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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.


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
nantly asexual, differing in how the cellular material
is apportioned between the daughter cells and
whether the cells separate or remain together as part
of a multicellular aggregation. Budding is character­
ized by an unequal division of cellular material. In
hyphae formation the cell elongates, forming rela­
tively long, generally branched filaments; crosswall
formation results in individual cells containing com­
plete genomes.

Bacterial Spore Formation (p. 288)

• Sporulation results in the formation of specialized re­
sistant resting cells or reproductive cells called
spores. Endospores are heat-resistant nonreproduc-
tive spores that are formed within the cells of a few
bacterial genera; cysts are dormant nonreproductive
cells sometimes enclosed in a sheath. Myxobacteria
form fruiting bodies within which they produce prog­
eny myxospores that can survive transport through
the air. Arthrospores are spores formed by hyphae
fragmentation that permit reproduction (increase in
cell number) of some bacteria.

Bacterial Growth (pp. 288-291)

• Binary fission leads to a doubling of a bacterial popu­
lation at regular intervals. The generation or doubling
time is a measure of the growth phase.

Generation Time (pp. 288-290)

Bacterial Growth Curve (pp. 291-292) • The normal growth curve for bacteria has four phases: lag, log or exponential, stationary, and death. During the lag phase, bacteria are preparing for re­production, synthesizing DNA and enzymes for cell

• 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
which new materials are not added. In continuous
culture, fresh nutrients are added and end products
removed so that the exponential growth phase is

Bacterial Growth on Solid Media (p. 291)

• On solid media, bacteria do not disperse and so nu­
trients become limiting at the center of the colony;
bacterial colonies have a well-defined edge. Each
colony is a clone of identical cells derived from a sin­
gle parental cell.

Enumeration of Bacteria (pp. 292-296)

Viable Count Procedures (pp. 292-294)

• Numbers of bacteria are determined by viable plate
count, direct count, and most probable number deter­
minations. In viable plate counts, serial dilutions of
bacterial suspensions are plated on solid growth
medium by the pour plate or surface spread tech­
nique, incubated, and counted. Since each colony
comes from a single bacterial cell, counting the colony
forming units, taking into account the dilution fac­
tors, can determine the original bacterial concentra­

Direct Count Procedures (pp. 294-295)

• Bacteria enumerated by direct counting procedures
do not have to be grown first in culture or stained.
Special counting chambers are often used.

Most Probable Number (MPN) Procedures (pp. 295-296)

• In most probable number enumeration procedures,
multiple serial dilutions are performed to the point of
extinction. Cloudiness or turbidity can be the crite­
rion for existence of bacteria at any dilution level; gas
production or other physiological characteristics can
be used to determine the presence of specific types of

Factors Influencing Bacterial Growth (pp. 296-306)

• Environmental conditions influence bacterial growth
and death rates. Each bacterial species has a specific
tolerance range for specific environmental parame­
ters. Changing environmental conditions cause popu­
lation shifts. Laboratory conditions can be manipu­
lated to achieve optimal growth rates for specific or­

Temperature (pp. 296-301)

• There are maximum and minimum temperatures at
which microorganisms can grow; these extremes of

Date: 2015-02-28; view: 704

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