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Microbial growth causes an increase in a microorganism's size and population number and can be studied using batch and continuous culture mediums (Lansing M. Prescott et al. 1990, 1993, p. 112)
Nutrient consumption and build up of toxic by-products slow down and stop the growth of bacteria. One method to avoid this is to use batch culture system. This involves culturing bacteria in liquid medium within a closed system where no fresh medium is added during the incubation period and hence the nutrient concentration declines and waste products accumulate during the growth of bacteria (Slonczewski & Foster 2009, p. 137).
The microorganisms reproduce by binary fission and their growth can be expressed as the logarithm of cell number and can be plotted against incubation time. This result in a curve that has 4 distinct phases as shown in Figure 1: (Lansing M. Prescott et al. 1990, 1993, p. 113)
Figure 1: Curve showing phases of growth for batch cultures (taken from Tortora et al. 1992)
Lag Phase- when microorganisms are introduced into fresh culture medium no increase in cell number or mass occurs. This marks the beginning of the Lag Phase which is necessary as the cells may be old and depleted of ATP, certain essential cofactors and ribosome, the medium may be different from the one that the microorganism was growing in previously, or the microorganism may have suffered injuries and may need time to recover (Lansing M. Prescott et al. 1990, 1993, p. 113).
Length of the Lag Phase varies depending on factors such as age of culture, temperature changes and differences between old and new media. When cells are transferred from a complex medium to a fresh complex medium lag phase is very short, whereas when cells are transferred from complex medium into a minimally defined medium, lag phase is protracted (Slonczewski & Foster 2009, p. 137).
Exponential Phase- during this phase the rate of growth of microorganisms is constant and they divide and become double in number at regular intervals and at the maximal rate possible given their genetic potential, nature of the medium and the conditions of growth (Lansing M. Prescott et al. 1990, 1993, p. 113).As batch cultures are not synchronous every cell has an equal generation time and each cell divides at a different moment, as hence the cell number rises smoothly. When the medium is abruptly changed nutritional downshifts or nutritional upshifts cause cells experiencing balanced exponential growth to be thrown into metabolic chaos that is unbalanced growth (Slonczewski & Foster 2009, p. 138). Exponential phase cultures are used in biochemical and physiological studies as the population is most uniform in terms of chemical and physiological characteristics during this phase (Lansing M. Prescott et al. 1990, 1993, p. 113).
Stationary Phase- during this phase the growth curve becomes horizontal as growth of the population ceases. Bacteria enter this phase at a population level of around cells per ml. The total number of viable organisms remains constant due to a balance between cell division and cell death or because the population ceases to divide but still remains metabolically active. Microbial populations enter the stationary phase due to reasons such as nutrient limitations, oxygen availability and accumulation of toxic waste products (Lansing M. Prescott et al. 1990, 1993, p. 114).
Death Phase- characteristic feature of this phase is decline in the number of viable cells due to nutrient deprivation and buildup of toxic wastes. The death of a microbial population is logarithmic. The extended survival of particularly resistant cells may reduce the death rate (Lansing M. Prescott et al. 1990, 1993, p. 114). In this phase the number of cells that die during a given time period is proportional to the number of cells that existed at the beginning of the time period (Slonczewski & Foster 2009, p. 138).
Advantages of using batch cultures:
They allow versatility and can be used for many different reactions (Nielsen & Villadsen 1994, p. 344).
They are safe and do not pose a threat of strain mutation (Nielsen & Villadsen 1994, p. 344).
They ensure complete conversion of substrate (Nielsen & Villadsen 1994, p. 344).
The changing conditions during the use of batch cultures affect the physiology and growth of bacteria and hence highlight the ability of bacteria to adapt to its environment (Slonczewski & Foster 2009, p. 137).
Disadvantages of using batch cultures:
Use of batch cultures requires highly skilled labor thus increasing labor costs (Nielsen & Villadsen 1994, p. 344).
Use of batch cultures is time consuming (Nielsen & Villadsen 1994, p. 344).
According to Xuezhen Kang (2000) changing concentrations of products and reactants, varying pH and oxidation-reduction potential makes interpretation of results difficult.
According to Xuezhen Kang (2000) complicated mix of growing, dying and dead cells also makes interpretation difficult.
In a continuous culture system the microorganisms are grown in an open system where constant environmental conditions are maintained through continuous provisions for new nutrients and removal of waste. This allows the microbial population to remain in exponential growth phase and at a constant biomass concentration for an extended period of time (Lansing M. Prescott et al. 1990, 1993, p. 120).
2 main types of continuous culture systems:
Chemostat- this system ensures that sterile medium containing essential nutrients in limiting quantities is fed into the culture vessel at the same rate as the medium containing the microorganism is removed (Lansing M. Prescott et al. 1990, 1993, p. 120).
The growth rate of microorganisms is determined by the rate at which new medium is fed into the growth chamber. The final cell density depends on the concentration of the limiting nutrient (Lansing M. Prescott et al. 1990, 1993, p. 120).
Dilution rate "D" is used to express the rate of nutrient exchange and is defined as the rate at which medium flows through the culture vessel relative to the vessel volume." f " represents flow rate and "V "represents volume of the vessel (Lansing M. Prescott et al. 1990, 1993, p. 120).
f (ml/hr) and V(ml)(Lansing M. Prescott et al.1990, 1993, p. 120)
Microbial population levels and generation time depend on the dilution rate. As the dilution rate increases generation time shortens and growth rate rises. Under these conditions the limiting nutrient will be almost completely depleted and it begins to rise at higher dilution rates as there are fewer microorganisms present to use it. At low dilution rate a rise in both cell density and growth rate occur (Lansing M. Prescott et al. 1990, 1993, p. 120).
Turbidostat- involves a photocell that measures the turbidity of the culture in the growth vessel. The rate of flow of the media through the growth vessel is automatically regulated to maintain a predetermined turbidity. Here the dilution rate remains constant and there is no limiting nutrient in the culture. The turbidostat operates best at high rates of dilution (Lansing M. Prescott et al. 1990, 1993, p. 120).
Advantages of using continuous cultures:
They allow good utilization of the bioreactor and ensure low labor costs (Nielsen & Villadsen 1994, p. 344).
They are efficient and ensure high and constant productivity due the autocatalytic nature of microbial reaction taking place (Nielsen & Villadsen 1994, p. 344).
They allow detailed analysis of microbial physiology at different growth rates as all cells of the population achieve a steady state and hence has significant industrial and research applications (Slonczewski & Foster 2009, p. 139).
The situation in a chemostat resembles the growth of bacteria in nature where the growth rates are very low (Slonczewski & Foster 2009, p. 140).
According to Xuezhen Kang (2000) their use eliminates the lag the organism experiences before going into high productivity.
Disadvantages of using Continuous Cultures:
Sometimes they fail to produce results due to infection and mutations of microorganisms that result in production of non producing strains (Nielsen & Villadsen 1994, p. 344).
Use of these cultures requires downstream equipments to be designed for low volumetric rates and continuous operation (Nielsen & Villadsen 1994, p. 344).
In conclusion, we see that both culture mediums have their advantages and disadvantages and while using either one, these should be kept in mind.