Inoculum is a term used in biology and microbiology to refer to the source material used for inoculation. Inoculum can be defined as the population of microorganisms or cells that is introduced in the fermentation medium or any other suitable medium. In fungi, the inoculum may be spores or sclerotia, which are compact masses of hardened mycelium. Inoculum can also refer to a biological material, such as a virus or toxin, that is injected to induce or increase immunity to a particular disease. Inoculum is used in laboratories to study the growth of microorganisms under certain environments with certain species and strains of microorganisms. Inoculum size is the needed concentration of microorganisms for a standard test. Inoculum can also refer to the beneficial introduction of microbes to improve plant health. Inoculum is prepared by picking one colony of bacteria from an agar plate and inoculating it into a tube containing a suitable medium.