THINK
IT
OVER
!
Why
Rods
and
Cocci
Bacteria exhibit a wide variety
of
shapes
but
the commonly
studied species
of
bacteria are generally either spherical in shape
which are called
cocci
(singular coccus)
or
have a cylindrical
shape
and
are called
rods
or bacilli (singular bacillus).
In
reality
rods and cocci are the ends
of
a continuum. Sonle
of
the cocci are
slightly elongated
and
some
of
the rods are
so
short that they
look almost like cocci. Sometimes a term coccobacillus is used to
describe them.
Why did bacteria get different shapes? Why aren't all
of
them
spherical
or
all rod shaped? Different shapes are likely to give
different selective advantages. A rod shaped cell has a streamlined
body which would be advantageous in swimming, just like the
slender body
of
a fish, a boat
or
a plane. A spherical body will
experience greater frictional resistance.
This
hypothesis is
supported by the fact that among the flagellated motile bacteria
almost all are rod shaped. Only exceptionally
few
cocci are
motile.
This
hypothesis, however,
is
not
adequate since a large
number
of
species
of
bacteria are non-motile.
A rod shape can confer another advantage, that
of
having a large
surface area per
unit
volume. Intake
of
nutrients takes place
through the cell surface.
The
rate
of
bacterial growth depends
largely on the rate
of
intake
of
nutrients. Therefore having a
large surface area can confer great selective advantage whenever
there is competition for nutrients.
The
obvious question that follows
is
that
if
a rod shape confers
many advantages, then why did spherical cells evolve at all?
Among all possible shapes, a sphere has the minimum ·surface
area per
unit
volume. Having a larger surface area might be good
for intake
of
nutrients,
but
it
may prove detrimental under some
other conditions such
as
desiccation or osmotic shocks.
If
a cell
Milind
Watve
M.E. Society,
Abasaheb
Garware College,
Karve
Road
Pune
411
004, India.
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