Antimicrobial agents are agents that helps fight against microorganism. The couple occur in natural or synthetic form.
Antibiotics are synthesis from natural source such as mold, bacteria to fight against various diseases.
Cephalothin produce by mold is an example of a natural antimicrobial that is produce by mold to kill microorganism causing various illnesses it could be further process into a synthetic antimicrobial agent and can be used as antibiotics.
A bunch of the same critters. Nah I am just kidding, you will find a lack of resources because if they are all the same animals they will all eat the same thing. I hope this helps you.