An antimicrobial is an agent which destroys microbes or inhibits their growth, or prevents or counteracts their pathogenic action.

Sulfonamides

Chloramphenicol

In the disk diffusion test (also called the Kirby Bauer test), disks containing an antimicrobial agent are placed on the surface of an agar plate containing a medium that has been inoculated with the disease agent being tested, which will grow and fill the disk. The antimicrobial agent diffuses into the medium, killing some of the disease agent around where the anitmicrobial agent was innoculated, depending on how susceptible the disease agent is to the antimicrobial agent. The size of the area cleared of the disease agent shows how effective the antimicrobial agent is.

Broad spectrum.