Fermentation

The bacterium responsible for curd formation is Lactobacillus. It ferments lactose in milk to produce lactic acid, which causes the milk to coagulate into curd.

Step 1: Role of Bacteria in Curd Formation.
The bacterium responsible for the formation of curd is Lactobacillus. This bacterium ferments the lactose in milk, producing lactic acid, which causes the milk proteins to coagulate and form curd. Step 2: Other Bacteria.
Bacillus subtilis and Escherichia coli are not involved in curd formation.
Clostridium is a genus of bacteria that causes diseases like botulism.
Step 3: Conclusion.
Thus, the bacterium responsible for the formation of curd is Lactobacillus.