The value of cultures to modern microbiology.

Since the late nineteenth century, pure cultures have been regarded as the cornerstone of bacteriology. However, not all bacteria will multiply sufficiently to produce visible colonies on solid media; some cells will produce micro-colonies that are invisible to the naked eye. Moreover, the proportion of culturable cells that produce visible growth will vary according to the species and the state of the cells-are they actively growing or comparatively inactive? The latter have a poorer rate of recovery in terms of cultivability. It is unclear whether or not an individual colony is always derived from a single cell; it is possible that organisms in close proximity to each other may multiply and come together to produce single colonies. Then, the resultant growth will most certainly be derived from more than one initial cell. Although it is generally assumed that streaking and re-streaking on fresh media will purify any culture, there is evidence for microbial consortia interacting to form what appear to be single pure cultures. As so-called pure cultures underpin traditional microbiology, it is relevant to understand that the culture does not necessarily contain clones of identical bacteria, but that there may be variation in the genetic potential of the component cells, i.e. the cells are not homogeneous. Certainly, many bacteria change rapidly upon culturing, with some becoming bigger and less active. It is difficult to be sure if these changes reflect a loss or change of DNA or whether standard culturing methods select faster growing cells that are effectively not representative of the environment from which they were derived. These concepts are reviewed with an emphasis on bacterial fish pathogens.