Bacillus is best described as which of the following?

• A. Spore forming, gram-positive, sometimes mobile that can cause systemic damage
• B. A non-spore-forming gram-negative bacterium
• C. A virus that infects bacteria
• D. A fungus with pseudohyphae

Answer: (A)

Bacillus is best described as a spore-forming, gram-positive bacterium that is often rod-shaped and may be motile. The ability to form endospores lets these bacteria survive harsh conditions, and being gram-positive reflects their thick peptidoglycan cell wall. Some Bacillus species, such as Bacillus anthracis, can cause systemic disease, which is why systemic damage is noted in the description. The other options describe organisms that are not Bacillus: non-spore-forming gram-negative bacteria do not fit Bacillus, viruses are bacteriophages that infect bacteria rather than being bacteria themselves, and fungi with pseudohyphae are fungal, not bacterial.