Ansamycin

Ansamycins is a family of bacterial secondary metabolites that show antimicrobial activity against many Gram-positive and some Gram-negative bacteria, and includes various compounds, including streptovaricins and rifamycins. In addition, these compounds demonstrate antiviral activity towards bacteriophages and poxviruses.

They are somewhat similar in structure to macrolide antibiotics, but because they have a lactam instead of a lactone, they do not belong in the class of macrolides.