 |
|
Nomads
are wanderers. They come in different flavors.
Hunter-gatherers
follow the animals as they graze in
different
places. Pastoral nomads have animal herds and
move
them around to where the grazing is best. But the
interesting
ones are the peripatetic nomads. These are
people
that move around within cities and other
populated
areas, often to sell services or trades. Romanis,
or
gypsies as they are sometimes called, are a
group
of peripatetic nomads.
|
Our current series has been talking about flagella and how they help bacteria become motile (amongst other things). A relatively new
discovery has opened our eyes to an exceptional movement by flagellated
bacteria, swarming.
Swimming is when a bacterium on a liquid/surface interface
or in liquid moves around by itself using its flagella as a propeller. But
groups of bacteria can use their flagella to create a swarm; a mass of bacteria
moving as one unit, often faster than the individuals can move on their own.
Bacteria moving as a unit is like tribes of humans moving from
one place to another. But there are also those bacteria that choose to hunker
down in one location and build a “city.” This prokaryotic Gotham is called a biofilm. We should do a whole series on biofilms, but for now
let’s just talk about them in general.
When a number of bacteria of the same type, or sometimes
even of different types, are in the same place at the same time, they may begin
to form a biofilm. Certain bacteria will secrete proteins as filaments, polysaccharides
in the form of slime, and some other structures. All of these together
form a network of tunnels, tubules, cavities, and surfaces onto which the
bacteria adhere. The biofilm also adheres to whatever surface is nearby. It’s a
bacterial city.
 |
|
The
plaque on your teeth is a biofilm. The saliva and
crevicular
fluid (between root and gum) provides some
proteins
and sugars to build the film. Above is a
photomicrograph
of plaque showing that yeast and
bacteria
are both involved in mature plaque.
|
Some bacterial colonies settle to form cities and some move
on en masse to another location – it really does sound like humans tribes. But biofilms and swarming are not mutually
exclusive, in some cases you will see the bacteria at the edge of a biofilm
start to swarm and expand, just like urban sprawl creates bigger cities.
There's organization to the swarm as well. Swarming isn’t an, “Everybody run!” kind of movement.
Swarming requires controls, regulation, and numerous gene
products spread out over the colony. Even though they work as a group, the
bacteria might not all go the same direction.
Since bacteria divide by binary fission (one form of asexual reproduction), they tend to form masses in
one location, often circular. When they give the signal(s) to swarm, some
may take off in this direction, and some in another, based on where they are in
the circle. Look at the picture below and right. Pretty, but it shows that colony swarm has multiple leading edges that will travel out into the
unknown, and part of the colony will follow behind each.
Every once in a while, a new leading edge might branch off
and swarm in a different direction, taking some followers with it. Other
types of bacteria seem to swarm equally in all directions, forming concentric circles of
new colonies.
 |
|
This
is a false color image showing the branching of a
bacteria
colony in a swarm. Dr. Eshel Ben-Jacob from
Tel
Aviv University produces these images as science
and
art. See many of his images at this site.
|
Many behaviors
occur in swarming bacteria that don’t occur in swimming bacteria. The leading
edge cells may secrete surfactant, a combination of chemicals that reduce the
surface tension on the plane so that the bacteria can move with less
resistance.
The leading edge bacteria grow extra flagella,
become elongated, and secrete slime for easy movement - but only the leading edge cells. They band together, becoming like rafts; in fact that’s what
they’re called, rafts. The movement of the leading edge plows a furrow in the
material they're moving across. This is partly due to the leading edge cells,
but it has more to do with the cells behind them. The following cells form roiling masses,
and together they push the leading edge along, like pushing a plow to form a
ditch for planting seeds.
The furrows are then followed and expanded by the cells
behind the leading edge, growing larger and easier to follow. That way, they
can push the leading edge better. All these changes and functions lead to
faster movement, which is why the swarm can move faster than individuals.
One amazing thing discovered in a 2013 series of experiments was that the leading edge cells secrete DNA. This nucleic acid
doesn’t function as genetic material, but is apparently important for keeping the
leading edge cells together and moving in the same direction, as well as
stimulating movement at all. In experiments where this DNA was chewed by
enzymes, the swarming movement stopped completely. Amazing - if they were a marching band in a parade, the DNA would be the banner carried by the drum majors that's emblazoned with their school and nickname. Everybody follows the banner and the drum major.
