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The
standard menu of prokaryotic shapes may seem boring, but they
really
perform specific functions. The underlined types are those we
hear
about most often, perhaps you’ve heard of “bacillus” more often
called
“rod.” Bacillus is actually the name of one genus of rod-shaped
bacteria,
but the name has taken over in many circles, like Kleenex for
facial
tissue. Most likely, all the forms evolved from rods, and you can
see how this might be possible from the cartoon. |
Question of the Day –
What shapes can prokaryotes take and are their shapes important?
We are taught in school that bacteria have three shapes; spheres,
rods, and spirals, but there are actually many more. The second part of our
question is just as important to think about. Does it matter what shape a
bacterium takes? Do you really think it’s random… really? After all the things
we have discussed in this blog?
A 2007 paper summarized the evidence for why scientists
believe the morphology (shape) of bacteria must be important. Here's a brief
summary. One - genera of bacteria will always show the same subset of shapes.
Why spend the energy to maintain the genetic blueprint if it isn’t important?
We have talked before about how genes that are not necessary are allowed to
drift, but morphology genes don’t drift.
Two - changes in environmental conditions or pressures will
bring changes in the morphology of many prokaryotes. What is more, the same
change will be bring the same morphology alteration again and again. This
implies both regulation and specific functions for different shapes. Shape must
be important if it is worthy of controlled regulation.
And three - archaea and eubacteria are very different, as
different as you and a pine tree, but they tend to fall within the same types
of morphologies. Each representative morphology must be adaptive (important for
survival and propagation) if they turn up again and again in very unrelated
organisms.
This and other papers by Dr. Young also discuss the ways
morphology confer advantages. Our morphology discussion should also include size. Small
bacteria usually have shorter generation times. This is important because
faster developing bacteria usually out-compete larger ones for limited resources
and nutrients. Therefore, most bacteria are very small.
The typical classroom answer for small size is that bacteria
do not have intracellular transport systems, so all movement of nutrients and
important molecules must be by diffusion. A big cell means too slow a transit
time and death. Also, being small is a good way to maximize surface area to
volume, so lots of room is given for possibly contacting food, while keeping
diffusion time fast.
These are valid reasons to be small, but size is just as
important in reducing the chances of being eaten. There probably hundreds of
thousands of different single-celled eukaryotes that feed exclusively or mostly
on bacteria. But bacteria are cannibals as well. They'll eat other bacteria,
and if times are bad enough, may feed on their own kind.
To avoid being the midnight snack of some protozoan, bacteria
have several choices. You can be small and fast, or you might opt to become huge -
too large to ingest or even be recognizes as food.
 |
There
is a lot going on in this collage. On the left is the aggregates that
cocci
can form based on the plane in which they divide. The only form
possible
for rods is the strepto- form since they always divide through
their
short side. The pictures on the right are to illustrate the size
difference
between E. coli and T. nambibiensis. If E.coli were the size of
a tic tac candy, the crater seen below could not hold T. nambibiensis. |
But back to shape - what might different shapes do for
different bacteria? Let’s look at some amazing prokaryotic shapes in terms of
several factors: nutrient acquisition; predators; cell division; attachment;
dispersal; motility; and differentiation of function.
Coccus
Cocci (the plural of coccus, from the Greek kokkus = berry) are round bacteria. Spherical
is a safe shape since it gives the maximum surface area for a given volume.
However, spherical doesn’t necessarily mean small. Thiomargarita namibiensis is a spherical bacterium, but it is the
second largest prokaryote we know of. If an E.
coli cell was the size of a tic tac, T.
namibiensis would have a diameter a bit larger than the Barringer Meteor
crater in Arizona (see picture above).
Rod
The rod is probably the oldest prokaryotic morphology. It is most
probable that cocci were short rods that kept getting shorter, and that other
shapes we will talk about are also modifications of rods. So give the rods (of
which E. coli is one) the respect
they deserve – it may seem mundane later when we talk about weird shapes, but
the rod is the mother of all shapes.
Rods show that motility comes into play as a reason for
shape. The rod shape, longer than wide, is the fastest mover in response to
chemical signals (chemotaxis, chemo = chemical and taxis = arrangement), the chemical trail
left by a potential meal for example. Becoming longer and thinner is also a good way to
increase apparent size (reduce predation) and provide more surface area (for
food collection).
Spirals
Spiral shaped bacteria are faster through viscous fluids –
so this shape is probably an adaptation to allow movement in different fluids.
Many spiral bacteria live in environments thicker than water, so moving faster
than predators would be important.
The spiral shape of spirilla is usually thicker and flatter
than that of spirochetes, and another difference is that spirochetes often have
different attachments for their flagella (whip-like oars for movement).
Predation may also play a different role in spiral shape
development. Arthrospira platensis (a cyanobacteria) grows as a spiral. It also known as spirulina, a potential
superfood, but promoters usually say it’s a blue-green algae, not a bacterium.
