What characteristic defines gram-positive bacilli?

The defining characteristic of gram-positive bacilli is their possession of a thick peptidoglycan layer in their cell wall. This thick layer retains the violet stain used in the Gram staining procedure, which is why gram-positive bacteria appear purple under a microscope after the staining process. The peptidoglycan layer provides structural support and helps protect the bacteria from environmental stresses, contributing to their ability to survive in various conditions.

In contrast, while some bacteria may be spiral-shaped, this does not apply specifically to gram-positive bacilli, as they are rod-shaped. The assertion that gram-positive bacilli can only survive in oxygen-rich environments is not accurate, as many gram-positive bacteria are facultative anaerobes or obligate anaerobes, indicating they can thrive in a range of oxygen conditions. The size comparison to gram-negative bacteria is also misleading; the size of bacteria can vary widely among species and is not a definitive characteristic for distinction between gram-positive and gram-negative types. Thus, the thick peptidoglycan layer is the key feature that uniquely characterizes gram-positive bacilli.