Page 18 - Science Class 08
P. 18
CHIEF MICROBE GROUP Types : In terms of shape bacteria are of four
As stated previously, there are five chief group types:
of microbes— bacteria, fungi, algae, protozoa v bacilli (rod-shaped)
and viruses. v cocci (spherical)
Let us read about them one-by-one: v spirilla (spiral-shaped)
Bacteria v vibrios (curved)
Bacteria are unicellular microbes. They are
Bacteria are also classified on the basis of
among the smallest organisms that exist
Gram's method. This method was developed by
everywhere. There are thousands of kinds of
a scien st, named Gram, for differen a ng
bacteria most of which are harmless. Bacteria
bacteria by staining them with a dye, then
were observed by a scien st named Anton van
a emp ng to remove the stain with a solvent
Leeuwenhoek in 1675. The average size of a
like iodine solu on and then washing it with
bacterium is 1/1000 mm.
water.
Most bacteria are not dangerous for human
According to Gram, bacteria fall into two
beings. Many of them even live on or in our
groups:
body and help us to stay healthy. For instance,
v Gram-posi ve : that retain the dye-stain.
lac c acid bacteria in the bowel support
diges on. Other bacteria help the immune v Gram-nega ve : that don’t retain the stain.
system by figh ng germs. Some bacteria are Nutri on : In view of nutri on, bacteria have
also needed in order to produce certain types two groups:
of food.
v Autotrophic : These bacteria synthesise
For example, yoghurt, sauerkraut or cheese. their own food using water, carbon dioxide
Structure : A bacteria is unicellular, but its cell and sunlight or by reac ng with chemicals
has no organelles and it has no organised of inorgainc compounds in place of
nucleus either. Its cell-wall is rigid and its sunlight. So, autotrophic bacteria fall into
cytoplasm is granular and viscous. Its nature is two sub-groups– photosynthe c and
colloidal (consis ng of ultramicroscope par cles) chemosynthe c.
with 70-85% moisture-content. Some types of v Heterotrophic : These bacteria feed on the
bacteria secrete a s cky material on their outer food made by other organisms or by ea ng
surface that some mes turns into a slimy layer. dead or decayed ma er. So, these bacteria
also fall into two sub-groups—parasi c and
saprophy c.
Reproduc on : Bacteria reproduce by binary
fission. In this process, the bacterium, which is
a single cell, divides into two iden cal daughter
cells. Binary fission begins when the DNA of the
bacterium divides into two (replicates). The
bacterial cell then elongates and splits into two
daughter cells each with iden cal DNA to the
Bacteria
18 Science-8
