Continuous Culture
It is possible to have culture of bacteria in the exponential growth phase and hold them there. This is done by continuously adding more nutrients and remove wastes. This is done in a chemostat. Appendix 7 shows a chemostat.
Inputs to a chemostat are sterile media and sterile air. Outputs are spent media which contains waste products, some nutrients and some bacteria.
Flow rate of media and nutrient concentration are controlled. This allows control of the specific growth rate over a wide range. If the flow is too fast, the microorganisms cannot keep up. They leave the chemostat more rapidly than they are produced. Eventually they are all gone. This condition is called ‘washout’. If the nutrient flow is too slow, then it behaves much like a batch culture.
Batch culture means that the volume of media is constant and there is no media added or taken away.
Counting Microbial Cells
Microbial cells can be counted directly or cell count plating methods can be used.
In the direct count method a Petroff-Hauser counting chamber is used. This is glass slide that fits on the stage of a microscope. It has a small depression of known volume. There is a grid on the slide. By determining the number of organisms per square in the grid, an estimate of the number of microorganisms can be made.
There are a number of problems with this method.
1) Dead cells cannot be distinguished from live cells.
2) Small cells are difficult to see.
3) Precision is difficult to attain ภาษาไทย