Of, pertaining to, or caused by, microbes; as, microbial growth; the microbian theory; a microbian disease.

like something that can only be seen with a microscope.

Microbial means of or relating to microbes (microscopic organisms such as bacteria or viruses).

see microorgansim.

of or involving or caused by or being microbes; "microbial warfare"

minute forms of life such as bacteria, fungi or viruses.

Minute living being.