Download Bacteria

2/6/11
Life and Diversity: Bacteria
Dr. Waggoner
BIOL 1400
Most bacteria, like most archaea, are at most a few
microns (millionths of a meter) in size. Even under the
best light microscopes, most bacteria look like dots, and
can’t be seen very well without staining.
We discussed archaea
(top) and bacteria
(bottom), which are
both prokaryotes:
cells without nuclei.
Exceptions to this rule are the cyanobacteria, a.k.a.
“blue-green pond scum”, whose cells are much
larger. Note that there’s not much detail visible
inside; these are still prokaryotes. . . 1
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Electron
microscopes,
however, have
revealed much of
what we know
about bacterial
structure. This
cross-section
through a
bacterium shows
that there’s not an
awful lot of
complex parts
inside it. . . .
A few bacteria, called rickettsias, have no cell wall.
(Rickettsias are of interest in part because they include
organisms that cause typhus, Rocky Mountain spotted
fever, and a few other diseases.)
Surrounding the cell is
a flexible cell
membrane, and
usually a stiffer cell
wall just outside the
membrane. Cell wall
composition is
different in different
types of bacteria, but
usually includes at
least some of a
protein-sugar
compound called
peptidoglycan.
Outside of the cell wall there may be a layer of slime, the
capsule (which can be important in determining whether the
bacterium causes infection or not). These capsules are
visible under the light microscope.
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Reproduction is typically by fission (splitting in
two). This is a cross-section through a bacterium
caught in the act of fission . . .
. . . and here’s the happy couple afterwards. Note that a
form of sexual reproduction can also take place.
Two clumps
of DNA
Furrow where
cells are pinching in two
Some bacteria are able to swim, with long,
threadlike, corkscrew-shaped flagella.
Finally, some bacteria can form resistant
endospores when life gets rough (seen here as
bright areas within the cells).
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Endospores are extremely
tough and thick-walled
capsules that can
withstand almost any
conditions. When
conditions improve, the
endospore “comes back to
life”. (Since endospores
can resist heat, some
bacteria can survive the
canning process and grow
in canned foods,
potentially causing food
poisoning.)
These are bacilli, or rodshaped bacteria. . .
Classifying bacteria
can be a complex
business, but it’s often
useful to use the shape
of the cells as one
criterion. These are
cocci, or dot-shaped
bacteria. . .
And these are
spirilla (spiralshaped bacteria)
and a vibrio (a
comma-shaped or
bean-shaped
bacterium).
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Very long spiralshaped bacteria are
known as
spirochaetes. The
bacterium that
causes Lyme
disease (that
disease you can
catch from tick
bites) is a
spirochaete. So is
the bacterium that
causes syphilis.
Bacteria can further
be identified by how
they group together
during growth. These
are cocci growing in
bunches, or
staphylococci. . .
. . . these are cocci
growing in pairs, or
diplococci. . . . . . and these are
cocci growing in
chains, or
streptococci.
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How bacteria make you sick
WHO CARES?
• Bacterial shape, size, growth form, and cell
wall structure are all used to identify
pathogens (disease-causing agents) in
human patients, animals, plants, food,
water, sewage, and so on.
The 19th-century
German bacteriologist
Robert Koch came
up with rigorous
experimental
procedures for
determining whether
a bacterium causes a
particular disease.
• A few bacterial diseases: Syphilis, gonorrhea, stomach
ulcers, cholera, botulism, bubonic plague, Lyme
disease, legionnaire's disease, tuberculosis, strep
throat, diphtheria, impetigo, leprosy. . . • Often, it's not so much the bacteria themselves that
cause illness, but toxins that the bacteria produce. – Exotoxins are released by bacterial cells (examples: tetanus,
botulism, staph, diphtheria)
– Endotoxins are certain components of bacterial cell walls
(example: salmonella)
Koch’s Postulates
• The same bacteria must be present in every human
or animal with a particular disease.
• The bacterium must be grown in the laboratory in
a pure culture -- one with no other cells present.
• The disease must appear in experimental animals
when the cultured bacteria are injected into them.
• The same bacteria must be isolated from the
experimental animals after they get the disease.
Robert Koch (1843-1910)
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Important Disclaimer
Koch used these postulates to
pinpoint the bacteria that cause
anthrax (his drawings of
anthrax bacteria are shown at
left), cholera, tuberculosis, and
others—which led to many
treatment and prevention
methods, and which won him a
Nobel Prize in 1905. . .
Beneficial bacteria
• Escherischia coli bacteria in your intestines help you
digest; they also make vitamin K and vitamin B12.
• Bacteria living inside the roots of plants, such as
alfalfa, take up nitrogen gas from the air and convert
it into a form the plant can use (nitrates)
• A few bacteria produce antibiotic drugs, such as
streptomycin and nocardicin.
• Bacteria used in the food industry convert milk to
buttermilk and yogurt, and wine to vinegar. They
also put the holes (and the taste) in Swiss cheese.
• Bacteria don’t only cause diseases—in fact, the
overwhelming majority of bacteria are completely
harmless
• Bacteria are everywhere—water, air, soils, plant and
animal bodies. . .
– Many bacteria require oxygen to live; these are called
aerobes.
– Other bacteria do not need oxygen, or may even be poisoned
by oxygen. These are anaerobes. – Many bacteria are saprotrophic—they break down dead
organisms, causing decay and releasing nutrients back into
the environment.
The lumps on these
soybean roots are
formed by bacteria that
live inside the plant's
roots—which take
nitrogen gas and
convert it into a form
that the plant can use.
This is common in
legumes (beans, peas,
clover, alfalfa, etc.)
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Beneficial bacteria
• Bacteria break down wastes in sewage treatment and
in septic tanks.
• Strains of bacteria exist that can feed on petroleum.
These have been used to clean up oil spills. – Others can extract metals from mining waste, and are
becoming used in mining and in environmental clean-up.
• As a byproduct of photosynthesis, blue-green
bacteria, or cyanobacteria, produce much of the
oxygen that we and other organisms breathe.
Marine bacteria such as
this recently discovered
species, Alcanivorax
borkumensis, are capable
of breaking down crude
oil. They exist around
natural crude oil seeps on
the seafloor, but become
extremely abundant in the
wake of disastrous oil
spills. It’ll probably take
a while, but bacteria like
this will eventually clean
up the Gulf of Mexico. . . 8