| Bacterial Slime Cities |
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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Many bacteria are now known to form multicellular structures and possibly all bacteria are capable of this. The majority of
bacteria form organised structures called biofilms. The bacteria in a biofilm are not electrically connected as are the cells
in animals, plants, some algae and fungi and indeed, they are rarely physically connected directly. In this way a biofilm is
not like a multicellular organism, but it is a multicellular colony of a less well-organised type. The bacteria are connected
by a mass of slime which they secrete and in which they are embedded. This slime forms the bulk of the biofilm. We can
think of a biofilm as a 'slime city'. Although a single species is capable of biofilm formation, in nature biofilms are often
multi-species and eukaryotic cells such as yeasts may also be incorporated.
A biofilm is founded when one or more planktonic cells adheres to a suitable substrate. The planktonic cells are the single
cells we normally think of as bacteria. In many species the planktonic cells are flagellated and so capable of swimming and
then they are also called swarmer cells. The adherent cells secrete slime and multiply by binary fission to form a small
microcolony. The biofilm consists of a surface sheet of slime from which emerge various columns or tower-like structures
that vary in form. As nutrients become used-up, the cells in the centre of these columns become deprived of nutrients and
undergo changes - they transform into swarmer cells, the column ruptures and the swarmer cells escape.
What is the function of biofilms?
The swarmer cells are essentially functioning as spores. Biofilm columns are typically 300-400 micrometres tall and this
height is sufficient to break the boundary layer of stagnant fluid that develops over surfaces in typical conditions. This
allows the bacteria to access more oxygen and nutrients and also aids swarmer cell dispersion. In this way, bacteria can
more efficiently utilise a food source and then colonise new areas when this food source becomes exhausted.
Other types of multicellularity in bacteria
Many bacteria form filaments, in which the cells are connected end-to-end in a chain. These chains forms when cells fail to
separate after binary fission and as the cells divide the chains lengthen. The cells may be joined at their end-walls or by a
common slime sheath. This form of multicellularity reaches its greatest complexity in the cyanobacteria (blue-green
bacteria). These bacteria are photosynthetic and the chains may be enclosed in larger colonial structures - they form
easily visible filamentous strands and nets, where each fibre is many cellular filaments together, or they may form
spherical slime-filled capsules, 1-2 cm in diameter and they may even form reef-like structures called stromatolites which
were abundant in prehistoric times, in the Age of the Prokaryotes when bacteria were the dominant life-form on Earth, and
are still frequent in the Dead Sea which is too salty for many other forms of life. In stromatolites, generations of
cyanobacterial biofilms trap sand grains and the like, cementing them together to build stony column-like or
mushroom-shaped structures which may reach several metres in height and diameter - quite a feat of construction for a
microbe!
A Pov-Ray 3D computer model of a filamentous cyanobacterium, of the Anabaena type, is shown below:
bacteria form organised structures called biofilms. The bacteria in a biofilm are not electrically connected as are the cells
in animals, plants, some algae and fungi and indeed, they are rarely physically connected directly. In this way a biofilm is
not like a multicellular organism, but it is a multicellular colony of a less well-organised type. The bacteria are connected
by a mass of slime which they secrete and in which they are embedded. This slime forms the bulk of the biofilm. We can
think of a biofilm as a 'slime city'. Although a single species is capable of biofilm formation, in nature biofilms are often
multi-species and eukaryotic cells such as yeasts may also be incorporated.
A biofilm is founded when one or more planktonic cells adheres to a suitable substrate. The planktonic cells are the single
cells we normally think of as bacteria. In many species the planktonic cells are flagellated and so capable of swimming and
then they are also called swarmer cells. The adherent cells secrete slime and multiply by binary fission to form a small
microcolony. The biofilm consists of a surface sheet of slime from which emerge various columns or tower-like structures
that vary in form. As nutrients become used-up, the cells in the centre of these columns become deprived of nutrients and
undergo changes - they transform into swarmer cells, the column ruptures and the swarmer cells escape.
What is the function of biofilms?