Spirulina is eaten by a
protist that can turn left on its long axis up to six times to ingest the A.
platensis. Low and behold - A. platensis can reverse it spiral
direction in the face of predation so that the ciliate would have to spin right
to eat it, and it can’t do that.
 |
On
the left is Caulobacter
crescentus, a crescent shaped prokaryote. It
takes
two forms, a swarming crescent with flagella for times of
plenty
and a stalked crescent for times when food is scarce. In some
parts
of the image, you can see the stalk. Atopobium rimae is on the
right,
a coccobacillus. A. rimae is a
constituent of normal oral flora,
but
can cause periodontal disease as well. Its looks remind me of a
microscopic Easter egg hunt. |
I call these bacteria elliptical; one nice example is Atopobium rimae. They look like
footballs, not quite a rod (bacilli are one genus of rod shaped bacteria), and
not yet reduced to a sphere.
Many coccobacilli are pathogenic, including the organisms that
cause chancroid STDs, brucellosis, pneumonia, infectious blindness, bacterial
flu, and whooping cough. However, a link between shape and disease causation
escapes me. I haven’t found any evidence that someone has even asked the
question. Maybe you can.
Crescent shaped
These are rod shaped bacteria that have become curved.
Vibrio bacteria are an important group of crescents (named because they looked
like they vibrated as they moved)….. oh, and because they cause a lot of
disease.
The crescent shape can be important for movement. Vibrio alginolyticus swims forward just
fine, but when it encounters a flat surface – an area where food might be gathered
– it swims backward. Its crescent shape keeps it bumping into the flat surface.
This keeps it longer in the area of food. In some crescents, a mutation making
them straight means that they lose their motility and ability to find food, so
it must be important.
Triangular and square
Yes, there are prokaryotes that look very much like triangles
or squares. I am listing them together because the thing they have most in come
is that they are usually halophiles
(halo = salt, and philic = loving) or are halotolerant.
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Both
of the bacteria above are halophilic, meaning they love salt.
Haloarcula japonica (left) and Holoquadratum walsbyi (right)
for
perfectly shaped to form contiguous sheets of cells. This
helps
them float parallel to the surface of the water they live in.
These
are archaeal extremophiles, but it is interesting that
archaea and bacteria use the same sets of shapes.
|
The triangular example I have for you is Haloarcula japonica, so named because it was discovered in a Japanese salt evaporation field. Another member of the same genus is H. quadratum, which you might guess, is square.
But it isn’t just this genus that take on definite geometric
shapes, there is another salt-loving arachaean called Haloquadratum walsbyi that is also square. Mind you, these are not
cubiodal bacteria, and the H. japonica
is not a triangular prism. They are very flat; H. walsbyi, for example, is 5 µm square but only 0.1 µm thick. The
same can be said for the triangular H.
japonica.
They all tend to grow as flat masses, like little floating
mosaics. Their flat shapes keep them buoyant and floating parallel to the
water’s surface. Their geometry then provides the largest surface toward the
sun, in order to pick up the most heat and energy. Sounds logical to me.
Star
 |
These
are my favorites, the stars of the bacterial world. The far left is
Stella vacuolata. Those bubbles in the
middle are gas – yes, bacteria get
gas
too. The middle photomicrograph is Prosthecomicrobium.
Prostheacae
are the arms, and are right in the name, even though these
are
very short prosthecae. On the right is Ancalomicrobium
adetum. Its
prosthecae
are much longer and are used to deter predators, amongst
other
things. There is even one very long rod bacterium that has a
star cross section. |
There are also bacteria that have projections from their
cell body that make them look like fireworks as they explode; see the picture
of Ancalomicrobium adetum. The
projections are called prosthecae
(Greek for appendage), and can serve
many functions. They can increase surface area without increasing mass for better
diffusion. They can make a bacterium large enough to not be eaten. They may
also catch more water, to help non-motile bacteria be moved along by the
current.
Other
We have only touched the surface of the morphologies that
prokaryotes can assume. There are others that look like Y’s (bifid bacteria),
some that look like connected lollipops, some that look like segmented worms, and
at least one that builds a net of connected tubules near black smokers at the
bottom of the ocean (Pyrodictum abyssi).
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Among
the many other shapes possible for bacteria, the bifids are
interesting
(left). Certain proteins are produced only in the ends of
the
bacteria, and if they need more of that protein, especially for
attachment,
one way to get it is to have more ends. On the
right
is an archaea found at the bottom of the ocean. The fine lines
are
actually tubular projections with fine nets of bacteria cell
inside.
The cell bodies are the nodular areas seen nonsymmetrically.
|
A 2011 paper shows that zinc metal is essential for proper
morphology. Zinc is an important part of several proteins that control which
DNA is read to make proteins (zinc finger transcription factors), so this is
evidence that discrete controls are in place to define a bacterium’s
morphology.
A 2013 study shows that E. coli rod shape is determined
mostly by its cell wall. If the cell wall was removed, it took 4-6 generations
for the rod shape to be recovered. If mutations in certain lipoproteins or
penicillin binding proteins were present, the bacteria progeny would always
remain spherical. These genes are not even used in producing the cell wall, so
it is apparent that many genes are needed just to maintain cell shape. My current research concerns the ability to bend rod bacteria into tiny balloon animals.
Next week we will start our series of exceptions and core concepts for the year. We begin by looking at the elements of life - there's more than you think, and then we'll look at the four types of biomolecules.
Young KD (2007). Bacterial morphology: why have different shapes? Current opinion in microbiology, 10 (6), 596-600 PMID: 17981076
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