What is the rod shaped bacteria called?

Correct option is B

Rod-shaped bacteria are called bacilli. These bacteria have a cylindrical structure, which may be straight or slightly curved. Bacilli can exist singly or in chains depending on the species. Common examples include Bacillus anthracis and Escherichia coli. Many bacilli are known to cause diseases, while others are beneficial in industrial and medical processes. Their shape provides a larger surface area-to-volume ratio, aiding nutrient absorption and motility.
Explanation of options: (a) Coccus – Cocci are spherical-shaped bacteria, not rod-shaped. They may occur singly, in pairs (diplococci), chains (streptococci), or clusters (staphylococci). Common examples include Streptococcus pneumoniae and Staphylococcus aureus. These bacteria are often associated with infections like pneumonia, skin infections, or sore throat. Since the question asks about rod-shaped bacteria, this option is incorrect.
(b) Bacillus – This is the correct answer. Bacilli are rod-shaped bacteria that may be found singly or in chains. They are involved in several functions ranging from nitrogen fixation to causing diseases. Some bacilli, like Bacillus subtilis, are used in biotechnology, while others, like Bacillus anthracis, cause anthrax. Their elongated shape assists in motility and efficient colonization of environments.
(c) Vibrio – Vibrio are comma-shaped bacteria, meaning they have a curved rod appearance. The most well-known example is Vibrio cholerae, which causes cholera in humans. While they may look slightly rod-like, they are not classified as true bacilli due to their curved structure. Vibrio species are mostly aquatic and often motile due to flagella. They are different from straight rod-shaped bacilli.
(d) Spirillum – Spirilla are spiral or helical-shaped bacteria, not rods. Their shape resembles a corkscrew or twisted coil, which enables them to move through viscous environments using flagella. Spirillum minus is an example that can cause rat-bite fever. Their morphology is adapted for swimming motion, especially in liquids. Hence, spirillum does not refer to rod-shaped bacteria.