Cholera and its causative agent:

Cholera is an infectious disease caused by the bacterium Vibrio cholerae. It is primarily transmitted through contaminated water and food, and can lead to severe diarrhea and dehydration if left untreated.

The causative agent - Vibrio cholerae:

Vibrio cholerae is a gram-negative bacterium that belongs to the family Vibrionaceae. It is a rod-shaped bacterium with a single polar flagellum, which allows it to move in a characteristic darting motion. V. cholerae is well adapted to survive in aquatic environments, particularly in warm, brackish water.

Why Vibrio cholerae causes Cholera:

Vibrio cholerae causes cholera through the production of a toxin called cholera toxin. This toxin is responsible for the severe watery diarrhea characteristic of the disease. When Vibrio cholerae bacteria colonize the small intestine, they produce cholera toxin, which binds to the cells lining the intestine. The toxin then stimulates the cells to release large amounts of water and electrolytes into the intestine, leading to the characteristic watery diarrhea.

Other bacteria and their roles:

a) Streptococcus: Streptococcus is a genus of bacteria that includes several species. While some species of Streptococcus can cause diseases such as strep throat and pneumonia, they are not associated with causing cholera.

b) Clostridium: Clostridium is a genus of bacteria that includes several species, some of which can cause diseases such as botulism and tetanus. However, they are not responsible for causing cholera.

c) Pasteurella: Pasteurella is a genus of bacteria that includes several species, some of which can cause infections in animals and humans. However, they are not associated with causing cholera.

Conclusion:

Among the given options, the bacterium that causes cholera is Vibrio cholerae. Streptococcus, Clostridium, and Pasteurella are not the causative agents of cholera. It is important to understand the specific bacteria responsible for different diseases to effectively prevent, diagnose, and treat them.