An antimicrobial additive is added to the polymer when the utensil is moulded. The most commonly used additives are silver ion-based. This is because silver is a natural antibacterial metal and doesn't contain the toxins present in some other metals.
Silver has been used to prevent the spread of germs for centuries, so how does it work in food contact equipment? Put simply, when bacteria comes into contact with the silver antimicrobial agent, it's growth is disrupted. This prevents it from multiplying and spreading.