The phases of growth described above apply to a batch culture. In this form of culture, appropriate nutrients and other conditions are provided for growth, then an inoculum is added and the culture incubated. No further nutrients are added and no waste products are removed, thus conditions in the culture are continually changing. This results in active growth being of limited duration for the reasons outlined above.
Sometimes it is desirable to keep the culture in the logarithmic phase, for example if the cells are being used to produce alcohol or antibiotics. In a continuous culture, nutrient concentrations and other conditions are held constant, and the cells are held in a state of exponential growth. This is achieved by continuously adding fresh culture medium and removing equal volumes of the old. Parameters such as pH can also be monitored and adjusted. The equipment used to do this is called a chemostat (Figure 5.10); it
Air supply controller controller
Acid/alkali addition to adjust pH
Figure 5.10 Continuous culture of microorganisms: the chemostat. By the continuous addition of new nutrients and removal of waste, the culture is kept in a steady state, with both cell numbers and nutrient composition remaining constant electrode pH
Figure 5.10 Continuous culture of microorganisms: the chemostat. By the continuous addition of new nutrients and removal of waste, the culture is kept in a steady state, with both cell numbers and nutrient composition remaining constant produces a steady-state culture whose population size is kept constant by careful control of flow rates and nutrient concentrations.
If uninterrupted, growth in fungi proceeds radially outwards from the initiating spore (Box 5.7), allowing the fungal colony to colonise new regions potentially rich in nutrients (Box 5.8). Actual growth occurs solely at the hyphal tip; as this happens, the terminal cell grows longer, until eventually a new cross wall or septum is formed. Cells away
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