| Bacteria |
Bacteria (singular bacterium) are minute organisms that often consist of single cells, like the rod-shaped cell
shown above which is about one thousandth of a millimetre (mm) in diameter (that is one micrometre or about a
tenth the diameter of one of your cells or about one thousandth the volume of a human cell), but may exist as
filaments (chains of cells) or cubes of cells or as large 'slime cities' or biofilms that may be easily visible to the
naked eye. Many bacteria are now known to alternate between single-celled swarmers that disperse to find new
habitats, and colonies or 'cities' held together by slime. The pictures above are 3D computer models produced in
Pov-Ray.
Bacteria come in a variety of shapes including: rods (bacilli), spheres (cocci), squares, star-shapes, coma-shapes
and long corkscrew-shapes.
shown above which is about one thousandth of a millimetre (mm) in diameter (that is one micrometre or about a
tenth the diameter of one of your cells or about one thousandth the volume of a human cell), but may exist as
filaments (chains of cells) or cubes of cells or as large 'slime cities' or biofilms that may be easily visible to the
naked eye. Many bacteria are now known to alternate between single-celled swarmers that disperse to find new
habitats, and colonies or 'cities' held together by slime. The pictures above are 3D computer models produced in
Pov-Ray.
Bacteria come in a variety of shapes including: rods (bacilli), spheres (cocci), squares, star-shapes, coma-shapes
and long corkscrew-shapes.
Above a single-celled helical or corkscrew-shaped bacterium, called a spirochaete. This resembles the bacteria
that live in thick mud, and those that cause diseases in humans such as Lyme's disease and syphilis. As they
move they rotate like a corkscrew, and this allows them to bore their way into thick mud, or into your tissues!
These types can be quite long cells, up to about one fifth of a millimetre in length, but it is quite narrow.
that live in thick mud, and those that cause diseases in humans such as Lyme's disease and syphilis. As they
move they rotate like a corkscrew, and this allows them to bore their way into thick mud, or into your tissues!
These types can be quite long cells, up to about one fifth of a millimetre in length, but it is quite narrow.
Although not as complex as animal or plant cells, bacteria still contain machinery consisting of thousands of
working parts, but you need to zoom in to the nanometre scale (one nanometre, 1 nm, is one millionth of a
millimetre) to see this complex machinery!
Let's take a slightly closer look at a section through a bacterium (click to enlarge):
working parts, but you need to zoom in to the nanometre scale (one nanometre, 1 nm, is one millionth of a
millimetre) to see this complex machinery!
Let's take a slightly closer look at a section through a bacterium (click to enlarge):
Above: a section through a single-celled rod-shaped bacterium. Shown in purple is the tough capsule or wall
of the bacterium (this is a type of bacterium known as Gram positive, but there are other structural types with
different wall arrangements). This capsule protects the bacterium. Shown in light blue is the peripheral
cytoplasm which is bounded by a cell membrane which you can just see underneath the capsule. The
cytoplasm consists mostly of water and proteins (including ribosomes that manufacture other proteins). This
membrane is similar to the membrane that forms the 'skin' of your own cells, and is also made up of mostly
fats and proteins. The central swirly region is the nucleoplasm. The red threads are the DNA, because the
nucleoplasm contains the genes and forms the central computer of the bacterial cell. (Unlike animal cells,
there is no nuclear membrane). Note that the bacterial cell has fewer internal compartments than the animal
cell.
Now, what about the long black coils? These are the engines of the bacterium and are called flagella
(singular flagellum) which means 'whip' in Latin, which is a bit of a misnomer for bacteria because these
flagella do not lash about but rotate like corkscrews. These motors are very fast, but not as good as drilling
into thick materials like what the spirochaete can do by turning its whole body into a corkscrew!
If you look carefully at the roots where the flagella join the cell body, then you will see what look like little
wheels, indeed that is exactly what they are, but instead of driving the cell along as do the wheels on a car
drive the car along, they drive the flagella and cause them to rotate like corkscrews, propelling the cell along.
You may notice the flexible hook that joins the flagellum to this system of wheels. We shall have a closer look
at these wheels and hooks shortly. Note then, that bacteria 'invented' wheels billions of years ago! Click here
to see how these wheels work.
