Broth tubes
The only difference between broth and agar media is that broths do not contain an agar component. We use broth tubes primarily for specific assays, or (rarely) for bacteria that will not form colonies on a solid surface. In broth a species may display motility and/or a characteristic pattern of association among individual cells, such as chains or clusters, that is not as obvious in agar cultures. To prepare broth a dry medium is layered onto the surface of a measured volume of water as with agar media, mixed, and distributed into individual loosely capped or vented capped tubes in racks. Heating to dissolve components is sometimes required, but not always. Racks are steam sterilized and then allowed to cool, and caps tightened to prevent evaporation. Unlike preparation of agar plates, tubes are prepared with media already in the incubation vessel. A large volume syringe can facilitate distribution of media into individual tubes.