What general feature is necessary to consider an organism a microbe?

to be unicellular.

Explanation:

A microbe, is a very small (microscopic) unicellular organism that can only be seen with a microscope. In some cases these organisms form colonies (aggregates of unicellular organisms). Microbes are part of two of the the three domains of life and are highly diverse and inhabit almost every habitat in the planet earth.

it is a learned trait

explanation:

the generally accepted parts of modern cell theory include: all known living things are made up of one or more cells. all living cells arise from pre-existing cells by division. the cell is the fundamental unit of structure and function in all living organisms.