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## GROWTH CYCLE

When we think of growth, this is usually in reference to changes in weight (how many ounces has a baby gained?) or height (how many inches has a child grown?). However, thinking about bacterial growth usually refers to the number of bacteria. Bacteria reproduce by binary fission, a process by which one cell divides into two cells, two divides into four, four divides into 8, etc. This is referred to as exponential growth (logarithmic growth), which is illustrated by the following relationship:

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 Number of cells 1 2 4 8 16 Exponential 20 21 22 23 24

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 to 24 hours for Mycobacterium tuberculosis. The concept of exponential growth (and the short doubling time of some organisms) explains how very large numbers of bacteria can occur in short periods of time. For example, 1 E. coli can produce over 1 million in about 7 hours. 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 phases. Figure 3–1 illustrates the typical phases of a standard growth curve when a small number of bacteria are inoculated into a liquid medium and the progeny are counted at frequent intervals.

1. Lag phase: Metabolic activity occurs but cells do not divide. This can last for a few minutes up to many hours.

2. Log (logarithmic or exponential) phase: 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 when they are actively dividing.

3. Stationary phase: Nutrient depletion or toxic products cause growth to slow until the number of new cells produced balances the number of cells that die (also called steady state).

4. Death phase: A decline in the number of viable bacteria.

###### FIGURE 3–1

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; 1992.)

## OBLIGATE INTRACELLULAR GROWTH

Most human bacterial pathogens can be cultivated on artificial media in the laboratory. The medium is typically composed of purified chemicals such as sugars, amino acids, and essential salts, and often contains sheep’s blood, which supplies other essential nutrients.

However, certain human bacterial pathogens, notably Chlamydia and Rickettsia (see Chapters 25 and 26, respectively) and Ehrlichia...

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