What characteristic describes gram-positive bacilli?

Gram-positive bacilli are characterized by their ability to retain the crystal violet stain during the Gram staining process. This property is primarily due to the thick peptidoglycan layer present in their cell wall, which traps the dye. When a Gram stain is performed, the thick peptidoglycan layer absorbs the crystal violet stain, resulting in the cells appearing purple under a microscope. This characteristic is significant in microbiology for distinguishing between different types of bacteria and is essential for identification and treatment purposes.

In contrast, spiral-shaped bacteria do not fall under the bacilli classification, and the absence of a cell wall is characteristic of other types of bacteria, such as mycoplasmas, which do not retain the Gram stain. Additionally, cocci, which are spherical bacteria, are distinctly different from bacilli, which are rod-shaped. Understanding these distinctions helps in the classification and identification of bacterial pathogens.