Index >> Microbiological Methods >> Pure Culture Methods

Earlier methods depended upon a dilution of the culture until an aliquot was likely to contain theoretically a single cell as judged from an initial count. However, such methods are tedious, unreliable and can only be used for the dominant organisms. These methods are now rarely used. Microbiologists began to find the possibility of diluting on a solid surface.
The sample is placed at one point on a sterile solid growth medium and then, using a sterile needle, the sample (or inoculum) is drawn several times over the surface (as in streak-plate method). Each streak represents a dilution process and eventually single cells are obtained along the streak. Each of these, on incubation, grows up into a separate colony, , which can be used for pure culture. This, streak method was pioneered by Robert Koch. An alternative to streak method is pour plate method. Here the diluted sample is mixed with a previously melted agar growth medium at a temperature just above the solidifying point. The mixture is poured into a suitable vessel and is incubated. Each cell produces a colony within the agar.