Having appeared about 3,500 million years ago, bacteria are one of the oldest and smallest group of organisms with cellular length ranging from 0.1 to 10 um and possessing an average diameter of about 1 lum. Found in soil, dust, water, air, in and on plants and animals, most bacteria are enclosed by a tough protective layer called cell wall. Some species are enclosed by a capsule, a slimy layer making the wall resistant to destructive chemicals. All bacteria have a cell membrane, an elastic bag -like structure just inside the cell wall. Small molecules of food enter the cell through pores in this membrane, but larger molecules cannot pass through.

Inside the membrane is cytoplasm, a jelly-like substance containing chemicals called enzymes, required for food breakdown and building cell parts. These cells also contain DNA, vital for growth, reproduction and other activities. It forms an area of the cytoplasm called nuclear body. Other parts of  bacterias cell structure are mesosomes – unfoldings of cell surface membrane; ribosomes sites for protein synthesis pilus numerous fine protein rods concerned with attachment to specific cells or surfaces; plasmidssmall, self-replicating circles of extra DNA, processing a few genes that give extra survival and flagellum-the tail-like structure enabling movement of bacteria cells.


  • Archaebacteria differ from other bacteria having a different cell wall structure and this feature is responsible for survival in extreme conditions. 
  • Methanogens are present in the guts of several ruminant animals such as cows and the produetion of methane (biogas) from the dung of the animals. 
  • These bacteria are special, they live in the harsh habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy are as (methanogens)


  • There are thousands of different eubacteria or’ true bacteria’.
  • They are characterized by the presence of a rigid cell wall, and if motile, a flagellum.
  • The cyanobacteria (blue-green algae) have chlorophyll a similar to green plants and are photo synthetic autotrophs.
  • Heterotrophic bacteria are the most abundant in nature. The majority are important de composers.
  • They are helpful in making curd from milk, production of antibiotics, fixing nitrogen in legume roots etc.
  • Some are pathogens causing damage to human beings, crops, farm animals and pets
  • Cholera, typhoid, tetanus, citrus canker is well known diseases caused by bacteria.
  • Bacteria reproduce mainly by fission under unfavorable conditions, they produce spores.
  • They reproduce by a sort of sexual reproduction by adopting a primitive type of DNA transfer from one bacterium to the other.
  • The Mycoplasmas are organisms that completely lack a cell wall.
  • The cyanobacteria are unicellular, colonial us, marine or terrestrial algae.
  • The colonies are surrounded by gelatinous sheath. They form blooms in polluted water bodies.
  • Some of the organisms can fix atmospheric nitrogen in specialized cells called heterocysts, e.g., Nostoc and Anabaena.
  • Chemosynthetic autotrophic bacteria oxidise various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production.
  • They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.
  • They are the smallest living cell known and can survive without oxygen. Many mycoplasma are pathogenic in animals and plants.