| Building Bodies from Slime |
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Bacteria that live in slime cities
Some single celled organisms, such as the amoeba, look like wandering masses of slime (most are microscopic
but the largest are slime sheets up to over one metre across) but some multicellular creatures also build bodies
out of slime. Indeed, this seems to be one of the simplest multicellular body types and represents one of the
most ancient of Nature's experiments in multicellularity. Amoeba are not bacteria, they belong to a group of
protoctistans called the Protozoa and resemble animal cells. However, even the much smaller and genetically
much simpler cells of bacteria can form multicellular slime bodies!
The picture above shows part of a bacterial biofilm. This is a sheet of bacterial cells encased in slime. The
bacterial cells may be tightly packed into groups (as in some myxobacteria) or they may be loosely dispersed
within the slime. The edges of the slimy mass are often motile and slowly advance over the surface, consuming
any suitable food items that are in their path, which in the case of the predatory myxobacteria are other
bacteria. Most bacterial biofilms are composed of rather loose slime which is easily fragmented when handled
and so most of these structures escaped scientific discovery for a surprisingly long time and were only
discovered a few years ago. Others, like those of the myxobacteria, are firmer and more rigid and have been
known for decades. It is now known that many (if not the majority) of bacteria that have been investigated have
a multicellular slime stage, called a biofilm or 'slime city'. Prior to these recent discoveries it was thought that
most bacteria spent their lives as single cells. Now it is known that bacteria alternate between single cells that
disperse to new habitats, and are often highly motile (so called swarmer cells, which may have flagella) and
slime colonies that colonise solid surfaces and the surface of stagnant water.
There are several advantages for bacteria working together in multicellular slime cities, first they are more
resistant to chemical attack, for example, by antibiotics produced by other micro-organisms to kill their rivals.
Secondly, the bacteria can form towering structures (usually no more than one a millimetre in height, but
sometimes several centimetres tall) that enable them to lift themselves up from the stagnant water near the
surface, into the flowing water above, in order to tap into the nutrients and oxygen carried by the water. It also
allows myxobacteria to hunt other bacteria in packs, surrounding their prey, dissolving it and then absorbing it.
Thirdly, it enables them to release spores or swarmer cells from the tops of these towers, into the water stream,
where they can be carried to new habitats for colonisation. When a swarmer cell finds a suitable surface, it will
attach and maybe form a new biofilm. Otherwise, swarmer cells can live and reproduce happily in the water
column or soil as single-celled organisms. The picture above shows several stalked colonies that will give rise
to spores or swarmer cells. These spore-producing 'sporangia' (singular sporangium) come in a tremendous
variety of forms, depending upon species, some are stalk-less domes, others are spheres on top of stalks, and
some form complex branching tree-like structures. Eventually the swollen ends of these sporangia will burst
open to release the spores or swarmer cells.
Some single celled organisms, such as the amoeba, look like wandering masses of slime (most are microscopic
but the largest are slime sheets up to over one metre across) but some multicellular creatures also build bodies
out of slime. Indeed, this seems to be one of the simplest multicellular body types and represents one of the
most ancient of Nature's experiments in multicellularity. Amoeba are not bacteria, they belong to a group of
protoctistans called the Protozoa and resemble animal cells. However, even the much smaller and genetically
much simpler cells of bacteria can form multicellular slime bodies!
The picture above shows part of a bacterial biofilm. This is a sheet of bacterial cells encased in slime. The
bacterial cells may be tightly packed into groups (as in some myxobacteria) or they may be loosely dispersed
within the slime. The edges of the slimy mass are often motile and slowly advance over the surface, consuming
any suitable food items that are in their path, which in the case of the predatory myxobacteria are other
bacteria. Most bacterial biofilms are composed of rather loose slime which is easily fragmented when handled
and so most of these structures escaped scientific discovery for a surprisingly long time and were only
discovered a few years ago. Others, like those of the myxobacteria, are firmer and more rigid and have been
known for decades. It is now known that many (if not the majority) of bacteria that have been investigated have
a multicellular slime stage, called a biofilm or 'slime city'. Prior to these recent discoveries it was thought that
most bacteria spent their lives as single cells. Now it is known that bacteria alternate between single cells that
disperse to new habitats, and are often highly motile (so called swarmer cells, which may have flagella) and
slime colonies that colonise solid surfaces and the surface of stagnant water.
