MICROBIOLOGY Study Guide - 2012

CHAPTER 8. MICROBIAL GROWTH

METABOLISM in a microbial Cell is accompanied by the Biosynthesis of Proteins, Nucleic Acids, Polysaccharides, and Other Compounds. These processes result in an increase in Cell size and biomass, which is defined as growth. The increase in cell number through Cell Division is referred to as reproduction. The growth rate is determined by dividing the biomass by The Cell count per unit volume over specific time intervals.

Bacteria multiplying within a closed volume of nutrient medium constitute a microbial population. In laboratory and industrial settings, microbial populations are cultivated using batch or continuous Methods.

8.1. BATCH CULTIVATION. THE GROWTH CURVE

Batch cultivation is defined as the propagation of microorganisms in a closed volume of nutrient medium. If no nutrients are added to the medium during cultivation and Metabolic waste products are not removed, Microbial growth will continue only until the concentration of a given component is depleted. Growth in such a closed system follows certain regularities and is described by the so-called "growth curve," which expresses the dependence of the logarithm of cell number on cultivation time (Fig. 23).

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Fig. 23. Growth curve of a microbial population under batch conditions

Several growth phases, succeeding one another in a specific sequence, can be distinguished on a typical growth curve:

✵ I — lag phase (or lag period);

✵ II — exponential phase (or logarithmic phase);

✵ III — stationary phase;

✵ IV — death phase.

The lag phase spans the time interval from the moment of inoculating the medium (inoculum addition) to the onset of microbial reproduction. During the lag phase, the cell count either remains unchanged or increases only very slightly. The growth delay at the beginning of the lag phase is attributed to adaptation to new environmental conditions. Nevertheless, this period is characterized by heightened metabolic activity: the intracellular content of RNA and total protein increases, and A number of Enzymes are synthesized. A noticeable increase in cell size is observed—Cells become 3 to 5 times larger than normal. The duration of the lag phase depends on the microorganism species, the COMPOSITION OF THE nutrient medium, as well as the quantity and age of the inoculum introduced. The lag phase is shortened when using a rich nutrient medium and a young inoculum.

Occasionally, two lag phases may be observed in a batch culture—a phenomenon known as diauxie. This biphasic growth pattern was first described in 1942 by the French scientist Jacques Monod using the sequential utilization of two different CARBOHYDRATES by Escherichia coli as an example. When E. coli is cultivated in a medium containing glucose and sorbitol, the cells consume glucose first, while the synthesis of enzymes required for sorbitol degradation is repressed. These enzymes are synthesized only after all the glucose has been exhausted.

The exponential (or logarithmic) phase is characterized by a maximum and virtually constant Cell Growth Rate. The number of cells during this phase increases in a geometric progression. The culture growth in this phase is balanced: substrate nutrients and metabolic waste products do not limit microbial reproduction.

In the exponential growth phase, the duration of which is relatively short for many microbial cultures in liquid media, nutrients are rapidly consumed, toxic metabolic byproducts accumulate, and the synthesis rate of intracellular components declines. All of this leads to a deceleration of culture growth and its transition into the next phase—the stationary phase.

The stationary phase corresponds to the period when the Number of viable cells reaches its maximum and ceases to increase. Many cells continue to divide, but the number of newly formed cells equals the number of dying cells. Consequently, the total size of the microbial population remains stable. Toward the end of the stationary phase, growth conditions continue to deteriorate, and the culture transitions into its final phase.

The death phase is characterized by the mass mortality of microbial cells. The rate of cell death significantly outpaces the rate of cell division. Under these adverse conditions, only isolated individuals survive, and so-called involution forms appear that are incapable of producing progeny.