Bacilli (singular form: bacillus) refers to a group or class of bacteria that are characterized by their rod-shaped morphology. The term "bacillus" is derived from the Latin word for "little stick" or "rod." These bacteria are classified under the phylum Firmicutes and exhibit a wide range of shapes, sizes, and arrangements.
A defining feature of bacilli is their elongated cylindrical or rod-like structure, which differentiates them from other bacterial shapes, such as spherical (cocci) or spiral (spirilla). Bacilli vary in size, typically ranging from 0.5 to 20 micrometers in length.
Bacilli are ubiquitous in nature, existing in various environments such as soil, water, and the human body. While some bacilli are harmless or even beneficial, others can be pathogenic and cause diseases in plants, animals, and humans. Examples of pathogenic bacilli include Bacillus anthracis, which causes anthrax, and Clostridium botulinum, the bacterium responsible for botulism.
Many bacilli are capable of forming endospores, which are highly resistant structures that allow the bacteria to survive harsh conditions such as extreme temperatures, desiccation, or exposure to disinfectants. This unique characteristic plays a crucial role in the persistence and transmission of some bacilli, as endospores can remain dormant for extended periods and then germinate under favorable conditions to resume active growth.
Bacilli, like other bacteria, have diverse metabolic capabilities, encompassing both aerobic and anaerobic respiration, as well as fermentation. They play important roles in nutrient cycling, decomposition, and other ecological processes, and are extensively studied by microbiologists due to their significance in both health and environmental contexts.
