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CHAPTER 10 BACTERIAL REPRODUCTION AND GROWTH OF MICROORGANISMS
FIG. 10-18Cells respond to osmotic pressure. In hypotonic solutions, cells may burst. reach a maximum. Although light intensities above this level do not result in further increases in the rates of photosynthesis, light intensities below the optimal level result in lower rates of photosynthesis. The wavelength of light also has a marked effect on the rates of photosynthesis. Different photosynthetic microorganisms use light of different wavelengths. For example, anaerobic photosynthetic bacteria use light of longer wavelengths than eukaryotic algae are capable of using. Many photosynthetic microorganisms have accessory pigments that enable them to use light of wavelengths other than the absorption wavelength for the primary photosynthetic pigments. The distribution of photosynthetic microorganisms in nature reflects the variations in the ability to use light of different wavelengths and the differential penetration of different colors of light into aquatic habitats. The rate of photosynthesis is a function of light intensity and wavelength. Exposure to visible light can also cause the death of bacteria (FIG. 10-19). Exposure to visible light can lead to the formation of singlet oxygen, which can result in the death of bacterial cells. Some bacteria produce pigments that protect them against the lethal effects of exposure to light. For example, yellow, orange, or red carotenoid pigments interfere with the formation and action of singlet oxygen, preventing its lethal action. Bacteria possessing carotenoid pigments can tolerate much higher levels of exposure to sunlight than nonpigmented microorganisms. Pig-1 mented bacteria often grow on surfaces that are exposed to direct sunlight, such as on leaves of trees. Many viable bacteria found in the air produce colored pigments. FIG. 10-19Pigmentation is important in the ability of bacteria to survive exposure to light. SUMMARY Bacterial Reproduction (pp. 287-288)Binary Fission (p. 287) • Binary fission is the normal form of asexual bacterial reproduction and results in the production of two equal-size daughter cells. Replication of the bacterial chromosome is required to give each daughter cell a complete genome. • A septum or crosswall is formed by the inward movement of the plasma membrane, separating the two complete bacterial chromosomes in an active protein-requiring process, physically cutting thechro-mosomes apart and distributing them to the two daughter cells. Cell division is synchronized with chromosome replication. Alternate Means of Bacterial Reproduction (pp. 287-288) • Other modes of bacterial reproduction are predomi Bacterial Spore Formation (p. 288) • Sporulation results in the formation of specialized re Bacterial Growth (pp. 288-291) • Binary fission leads to a doubling of a bacterial popu Generation Time (pp. 288-290)
• The time required to achieve a doubling of the population size is known as the generation time or doubling time. • The generation time is a measure of bacterial growth rate. • The generation time of a bacterial culture can be expressed as: division; there is no increase in cell numbers. During the log phase the logarithm of bacterial biomass increases linearly with time; this phase determines the generation or doubling time. Bacteria reach a stationary phase if they are not transferred to new medium and nutrients are not added; during this phase there is no further net increase in bacterial cell numbers and the growth rate is equal to the death rate. The death phase begins when the number of viable bacterial cells begins to decline. Batch and Continuous Growth (p. 291) • In batch cultures, bacteria grow in a closed system to Bacterial Growth on Solid Media (p. 291) • On solid media, bacteria do not disperse and so nu Enumeration of Bacteria (pp. 292-296) Viable Count Procedures (pp. 292-294) • Numbers of bacteria are determined by viable plate Direct Count Procedures (pp. 294-295) • Bacteria enumerated by direct counting procedures Most Probable Number (MPN) Procedures (pp. 295-296) • In most probable number enumeration procedures, Factors Influencing Bacterial Growth (pp. 296-306) • Environmental conditions influence bacterial growth Temperature (pp. 296-301) • There are maximum and minimum temperatures at Date: 2015-02-28; view: 1149
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