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Microbial Physiology

Dr. Ghada Younis
2018


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Oxygen requirements

4-20
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• Oxygen is a very reactive molecule and can affect cells in several ways. The effect of oxygen is often determined using thioglycollate broth, a special medium that contains a reducing agent (thioglycollate) that removes oxygen so that a gradient occurs within the tube.
• Obligately aerobic bacteria can obtain energy only through aerobic respiration and have to have oxygen available. Thus, they will grow only at the surface of thioglycollate broth.
• Obligately anaerobic bacteria die in the presence of oxygen and can only grow at the bottom of thioglycollate broth. Some anaerobes are so sensitive to oxygen that even thioglycollate broth is not anoxic enough to provide suitable anaerobic conditions.
• Microaerophiles require oxygen for growth but the 20% in air is too toxic. As a result, they grow near the top but beneath the surface of thioglycollate broth where the oxygen concentration is typically 4 – 10%.
• Facultative anaerobes can use oxygen for aerobic respiration but can switch to fermentative metabolism in the absence of oxygen. As a result, they will grow throughout thioglycollate broth. (Heavier growth at top.)
• Aerotolerant anaerobes are anaerobic bacteria that can grow in the presence of air. (Not shown in diagram.)



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Binary fission

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Figure 4.2

• Prokaryote cells grow by increasing in cell number (as opposed to increasing in size).
• Replication is by binary fission, the splitting of one cell into two
• Therefore, bacterial populations increase by a factor of two (double) every generation time.

Principles of Bacterial Growth

Growth can be calculated
Nt = N0 x 2n
(Nt ) number of cells in population
(N0 ) original number of cells in the population
(n) number of divisions
Example
N0 = 10 cells in original population
n = 12
4 hours assuming 20 minute generation time
Nt = 10 x 212
Nt = 10 x 4,096
Nt = 40,960



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