Why do bacteria use these 'corkscrews' to get about when larger animal cells use different propulsion
systems (they may crawl like the amoeba or use beating hairs called cilia or lash a long whip-like flagellum
which does not rotate as does the flagellum in bacteria but lashes from side-to-side)? Well, when one is as
small as a bacterium, water starts to behave oddly - it becomes thick and sticky (it has a high relative
viscosity due to the system having a very low Reynold's number) so the bacterium is effectively swimming
through treacle. Now imagine trying to swim through treacle with a paddle, push it to the left and you move
forward a bit, but push it back to the right and you move backwards a bit - in short you get nowhere, you just
oscillate about a bit!
This does not happen in water because the water flows away, so you are not pushing the same water
backwards and forwards. This is expressed by what we call time-reversibility. Reverse the motion of someone
swimming with a paddle and you can tell the difference because they go backwards! Now do the same to
someone paddling in treacle and you can't tell the difference. Now, to move forwards a swimmer has to break
this time-reversal symmetry so that the video looks different when played backwards. A corkscrew is ideal for
this, because a corkscrew is twisted either clockwise or anticlockwise. Reverse a corkscrew and it does make
a difference - try turning the corkscrew in the opposite direction next time you open a bottle of wine. This is
why a corkscrew can drill through thick materials like cork. Now, by using corkscrew-like engines the
bacterium overcomes the problem of its tiny size which makes water behave like treacle, and it easily ploughs
forward.
Before I get into more technical details of these rotary engines, let me just stress the importance of bacteria.
Most people think of them as germs, but the vast majority are not only harmless to humans, but beneficial.
The Earth's ecosystems would grind to a halt without bacteria, in fact bacteria are the foundations of life on
Earth, kill everything else but the bacteria and life continues, kill all the bacteria and the Earth dies. Bacteria
recycle nutrients into the biosphere, they break-down dead animal and plant remains and do all sorts of
wonderful chemistry. Some do cause disease, however. Examples include most bouts of food-poisoning,
tuberculosis, syphilis, hospital superbugs and others. However, they do not cause all infectious diseases!
Viruses, fungi, worms and other organisms also cause many diseases.
Killing all bacteria on Earth is easier said than done of course! Bacteria are extremely tough, partly because
they can reproduce so rapidly, the fastest double their number every 10-20 minutes in optimum conditions,
and partly because they are so adaptable - they evolve rapidly when they need to.
Did you know?
If a single bacterium (weighing ~10^-15 kg) doubled every 20 minutes, then the total population of bacteria
would weigh the same as about 10 000 Earth's after two days! Clearly this does not happen in Nature
because conditions for bacterial growth are seldom optimum - the bacteria may run out of food for one thing.
The appearance of superbugs that are resistant to almost every known drug is testament to the ability of
bacteria to undergo rapid genetic changes and evolve and adapt to changes in their environment. There are
even bacteria that grow inside nuclear reactors! Humans often boast at how their nuclear weapons could
sterilise their planet, but they can't, they would just kill all the big creatures like humans. Sometimes it pays to
be small - smaller creatures require fewer nutrients and so can reproduce rapidly. Bacteria grow so fast
because they are small and so can easily absorb nutrients and get the food to all their parts, in contrast,
larger animals require complex circulatory systems (heart and blood) and this limits the rate at which food
can be absorbed. By reproducing faster, smaller creatures evolve faster - imagine how humans have
changed in the past 10 000 generations (say 300 000 years), well bacteria pass through 10 000 generations
in as little as 20 weeks!
Click here to learn about bacterial slime cities!
of the bacterium (this is a type of bacterium known as Gram positive, but there are other structural types with
different wall arrangements). This capsule protects the bacterium. Shown in light blue is the peripheral
cytoplasm which is bounded by a cell membrane which you can just see underneath the capsule. The
cytoplasm consists mostly of water and proteins (including ribosomes that manufacture other proteins). This
membrane is similar to the membrane that forms the 'skin' of your own cells, and is also made up of mostly
fats and proteins. The central swirly region is the nucleoplasm. The red threads are the DNA, because the
nucleoplasm contains the genes and forms the central computer of the bacterial cell. (Unlike animal cells,
there is no nuclear membrane). Note that the bacterial cell has fewer internal compartments than the animal
cell.
