Industrial Microbiology

Growth of bacteria in closed and open systems

Written by MicroDok

Microbial cells can exhibit different growth patterns depending on the availability of growth nutrients in their immediate environment. In batch culture, microbial cells cannot grow continuously because the growth nutrients required for development is not renewed after it has been depleted. Thus exponential growth is limited to a few generations of microbial cells in batch cultures because the system is a closed system of microbial cultivation. Microorganisms cultivated in test tubes are typical examples of a batch or closed system of growth. But this is not always the case in an open system of culture or growth known as continuous culture. Only continuous culture can support the exponential growth of bacterial cells over a long period of time because such a system of microbial culture is an open system which is flexible and allows for nutrient replacement after their depletion.

Continuous culture system is a growth method that maintains a constant environmental growth conditions for microbial cells through the continual provision of nutrients and removal of toxic wastes from the growth medium. It is an open system of growth that maintains microbial population in the log or exponential phase; and one that maintains growth for extensive periods of time. Continuous culture is usually carried out in a specialized system known as chemostat (Figure 1). In a chemostat, fresh nutrient is supplied to meet up with the microbial growth. Excess microbial cells (i.e. spent cells) and toxic wastes are immediately removed from the culture medium as soon as they develop. The two methods used for carrying out a continuous culture system of microbial growth are chemostats and turbidostats. Turbidostats is an open system method of continuous culture in which nutrients are constantly provided and toxic wastes removed as they are formed (as obtainable in the chemostats). It is so named because it evaluates the turbidity of bacterial culture in a growth vessel during the log phase of growth.

Turbidity is the measurement of suspended solids (in this case, microbial cells) in a solvent e.g. water or normal saline. The turbidostats is similar to the chemostats except that the former functions at a higher dilution rate while the later runs at a lower dilution rate. A chemostat is simply a static chemical and/or biological environment in which a controlled chemical/biological reaction takes place. It is a bioreactor to which fresh medium for the growth of an organism is continuously added. It is however noteworthy that as the growth medium is continuously introduced into the vessel; the culture liquid generated in the process is also continuously removed from the bioreactor in order to keep the culture volume of the vessel constant and operating at an optimal condition.

By changing the rate with which fresh growth medium is added to the bioreactor (i.e., the chemostat) the growth rate of the microorganism being cultivated can be easily controlled in real time. A chemostat has fittings for effluents generated during the process; and portals for influents, agitation of the medium and source for the introduction of filtered air are also made available in it. With the chemostat, scientists can identify and better understand how cells especially microorganisms and other forms of life regulate their growth; and also to understand the adaptive evolution of living organisms. Chemostat enables precise control of the selective pressure under which organisms evolve and it facilitates experimental control of cell growth rate.

Figure 1: Structure of a chemostat showing other fitted components.

References

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