What are bacilli bacteria characterized by?

Bacilli bacteria are primarily characterized by their rod shape. This shape is an important feature that helps in their classification within the broader category of bacteria, which can exhibit various forms, including cocci (spherical) and spirilla (spiral). The rod-like structure of bacilli allows for a greater surface area, which can be advantageous for nutrient absorption and also influences how these bacteria move and behave in their environments. Their shape is significant in identifying them and understanding their biological functions.

In the context of the other potential answers, spirilla refers to bacteria with a spiral shape, which is distinct from bacilli. Pair formation is a classification more relevant to certain types of cocci that can exist in pairs, while single-cell refers to bacteria as individual organisms, which again does not specifically denote the characteristic shape that defines bacilli. Thus, the defining rod shape of bacilli is what distinguishes them in microbiological classification.