Download 357 CHAPTER 21 Nucleoid . Plasmids . SPORES

THE NATURE OF BACTERIA
CHAPTER 21
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of ribosomes varies directly with the growth rate of the cell. Except for the functions associated
with the cell membrane, all of the metabolic reactions of the cell take place in the cytosol.
Nucleoid
The bacterial genome resides on a single chromosome (there are rare exceptions) and
typically consists of about 4000 genes encoded in one, large, circular molecule of doublestranded DNA containing about 5 million nucleotide base pairs. This molecule is more
than 1 mm long, and it therefore exceeds the length of the cell by about 1000 times. Tight
packing displaces ribosomes and other cytosol components, creating regions that contain a
chromosome, coated usually by polyamines and some specialized DNA-binding proteins.
The double-helical DNA chain is twisted into supercoils and attached to the cell membrane
and/or some central structure at a large number of points. This creates folds of DNA, each
of which is independently coiled into a tight bundle. Each nuclear body corresponds to a
DNA molecule. The number of nuclear bodies varies as a function of growth rate; resting
cells have only one, and rapidly growing cells may have as many as four.
The absence of a nuclear membrane confers on the prokaryotic cell a great advantage for
rapid growth in changing environments. Ribosomes can be translating mRNA molecules
even as the latter are being made; no transport of mRNA from where it is made to where it
functions is needed.
Circular chromosome of
supercoiled double-stranded DNA
Attached to cell membrane and
central structures
Plasmids
Many bacteria contain small, usually circular, covalently closed, double-stranded DNA
molecules separate from the chromosome. More than one type of plasmid or several copies
of a single plasmid may be present in the cell. Many plasmids carry genes coding for the
production of enzymes that protect the cell from toxic substances. For example, antibiotic
resistance is often plasmid-determined. Many attributes of virulence, such as production of
some pili and of some exotoxins, are also determined by plasmid genes.
Plasmids are small, usually circular,
double-stranded DNA molecules
SPORES
Endospores are small, dehydrated, metabolically quiescent forms that are produced by
some bacteria in response to nutrient limitation or a related sign that tough times are coming. Very few species produce spores (the term is loosely used as equivalent to endospores),
but they are particularly prevalent in the environment. Some spore-forming bacteria are of
great importance in medicine, causing such diseases as anthrax, gas gangrene, tetanus, and
botulism. All spore formers are Gram-positive rods. The bacterial endospore is not a reproductive structure. One cell forms one spore under adverse conditions (the process is called
sporulation). The spore may persist for a long time (centuries) and then, on appropriate
stimulation, give rise to a single bacterial cell (germination). Spores, therefore, are survival
rather than reproductive devices.
Spores of some species can withstand extremes of pH and temperature, including boiling water, for surprising periods of time. The thermal resistance is brought about by the
low water content and the presence of a large amount of a substance found only in spores,
calcium dipicolinate. Resistance to chemicals and, to some extent, radiation is aided by
extremely tough, special coats surrounding the spore. These include a spore membrane
(equivalent to the former cell membrane); a thick cortex composed of a special form of
peptidoglycan; a coat consisting of a cysteine-rich, keratin-like, insoluble structural protein; and, finally, an external lipoprotein and carbohydrate layer called an exosporium.
Sporulation is under active investigation. The molecular process by which a cell produces
a highly differentiated product that is incapable of immediate growth but able to sustain
growth after prolonged periods of nongrowth under extreme conditions of heat, desiccation, and starvation is of great interest. In general, the process involves the initial walling off
of a nucleoid and its surrounding cytosol by invagination of the cell membrane, with later
additions of special spore layers (Figure 21–11). Germination begins with activation by
heat, acid, and reducing conditions. Initiation of germination eventually leads to outgrowth
of a new vegetative cell of the same genotype as the cell that produced the spore.
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Endospores are hardy, quiescent
forms of some Gram-positives
Spore-forming allows survival
under adverse conditions
Resistance of spore is due
to dehydrated state, calcium
dipicolinate, and specialized coats
Germination reproduces cell
identical to that which was
sporulated
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PART I I I
PATHOGENIC BACTERIA
Cell division
Wall
Free spore
Exosporium
I
Spore coat
Axial filament
VII
Cortex Lysis of
formation
Core sporangium,
Plasma
spore
membrane DNA
Spore coat
liberation
VI
II
Completion of
Septum
coat synthesis,
formation
increase in
and
refractility and
forespore
heat resistance
development
Exosporium
III
V
Coat synthesis
Cortex
FIGURE 21–11. Stages of bacterial spore formation. (Reproduced
with permission from Willey J, Sherwood L, Woolverton C (eds). Prescott’s
Principles of Microbiology. New York:
McGraw-Hill; 2008.)
Engulfment of
forespore
IV
Cortex formation
BACTERIAL GROWTH AND METABOLISM
Growth requires metabolism,
regulation, and division by binary
fission
Growth of bacteria is accomplished by an orderly progress of metabolic processes followed
by cell division by binary fission. This requires metabolism, which produces cell material
from the nutrient substances in the environment; regulation, which coordinates the progress of the hundreds of independent biochemical processes in an orderly way; and, finally,
cell division, which produces two independent living units from one.
BACTERIAL METABOLISM
Many of the principles of metabolism are universal. This section focuses on the unique
aspects of bacterial metabolism that are important in medicine. The need to compare bacterial and mammalian pathways is muted by the fact that much of what we understand about
human metabolism is derived from work with Escherichia coli.
The broad differences between bacteria and human eukaryotic cells can be summarized
as follows:
Speed. Bacteria metabolize at a rate 10 to 100 times faster.
Versatility. Bacteria use more varied compounds as energy sources and are much more
diverse in their nutritional requirements.
Simplicity. The prokaryotic body plan makes it possible for bacteria to synthesize macromolecules in a streamlined way.
Uniqueness. Some biosynthetic processes, such as those producing peptidoglycan,
lipopolysaccharide, and toxins, are unique to bacteria.
Bacterial metabolism is highly complex. The bacterial cell synthesizes itself and generates
energy by as many as 2000 chemical reactions. These reactions can be classified according
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