What it is
An antifungal or antibacterial borrowed from microbiology to keep shredded cheese loose in the bag.
Why your biology objects
Nisin is a bacteriocin (a peptide one microbe makes to kill others) and natamycin a fungal-derived antifungal — both effective and low-dose. The concern is subtler: routine antimicrobial use in food and possible cross-resistance. Block cheese with one ingredient doesn't need it.