The Collaborative International Dictionary
microbian
microbial \mi*cro"bi*al\, microbian \mi*cro"bi*an\, a. (Biol.) Of, pertaining to, or caused by, microbes; as, microbial growth; the microbian theory; a microbian disease.
Wiktionary
microbian
a. (context biology English) Of, relating to, or caused by, microbes; microbial.