There are several advantages for bacteria working together in multicellular slime cities, first they are more
resistant to chemical attack, for example, by antibiotics produced by other micro-organisms to kill their rivals.
Secondly, the bacteria can form towering structures (usually no more than one a millimetre in height, but
sometimes several centimetres tall) that enable them to lift themselves up from the stagnant water near the
surface, into the flowing water above, in order to tap into the nutrients and oxygen carried by the water. It also
allows myxobacteria to hunt other bacteria in packs, surrounding their prey, dissolving it and then absorbing it.
Thirdly, it enables them to release spores or swarmer cells from the tops of these towers, into the water stream,
where they can be carried to new habitats for colonisation. When a swarmer cell finds a suitable surface, it will
attach and maybe form a new biofilm. Otherwise, swarmer cells can live and reproduce happily in the water
column or soil as single-celled organisms. The picture above shows several stalked colonies that will give rise
to spores or swarmer cells. These spore-producing 'sporangia' (singular sporangium) come in a tremendous
variety of forms, depending upon species, some are stalk-less domes, others are spheres on top of stalks, and
some form complex branching tree-like structures. Eventually the swollen ends of these sporangia will burst
open to release the spores or swarmer cells.
Amoebae that form slime moulds
Click here for a more technical account of cellular slime moulds.
Most amoebae live as single-celled organisms in the water and soil, but some amoebae can also form
multicellular slime structures. Amoebae are not bacteria, rather they are micro-organisms belonging to a group
called the Protoctista. Amoebal cells are typically 10-100 times the diameter of bacterial cells and have the
structure typical of animal cells, but they are not animals because animals always form complex multicellular
bodies. Some amoebae (called myxamoebae), such as Dictyostelium, will live in the soil as single cells that feed
and reproduce for many generations, but if these cells start to run out of food in their neighbourhood, then they
send chemical signals to one another and the amoebae respond by streaming in long conveys to a common
rendezvous. When they arrive, these amoebae do something very strange, they form a multicellular mound or
aggregate that piles on new coming cells, getting taller and taller. Eventually the cells at the tip of the mound
form a nipple-like protuberance and this takes charge as it is designed to become the 'head' of our new
organism. All this happens on a small scale, these mounds are only a few millimetres in diameter.
Click here for a more technical account of cellular slime moulds.
Most amoebae live as single-celled organisms in the water and soil, but some amoebae can also form
multicellular slime structures. Amoebae are not bacteria, rather they are micro-organisms belonging to a group
called the Protoctista. Amoebal cells are typically 10-100 times the diameter of bacterial cells and have the
structure typical of animal cells, but they are not animals because animals always form complex multicellular
bodies. Some amoebae (called myxamoebae), such as Dictyostelium, will live in the soil as single cells that feed
and reproduce for many generations, but if these cells start to run out of food in their neighbourhood, then they
send chemical signals to one another and the amoebae respond by streaming in long conveys to a common
rendezvous. When they arrive, these amoebae do something very strange, they form a multicellular mound or
aggregate that piles on new coming cells, getting taller and taller. Eventually the cells at the tip of the mound
form a nipple-like protuberance and this takes charge as it is designed to become the 'head' of our new
organism. All this happens on a small scale, these mounds are only a few millimetres in diameter.
| Above a mound of assembling Dictyostelium amoebae on a glass slide, left, and a later stage with a tip, right. |
Eventually, this tipped mound falls over and starts crawling around like a slug, with the tip raised up like a snout
behind which the rest of the body follows, leaving a trail of slime behind it as it does so. So our single celled
creatures have all come together to form a temporary multicellular body! The reason is, that this way they are
bigger and so can move faster and further. The job of the snout is to find a suitable place high up in the light
and air, from which to release spores into the wind or running rain-water. This slug-like creature is called a
grex, and is one to a few millimetres long, which is not bad for something that started out as amoebae one
hundredth of a millimetre in diameter! Each grex contains about 100 000 separate amoebae, all encased in
slime and working together as a single unit! The grex of Dictyostelium discoideum is white and translucent, but
experimenters frequently add colouring agents to make the different cells in the grex apparent.
behind which the rest of the body follows, leaving a trail of slime behind it as it does so. So our single celled
creatures have all come together to form a temporary multicellular body! The reason is, that this way they are
bigger and so can move faster and further. The job of the snout is to find a suitable place high up in the light
and air, from which to release spores into the wind or running rain-water. This slug-like creature is called a
grex, and is one to a few millimetres long, which is not bad for something that started out as amoebae one
hundredth of a millimetre in diameter! Each grex contains about 100 000 separate amoebae, all encased in
slime and working together as a single unit! The grex of Dictyostelium discoideum is white and translucent, but
experimenters frequently add colouring agents to make the different cells in the grex apparent.