Now, what about the long black coils? These are the engines of the bacterium and are called flagella
(singular flagellum) which means 'whip' in Latin, which is a bit of a misnomer for bacteria because these
flagella do not lash about but rotate like corkscrews. These motors are very fast, but not as good as drilling
into thick materials like what the spirochaete can do by turning its whole body into a corkscrew!
If you look carefully at the roots where the flagella join the cell body, then you will see what look like little
wheels, indeed that is exactly what they are, but instead of driving the cell along as do the wheels on a car
drive the car along, they drive the flagella and cause them to rotate like corkscrews, propelling the cell along.
You may notice the flexible hook that joins the flagellum to this system of wheels. We shall have a closer look
at these wheels and hooks shortly. Note then, that bacteria 'invented' wheels billions of years ago! Click here
to see how these wheels work.
Why do bacteria use these 'corkscrews' to get about when larger animal cells use different propulsion
systems (they may crawl like the amoeba or use beating hairs called cilia or lash a long whip-like flagellum
which does not rotate as does the flagellum in bacteria but lashes from side-to-side)? Well, when one is as
small as a bacterium, water starts to behave oddly - it becomes thick and sticky (it has a high relative
viscosity due to the system having a very low Reynold's number) so the bacterium is effectively swimming
through treacle. Now imagine trying to swim through treacle with a paddle, push it to the left and you move
forward a bit, but push it back to the right and you move backwards a bit - in short you get nowhere, you just
oscillate about a bit!
This does not happen in water because the water flows away, so you are not pushing the same water
backwards and forwards. This is expressed by what we call time-reversibility. Reverse the motion of someone
swimming with a paddle and you can tell the difference because they go backwards! Now do the same to
someone paddling in treacle and you can't tell the difference. Now, to move forwards a swimmer has to break
this time-reversal symmetry so that the video looks different when played backwards. A corkscrew is ideal for
this, because a corkscrew is twisted either clockwise or anticlockwise. Reverse a corkscrew and it does make
a difference - try turning the corkscrew in the opposite direction next time you open a bottle of wine. This is
why a corkscrew can drill through thick materials like cork. Now, by using corkscrew-like engines the
bacterium overcomes the problem of its tiny size which makes water behave like treacle, and it easily ploughs
forward.
Before I get into more technical details of these rotary engines, let me just stress the importance of bacteria.
Most people think of them as germs, but the vast majority are not only harmless to humans, but beneficial.
The Earth's ecosystems would grind to a halt without bacteria, in fact bacteria are the foundations of life on
Earth, kill everything else but the bacteria and life continues, kill all the bacteria and the Earth dies. Bacteria
recycle nutrients into the biosphere, they break-down dead animal and plant remains and do all sorts of
wonderful chemistry. Some do cause disease, however. Examples include most bouts of food-poisoning,
tuberculosis, syphilis, hospital superbugs and others. However, they do not cause all infectious diseases!
Viruses, fungi, worms and other organisms also cause many diseases.
Killing all bacteria on Earth is easier said than done of course! Bacteria are extremely tough, partly because
they can reproduce so rapidly, the fastest double their number every 10-20 minutes in optimum conditions,
and partly because they are so adaptable - they evolve rapidly when they need to.
Did you know?
If a single bacterium (weighing ~10^-15 kg) doubled every 20 minutes, then the total population of bacteria
would weigh the same as about 10 000 Earth's after two days! Clearly this does not happen in Nature
because conditions for bacterial growth are seldom optimum - the bacteria may run out of food for one thing.
The appearance of superbugs that are resistant to almost every known drug is testament to the ability of
bacteria to undergo rapid genetic changes and evolve and adapt to changes in their environment. There are
even bacteria that grow inside nuclear reactors! Humans often boast at how their nuclear weapons could
sterilise their planet, but they can't, they would just kill all the big creatures like humans. Sometimes it pays to
be small - smaller creatures require fewer nutrients and so can reproduce rapidly. Bacteria grow so fast
because they are small and so can easily absorb nutrients and get the food to all their parts, in contrast,
larger animals require complex circulatory systems (heart and blood) and this limits the rate at which food
can be absorbed. By reproducing faster, smaller creatures evolve faster - imagine how humans have
changed in the past 10 000 generations (say 300 000 years), well bacteria pass through 10 000 generations
in as little as 20 weeks!