Integral to the concepts of swarming and biofilm development
is the idea of multicellularity in bacteria. They're all clones of one another
(except for mutation and any lateral gene transfer), but they work together and
may take on different jobs, structures, and morphologies. They are working
together to accomplish more than they could on their own. That sounds a lot like a
multicellular organism where the different cell specialize into different types
in order to perform different functions.
 |
|
On
the left is a cartoon that illustrates how the electron
donor
hydrogen sulfide can’t donate electrons unless
something
is available to accept them. The oxygen is the
acceptor,
and the bacteria provide the cable to connect
them.
The filaments of bacteria are shown on the right.
Photocredit
to Nils Risgaard-Petersen.
|
We don’t know all the bacteria that are capable of swarming, but it's probably many more than we have found so far. And we aren't sure just why do they do it. Perhaps
it's to leave an area of poor food value behind and strike out for better
hunting grounds. Moving faster than they would as individuals might be
important when trying to find, and then take advantage of a new food source.
Eat up before someone else finds it.
Perhaps swarming is for protection. Like for biofilms, there
is evidence that bacteria are less susceptible to antibiotics when swarming. Or
it may have something to do with the best way to achieve full biochemical
development. There are many studies that suggest that infectious organisms must
swarm in order to create disease. Please remember, they aren’t trying to cause disease, but
it shows that swarming must be important in their colonial development and a byproduct of this may be disease.
 |
|
Three
colonies of the same bacteria that were not clonal (not
from
same exact ancestor - A, B, and C) were grown on the same
plate
and they expanded in a swarm-like behavior. Where the
different
colonies meet is the Dienes line. On the right is a false
color
close up of a Dienes line, showing the battlefield. The
black
line is 50 µm long.
|
P. mirabilis has
the ability to produce a type VI secretion system that acts as a needle. It
punctures an adjacent bacterium and injects toxins. When a swarming colony
invades another colony, they all start to produce their type VI secretion
needles.
They attack any cell that makes contact with them, in a
preemptive sort of fashion. There are many friendly fire incidents, but kin will survive the attack while cells from the
other colony will be killed (they aren't immune to the specific toxin). The deeper invader is usually the dominant colony
and will kill off the other colony, even though they may be of the same strain. Man -
bacteria can be ruthless.
The key to both biofilm development and swarming is quorum sensing (quorum is from Latin qui meaning who, it means the number of members that must be present to
transact business). The bacteria sense when their numbers reach a certain
tipping point because the levels of certain chemicals reach critical
concentrations.
We aren’t sure just why one behavior happens instead of the
other, the situations that will induce either biofilm formation or swarming, but the
number of bacteria and the state of their environment is key. Therefore, if you
can stop the quorum sensing, you can stop swarming or biofilm formation, or
both. This would be key to battling some pretty nasty infectious organisms since we said they are often important for pathogenesis.
 |
|
Proteus mirabilis is a bacteria that swarms
in concentric
circles.
It causes urinary tract infections in both men and
women.
In the lower image you can see the many flagella
of
the organism – and this is before it starts to swarm and
leading
edge organisms differentiate.
|
We have discussed in prior posts about the amazing ability of cranberry to prevent UTIs. A 2013 paper shows that at least part of the cranberry's action
on UTI-causing P. mirabilis is
through the prevention of swarmer cell differentiation. Work with other bacteria shows that
it is quorum sensing that is disrupted by the cranberry compounds, so the
swarm in P. mirabilis might be
stopped via the bacteria not knowing how many of their brothers are around. Bacteria won't pick a fight unless they know their gang is big enough - it's West Side Story in your bladder.
Next week - some prokaryotes don't move. Just like couch potatoes, they wait for someone to bring them their
dinner.
For
more information or classroom activities, see:
Quorum
sensing –
Biofilms
–
Bacterial
swarming -
May I use your pictures of Proteus mirabilis for my college poster project? It will not be used for anything other then a good grade.
ReplyDelete