Bacteria reproduce by binary fission, a process by which one parent cell divides to form two progeny cells. Because one cell gives rise to two progeny cells, bacteria are said to undergo exponential growth (logarithmic growth). The concept of exponential growth can be illustrated by the following relationship:
Thus, 1 bacterium will produce 16 bacteria after 4 generations.
The doubling (generation) time of bacteria ranges from as little as 20 minutes for Escherichia coli to as long as 18 hours for Mycobacterium tuberculosis. The exponential growth and the short doubling time of some organisms result in rapid production of very large numbers of bacteria. For example, 1 E. coli organism will produce over 1000 progeny in about 3 hours and over 1 million in about 7 hours. The doubling time varies not only with the species, but also with the amount of nutrients, the temperature, the pH, and other environmental factors.
The growth cycle of bacteria has four major phases. If a small number of bacteria are inoculated into a liquid nutrient medium and the bacteria are counted at frequent intervals, the typical phases of a standard growth curve can be demonstrated (Figure 3–1).
The first is the lag phase, during which vigorous metabolic activity occurs but cells do not divide. This can last for a few minutes up to many hours.
The log (logarithmic) phase is when rapid cell division occurs. Many antibiotics, such as penicillin, are most efficacious during this phase because they act by disrupting biosynthetic processes carried out by the bacterial cell during active growth (i.e., when they are dividing). The log phase is also known as the exponential phase.
The stationary phase occurs when nutrient depletion or toxic products cause growth to slow until the number of new cells produced balances the number of cells that die, resulting in a steady state. Cells grown in a special apparatus called a “chemostat,” into which fresh nutrients are added and from which waste products are removed continuously, can remain in the log phase and do not enter the stationary phase.
The final phase is the death phase, which is marked by a decline in the number of viable bacteria.
Growth curve of bacteria: a, lag phase; b, log phase; c, stationary phase; d, death phase. (Reproduced with permission from Joklik WK, Willett HP, Amos DB: Zinsser Microbiology, 20th ed. New York, NY: McGraw-Hill Education; 1992.)
OBLIGATE INTRACELLULAR GROWTH
Most bacterial pathogens of humans are capable of growing on artificial media in the clinical laboratory. The term ...