Click here to learn about bacterial slime cities!
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Coming soon - more bacteria and nanotechnology...
The diagram below shows some transmission electron micrographs of bacteria (taken by Bot). These are
bacteria that have been cut into sections some 70 millionths of a millimetre thick and viewed under a powerful
electron microscope. The bottom picture shows a close up view of one cell in the act of splitting into two new
daughter cells. This is how most bacteria reproduce. Can you identify the cell envelope (cell wall, which has 4
layers in this type of bacterium, which is called the Gram negative type), the ribosome-rich peripheral
cytoplasm and the central DNA-rich nucleoplasm?
bacteria that have been cut into sections some 70 millionths of a millimetre thick and viewed under a powerful
electron microscope. The bottom picture shows a close up view of one cell in the act of splitting into two new
daughter cells. This is how most bacteria reproduce. Can you identify the cell envelope (cell wall, which has 4
layers in this type of bacterium, which is called the Gram negative type), the ribosome-rich peripheral
cytoplasm and the central DNA-rich nucleoplasm?
Click here to see a labelled version of this diagram for the answers! Notice that the peripheral cytoplasm
is granular (due mostly to ribosomes) and in the central nucleoplasm you can see strands of DNA (with
attached proteins). Notice that the surface of this bacterium type appears somewhat 'hairy', actually in
life these 'hairs' absorb water and form a slimy outer layer. An additional and much thicker slime capsule
may also form a sheath around the cell. The scale refers to the enlarged image and shows that the
width of each bacterial cell is about one micrometre or one thousandth of a millimetre across. Notice that
even this high magnification does not reveal most of the details of the complex nano-scale machinery
that exists within the cell.
is granular (due mostly to ribosomes) and in the central nucleoplasm you can see strands of DNA (with
attached proteins). Notice that the surface of this bacterium type appears somewhat 'hairy', actually in
life these 'hairs' absorb water and form a slimy outer layer. An additional and much thicker slime capsule
may also form a sheath around the cell. The scale refers to the enlarged image and shows that the
width of each bacterial cell is about one micrometre or one thousandth of a millimetre across. Notice that
even this high magnification does not reveal most of the details of the complex nano-scale machinery
that exists within the cell.
Bacterial motility - learn about one of the smallest known electric motors! Motility in spirochaetes - learn about the bacterial inventions of the corkscrew and drill! Chemokinesis - learn how bacteria use cunning strategies to find their way around. Bacterial slime cities -learn about biofilms. Pili - bacterial appendages with diverse functions - see more bacterial nanotechnology - bridges, springs and electric wires. Coming soon - bacterial sensory systems. bacterial communication systems. tough materials: the bacterial cell wall. A strange experiment - can you solve the mystery of the anomalous mutations? Coming soon - bacterial power generators. |
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Download a pdf on bacteria: Prokaryotes_motility
Above: a simplified version of the bacterial cell structure diagram, showing the structures most
frequently encountered in elementary biology courses.
frequently encountered in elementary biology courses.
Above: another Pov-Ray model of a flagellated bacillus. The bacillus cell-body is essentially a
cylinder with hemispherical end-caps, as modeled here. This version has the flagella
undergoing changes. The flagella of many bacteria have two wavelength modes, one usually
around 1 micrometre wavelength, and the other double this at 2 micrometres - the flagella
can switch between these two modes. They can also flex when they disengage from the
flagellum-bundle as shown here. The flagella have been generated with certain random
parameters, creating a more natural look.
cylinder with hemispherical end-caps, as modeled here. This version has the flagella
undergoing changes. The flagella of many bacteria have two wavelength modes, one usually
around 1 micrometre wavelength, and the other double this at 2 micrometres - the flagella
can switch between these two modes. They can also flex when they disengage from the
flagellum-bundle as shown here. The flagella have been generated with certain random
parameters, creating a more natural look.