Above: a slime mould grex (rendered with Pov Ray) crawling across a glass
slide, leaving a trail of slime behind it.
Left: when a grex finally finds a suitable place (or runs out of time) it will stop
moving, then form a mound which elongates into a relatively long stalk
(several millimetres long) with a rounded structure at the tip (the colour and
form varies tremendously depending upon species). This structure, called a
sporangium, will dry and break open, releasing amoebae in the form of
spores, into the wind or rain water to be carried off to new habitats. The
spores are dormant cells with tough walls to resist drying out. Hopefully some
of the spores will find a suitable place and germinate into single-celled
amoebae and live and reproduce happily, until they run out of food that is ...
then the cycle will start all over again!
Let's look at another creature that builds its body from slime, and may be over
one metre across!
slide, leaving a trail of slime behind it.
Left: when a grex finally finds a suitable place (or runs out of time) it will stop
moving, then form a mound which elongates into a relatively long stalk
(several millimetres long) with a rounded structure at the tip (the colour and
form varies tremendously depending upon species). This structure, called a
sporangium, will dry and break open, releasing amoebae in the form of
spores, into the wind or rain water to be carried off to new habitats. The
spores are dormant cells with tough walls to resist drying out. Hopefully some
of the spores will find a suitable place and germinate into single-celled
amoebae and live and reproduce happily, until they run out of food that is ...
then the cycle will start all over again!
Let's look at another creature that builds its body from slime, and may be over
one metre across!
Above: The biofilm life-cycle. Isolated cells adhere to the substrate (1) and move across the substrate until
they aggregate into groups. Once aggregated they secrete slime to form a small microcolony (2) and undergo
changes in gene expression, which suppresses flagellin synthesis and causes other changes to produce a
phenotype more suited to biofilm ‘city’ life. These microcolonies eventually grow upwards from the substrate as
mushroom-shaped or cylindrical columns (3). These larger microcolonies are exposed to higher fluid flows
above the lower boundary layer and this flow may be conducted through water channels in the base of the
biofilm (blue arrow). Some cells begin to differentiate into flagellated swarmer cells which are released high
above the substrate when the columns rupture (5). Columns may undergo several cycles of rupture and
swarmer dispersal, regrowth, and rupture and swarmer dispersal. The flagellated swarmer or planktonic cells
(6) are dispersed both passively by fluid flow and actively, and can even swim upstream (7) as discussed in the
text. Swarmer cells may undergo several stages of cell-division in the planktonic stage, but eventually adhere to
the substrate to complete the cycle (8). Additional mechanisms of cell dispersal from biofilms are discussed in
the text. (Note that this image is copyrighted and permission must be sought before using it elsewhere).
they aggregate into groups. Once aggregated they secrete slime to form a small microcolony (2) and undergo
changes in gene expression, which suppresses flagellin synthesis and causes other changes to produce a
phenotype more suited to biofilm ‘city’ life. These microcolonies eventually grow upwards from the substrate as
mushroom-shaped or cylindrical columns (3). These larger microcolonies are exposed to higher fluid flows
above the lower boundary layer and this flow may be conducted through water channels in the base of the
biofilm (blue arrow). Some cells begin to differentiate into flagellated swarmer cells which are released high
above the substrate when the columns rupture (5). Columns may undergo several cycles of rupture and
swarmer dispersal, regrowth, and rupture and swarmer dispersal. The flagellated swarmer or planktonic cells
(6) are dispersed both passively by fluid flow and actively, and can even swim upstream (7) as discussed in the
text. Swarmer cells may undergo several stages of cell-division in the planktonic stage, but eventually adhere to
the substrate to complete the cycle (8). Additional mechanisms of cell dispersal from biofilms are discussed in
the text. (Note that this image is copyrighted and permission must be sought before using it elsewhere).
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