In Swiss cheese, big holes are made by a

The correct option is "bacterium producing large quantities of carbon dioxide".

Step-by-Step Explanation:

1. Understanding Swiss Cheese Production:
Swiss cheese is well-known for its characteristic appearance, which includes large holes or "eyes". These holes are not made mechanically; instead, they are the result of a natural biological fermentation process during the cheese's ripening stage.

2. The Role of Microorganisms:
A specific bacterium named Propionibacterium freudenreichii (specifically the subspecies shermanii) is added to the milk during the cheese-making process along with other lactic acid bacteria.

3. Metabolic Fermentation:
During the aging and fermentation process, this bacterium consumes the lactic acid (or lactates) produced by other bacteria and converts it into propionic acid, acetic acid, and carbon dioxide gas (${\mathrm{CO}}_2$). The chemical reaction can be represented as:
$3\text{Lactate}\Rightarrow 2\text{Propionate}+1\text{Acetate}+1{\mathrm{CO}}_2+1H_{2}O$

4. Formation of Holes (Eyes):
Because the cheese is dense and has a semi-hard rind, the large quantities of carbon dioxide gas released during this metabolic activity cannot easily escape. As a result, the gas accumulates in pockets within the curd, expanding to form the classic round holes characteristic of Swiss cheese.