An antimicrobial such as isoniazid that is only effective against mycobacterium would be considered to be a drug.

Anti-bacterial drug

Explanation:

An antimicrobial such as isoniazid that is only effective against mycobacterium would be considered to be an anti-bacterial drug.

Mycobacterium is a pathogenic micro organisms which belongs to the class called Bacteria. Others include virus, Protozoa and fungi.

Since the drug is only effect against mycobacterium which is a bacteria it is called anti bacterial drug. Drugs which are effective against virus, Protozoa and fungi are known as antiviral, antiprotozoal and antifungal respectively.