Prosthecate Bacteria
Prosthecate bacteria are bacteria bearing one or more projections called
prosthecae (sing. prostheca) and occur in a variety of groups, in addition to
many forms typically being grouped together on their own. Each prostheca is
an outgrowth of the cell, containing cell wall material and a narrow core of
cytoplasm.

One example is
Prosthecomicrobium (left) which has about 20 short conical
prosthecae (each about 1 micrometre long, 0.2 micrometres wide). These
prosthecae function as flotation devices. For a tiny cell, water behaves like a
very viscous fluid (the Reynold's number, Re is much less than 1) and spines
and similar protuberances are an effective way of increasing the surface area
without adding much mass to the cell and they make economical flotation
devices. In addition,
Prosthecomicrobium typically possesses a large number
of gas vacuoles (the tubular structures inside the model of the cell, left). This
species divides by binary fission and some strains are motile (or is this a
feature of the life-cycle?).
Some bacteria, like Caulobacter crescentus, normally have a single prostheca, which serves an adhesive function. In
Caulobacter this prostheca functions as a stalk with a terminal holdfast of sticky adhesive material to anchor the cell to a
surface. Each stalked cell is about 8 micrometres long and 2 micrometres wide and produces a new cell by binary fission
at the nonstalked pole. This new daughter cell develops a single flagellum and becomes a swarmer cell, swimming away
to a new location, attaching to a surface and losing its flagellum and motility as the stalk grows at the attachment point.
The original stalked cell can go on to divide again, producing a series of swarmers, with a crossband forming in the stalk
each time it divides, which allows the age of the stalk cell to be estimated from counting the crossbands in its stalk.
Ancalomicrobium is another example that has irregular-shaped non-motile cells bearing up to 8 prosthecae, which are
longer (up to 3 micrometres long) occasionally branched and more cylindrical than those of
Prosthecomicrobium. This
species multiplies by budding. In budding, a small outgrowth of the cell forms and detached, becoming a new miniature
cell which then grows and matures. This contrasts to binary fission, which is more common in bacteria, in which each
mother cell divides equally into two daughter cells.

Although these prosthecae appear to function in flotation, they may also serve to increase the surface area to volume
ratio of the cells for nutrient absorption. It is suggested that they may appear and/or elongate under conditions of
nutrient starvation. This has been observed experimentally in several forms, including
Hyphomicrobium and
Caulobcater. In Ancalomicrobium, no prosthecae are produced in nutrient-rich conditions and the cells are spherical or
rod-shaped, but prosthecae develop in nutrient-poor conditions. Whether this is for absorption or to increase buoyancy
and so disperse to new areas (where there may be more nutrients) is not certain.
Above: Prosthecomicrobium.
micrometres, suggesting that the stalk is attempting to position the cell above the stagnant boundary layer or region of
adjacent biofilm into a region of better mixed water where more nutrients may be found. Wherever fluid encounters a
solid surface, a boundary layer of poorly mixed fluid lines the boundary (as the fluid sticks to the surface due to its
viscosity) and in water this boundary layer is about 100 micrometres, though it is not a precise boundary - mixing
gradually increases the further one is from the surface and 20 micrometres seems likely to make a significant difference.
In particular, it may lift the cell above competing micro-organisms adhering to the surface. In stationary phase (the stage
when nutrients are depleted and a bacterial population stops reproducing)
Caulobacter crescentus cells change from
the usual short curved rods to helical filaments, up to 30 micrometres long; the significance of which is not certain (it may
be adaptive or due to a failure of cell division).

Hyphomicrobium also produces cells with a single stalk attaching to a surface, but its mode of division is more variable: A
flagellated bud may appear on the end of a second prostheca growing from the pole opposite to the stalk, or a bud may
develop as a side-branch born on the stalk.

Studies have also shown that bacteria with long prosthecae, like
Ancalomicrobium, are more resistant to protozoan
grazing, since they are often simply too large for these predators to handle. Thus prosthecae have a clear defensive
function, though it is not certain whether the presence of predators induces prostheca development or enlargement.
Some bacteria sense the presence of predators and increase their size by forming straight or branched filaments (either
multicellular chains or elongated cells with multiple genomes) which are harder to ingest. Protozoan grazing has selected
Ancalomicrobium cells with long prosthecae (up to 4.5 micrometres long) and also Ancalomicrobium growing in chains
10-15 micrometres long.

Gas Vesicles and Gas Vacuoles

Gas vesicles are protein cylinders, 75 nm in diameter and 0.2 to 1.0 micrometres in length, with conical ends. They are
bounded by a single protein membrane 2nm thick (they are not bound by lipid membranes like the vesicles of eukaryotic
cells). Several gas vesicles may be grouped together in parallel arrays called gas vacuoles. Each vesicle is filled with
gas to which the membrane is freely permeable. The protein membrane is ribbed, with ribs 4.5 nm wide perpendicular to
the long axis, somewhat like the staves of a barrel. The ribs probably confer strength as the vesicle must withstand
certain pressures that may arise, for example from changes in depth. However, a sudden increase in pressure will
collapse the vesicles. Once collapsed they can not be re-inflated, must be re-synthesised.

Gas vesicles are hydrophobic inside, hydrophilic outside and so exclude water, but the internal gas is not pressurised
(the membrane is freely gas permeable). They lower the density of the cell and so increase its buoyancy, increasing
flotation and lengthening the time it takes a cell to sediment. They occur in many planktonic bacteria, including
prosthecate bacteria, cyanobacteria and archaebacterium (archaeon)
Halobacterium. In Halobacterium halobium they
are composed of two proteins, one encoded for on a large plasmid, the other on the main chromosome. Cyanobacteria
are photosynthetic and  synthesise fewer gas vesicles if the light intensity (photon flux density) becomes too high, and
synthesise more gas vesicles if light intensity becomes too low, and so are able to maintain an ideal depth with optimum
light intensities for photosynthesis.