The microbiologist has many choices to make if wishing to grow microorganisms in the laboratory. The biomass introduced into the growth medium is known as the inoculant, and one of the first questions that need to be addressed is whether to inoculate onto solid or into liquid media. The medium will be chosen to reflect the origin of the inoculant, which might be a mixed culture of many different species of microorganism, or a pure culture of only one species. Solid media are normally held in circular sterile plastic containers with lids (Petri dishes), the solidity being provided by agar. The culture is streaked or plated onto the surface of the medium using a sterile wire or plastic loop, or a sterile glass spreader, respectively.