Bacterial GI infections are usually contracted from contaminated food handled by someone who ignored the “Employees must wash hands...” sign in the bathroom. Some, like Shigella, are also infectious enough to be transmitted from person to person.
Calprotectin doesn’t give very useful information in most cases. It’s usually a good idea to try to identify the specific bacteria before treating with antibiotics. In any case these can run the gamut from clearing up by themselves to killing you (fortunately that’s unusual). It’s generally best to find an antibiotic that the specific bug is sensitive to, and treat with that.