Step 1: Bacterial action in curd formation.
The process of converting milk into curd is a fermentation process, where specific bacteria convert lactose (milk sugar) into lactic acid.
Step 2: Identify the bacteria.
The primary bacterium responsible for this process is Lactobacillus species, which includes Lactobacillus bulgaricus.
Final Answer:
\[
\boxed{\text{Lactobacillus}}
\]