Modern Industrial Microbiology and Biotechnology

Fermentors and Fermentor Operation 199exponential phase, the population doubles at a constant rate, in an environment inwhich the various nutritional requirements are present in excess. As the populationincreases, various nutrients are used up and inhibitory materials, including acids, areproduced; in other words the environment changes. The change in the environment soonleads to the death of some organisms. In the stationary phase the rate of growth of theorganisms is the same as the rate of death. The net result is a constant population. In thedeath phase, the rate of death exceeds the growth rate and the population declines at anexponential rate.If however during the exponential phase of growth, a constant volume is maintainedby ensuring an arrangement for a rate of broth outflow which equals the rate of inflow offresh medium, then the microbial density (i.e., cells per unit volume) remains constant.This is the principle of one method of the continuous culture in the laboratory, namely,the turbidostat.As discussed above, the stationary phase sets in partly because of the exhaustion ofvarious nutrients and partly because of the introduction of an unfavorable environmentproduced by metabolites such as acid. Either of these two groups of factors can be used tomaintain the culture at a constant density. Usually nutrients are used and their use forthis purpose will be discussed.In a batch culture the various nutrients required by an organism are usually initiallypresent in excess. If all but one of the nutrients are present in adequate amount, then therate of growth of the organisms will depend on the proportion of the limiting nutrient thatis added. Thus if 100 grams per liter of the limiting nutrients are required for maximumgrowth but only 90 grams per liter are added, then the rate of growth will be 90% of themaximum. It is then possible to control the growth at any given rate but which rate is lessthan the maximum possible, by letting in fresh nutrient at the same rate as broth isreleased and also supplying one of the nutrients at a level slightly less than themaximum. This principle is employed in the chemostat method of continuous growth.In both the chemostat and the turbidostat the rate of nutrient inflow and broth outflowmust relate to the generation time or growth rate of the organism. If the rate of nutrientaddition is too high, then sufficient time is denied to the organism to develop an adequatepopulation. The organisms are then washed out in the outflow. If on the other hand therate of nutrient addition is too low, a stationary phase may set in and the population maybegin to decline.The above is a simple non-mathematical description of the two basic procedureswhich have been employed in the laboratory study and industrial application ofcontinuous individual cultivation. More detailed studies are widely available in texts onmicrobial physiology.To summarize, in the turbidostat a device exists for ensuring that a constant volume ofa microbial culture is maintained at constant density or turbidity. All the nutrients arepresent in excess and the density or turbidity is monitored by a photo-cell whichtranslates any change to a mechanism which automatically reduces or increases the rateof medium inlet and broth output, as necessary.In the chemostat method a constant population is maintained in a constant volume bythe use of sub-maximal amounts of nutrient(s).