell wall structure, which affects how they interact with stains used in the Gram staining procedure. Gram-negative bacilli do not retain the crystal violet stain due to their thin peptidoglycan layer and the presence of an outer membrane composed of lipopolysaccharides. When subjected to the Gram stain process, they take up the counterstain, typically safranin, which gives them a pink appearance under the microscope.

In contrast, gram-positive bacilli have a thick peptidoglycan layer that retains the crystal violet stain even after the washing steps with alcohol or acetone, leading to their characteristic purple color. This fundamental difference in cell wall composition directly impacts the retention of the crystal violet stain, making the identification of these two groups of bacteria possible through Gram staining.

The other options can be clarified in the context of this understanding: retaining crystal violet is characteristic of gram-positive bacteria, not gram-negative; the spherical shape refers to cocci, not bacilli; and size can vary widely among different bacterial species regardless of their gram classification. Thus, the distinguishing characteristic of gram-negative bacilli is their inability to retain crystal violet stain during the Gram staining process.