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
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
maintained.

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.

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