The swarmer cells are essentially functioning as spores. Biofilm columns are typically 300-400 micrometres tall and this
height is sufficient to break the boundary layer of stagnant fluid that develops over surfaces in typical conditions. This
allows the bacteria to access more oxygen and nutrients and also aids swarmer cell dispersion. In this way, bacteria can
more efficiently utilise a food source and then colonise new areas when this food source becomes exhausted.
Other types of multicellularity in bacteria
Many bacteria form filaments, in which the cells are connected end-to-end in a chain. These chains forms when cells fail to
separate after binary fission and as the cells divide the chains lengthen. The cells may be joined at their end-walls or by a
common slime sheath. This form of multicellularity reaches its greatest complexity in the cyanobacteria (blue-green
bacteria). These bacteria are photosynthetic and the chains may be enclosed in larger colonial structures - they form
easily visible filamentous strands and nets, where each fibre is many cellular filaments together, or they may form
spherical slime-filled capsules, 1-2 cm in diameter and they may even form reef-like structures called stromatolites which
were abundant in prehistoric times, in the Age of the Prokaryotes when bacteria were the dominant life-form on Earth, and
are still frequent in the Dead Sea which is too salty for many other forms of life. In stromatolites, generations of
cyanobacterial biofilms trap sand grains and the like, cementing them together to build stony column-like or
mushroom-shaped structures which may reach several metres in height and diameter - quite a feat of construction for a
microbe!
A Pov-Ray 3D computer model of a filamentous cyanobacterium, of the Anabaena type, is shown below:
The filaments of some cyanobacterial species show differentiation - some cells are specialised for certain functions. In
the filament above, their two enlarged cells, the leftmost one is a heterocyst, which is specialised for nitrogen-fixation,
and the one on the left is an akinete - a dormant cell with a thickened wall that is resistant to harsh conditions.
In some cyanobacteria individual filaments live inside slime-tubes which they secrete and many tubes may be arrayed
together. The filaments can glide up and down inside their tubes and this enables the bacteria to adjust their height
above the surface to reach the light and oxygen and other materials they may need. Thus, being filamentous is an
advantage since again it allows bacteria to break free of the stagnant boundary layer and utilise resources in the
free-flowing water. Many cyanobacterial filaments can glide over the surface, enabling them to move up structures
projecting above the boundary layer.
Some filamentous bacteria form net-like biofilms which float in the water column. Thiovulum majus forms floating veils,
with the cells held together by slime threads, and the veils are ventilated by the many beating flagella of the
constituent cells. These nets can be moved up or down in the water column to optimise their position.
In some filamentous bacteria the cells are connected by electrical conjunctions and an electrical signal (such as the
flow of protons) can pass from cell to cell. This enables them to coordinate their gliding activity, so that they move
forwards or backwards in concert for maximum locomotive efficiency. This is the closest bacteria seem to get to
forming true multicellular tissues, even if these filaments are rather one-dimensional.
Download a pdf for more information about bacterial movement and filament formation: Prokaryotes_motility
the filament above, their two enlarged cells, the leftmost one is a heterocyst, which is specialised for nitrogen-fixation,
and the one on the left is an akinete - a dormant cell with a thickened wall that is resistant to harsh conditions.
In some cyanobacteria individual filaments live inside slime-tubes which they secrete and many tubes may be arrayed
together. The filaments can glide up and down inside their tubes and this enables the bacteria to adjust their height
above the surface to reach the light and oxygen and other materials they may need. Thus, being filamentous is an
advantage since again it allows bacteria to break free of the stagnant boundary layer and utilise resources in the
free-flowing water. Many cyanobacterial filaments can glide over the surface, enabling them to move up structures
projecting above the boundary layer.
Some filamentous bacteria form net-like biofilms which float in the water column. Thiovulum majus forms floating veils,
with the cells held together by slime threads, and the veils are ventilated by the many beating flagella of the
constituent cells. These nets can be moved up or down in the water column to optimise their position.
In some filamentous bacteria the cells are connected by electrical conjunctions and an electrical signal (such as the
flow of protons) can pass from cell to cell. This enables them to coordinate their gliding activity, so that they move
forwards or backwards in concert for maximum locomotive efficiency. This is the closest bacteria seem to get to
forming true multicellular tissues, even if these filaments are rather one-dimensional.
Download a pdf for more information about bacterial movement and filament formation: Prokaryotes_motility