Page 26 - Review of Medical Microbiology and Immunology ( PDFDrive )

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Growth mebooksfree.com
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mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com C H 3 E R mebooksfree.com
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A
P
T

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CHAPTER C ONTENT S

Growth Cycle
Obligate Intracellular Growth
Pearls
Aerobic & Anaerobic Growth
Self-Assessment Questions
Practice Questions: USMLE & Course Examinations
Fermentation of Sugars

GROWTH CYCLE c
mebooksfree.com mebooksfree.com 1 2 0 mebooksfree.com FIGURE 3–1 b Growth curve of bacteria: a, lag phase; b, log mebooksfree.com
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Log number of cells
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
d
undergo exponential growth (logarithmic growth). The
concept of exponential growth can be illustrated by the fol-
lowing relationship:
a
Time
8
4
2
Number of cells
16
4
1
2
3
2
2
2
Exponential
2
Thus, 1 bacterium will produce 16 bacteria after 4
from Joklik WK et al. Zinsser Microbiology. 20th ed. Originally published by Appleton
generations.
& Lange. Copyright 1992, McGraw-Hill.)
The doubling (generation) time of bacteria ranges from phase; c, stationary phase; d, death phase. (Reproduced with permission
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mebooksfree.com mebooksfree.com mebooksfree.com log phase is also known as the exponential phase. mebooksfree.com
as little as 20 minutes for Escherichia coli to as long as 18 hours
this phase because the drugs are effective when cells are
for Mycobacterium tuberculosis. The exponential growth
making peptidoglycan (i.e., when they are dividing). The
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
(3) The stationary phase occurs when nutrient depletion
in about 3 hours and over 1 million in about 7 hours. The
or toxic products cause growth to slow until the number of
new cells produced balances the number of cells that die,
doubling time varies not only with the species, but also
resulting in a steady state. Cells grown in a special apparatus
with the amount of nutrients, the temperature, the pH, and
other environmental factors.
called a “chemostat,” into which fresh nutrients are added and
The growth cycle of bacteria has four major phases. If a
small number of bacteria are inoculated into a liquid nutri-
remain in the log phase and do not enter the stationary phase.
ent medium and the bacteria are counted at frequent inter-
(4) The final phase is the death phase, which is marked
by a decline in the number of viable bacteria.
vals, the typical phases of a standard growth curve can be from which waste products are removed continuously, can
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mebooksfree.com mebooksfree.com mebooksfree.com Most bacterial pathogens of humans are capable of growing 15 mebooksfree.com
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demonstrated (Figure 3–1).
(1) The first is the lag phase, during which vigorous
OBLIGATE INTRACELLULAR GROWTH
metabolic activity occurs but cells do not divide. This can
last for a few minutes up to many hours.
(2) The log (logarithmic) phase is when rapid cell divi-
on artificial media in the clinical laboratory. The term,
sion occurs. β-Lactam drugs, such as penicillin, act during
artificial, means that the medium is composed of purified
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