ABSTRACT
The concept of the pure culture is central to classical bacteriology, and is a central tenet of Koch’s postulates (see Instant Notes in Medical Microbiology). If an organism can grow on agar, it can be streaked to obtain single colonies, each of which should have arisen as a result of a single prokaryotic cell (Figure 1). This works by means of the dilution effect of each round of streaking and sterilization. The first inoculum onto the plate might transport millions of bacteria to the agar but, each time the loop is dragged across the plate, these cells are removed further from their neighbors. Coupled with sterilization of the loop, fewer than 10 cells may be present in the last line of streaking before incubation.
