- Bacteria are the most abundant microorganisms.
- Hundreds of bacteria are present in a handful of soil.
- They also live in extreme habitats such as hot springs, deserts, snow & deep oceans. Many are parasites.
- Based on shape, bacteria are 4 types: Coccus (Spherical), Bacillus (Rod-shaped), Vibrium (Comma-shaped) & Spirillum (Spiral).
- Some bacteria are autotrophic (synthesize food from inorganic substrates). Majority are heterotrophs (they do not synthesize food but depend on other organisms or on dead organic matter for food).
- They live in harshest habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).
- Archaebacteria have a different cell wall structure for their survival in extreme conditions.
- Methanogens are present in the guts of ruminant animals (cows, buffaloes etc). They produce methane (biogas) from the dung of these animals.
- They have a rigid cell wall and a flagellum (if motile).
- They include Autotrophs (photosynthetic and chemosynthetic) and Heterotrophs.
a. Photosynthetic autotrophs (E.g. Cyanobacteria):
- They have chlorophyll a similar to green plants.
- The colonies are generally surrounded by gelatinous sheath.
- They often form blooms in polluted water bodies.
- Some of them fix atmospheric nitrogen in specialized cells (heterocysts). E.g., Nostoc & Anabaena.
b. Chemosynthetic autotrophs:
- They oxidize inorganic substances such as nitrates, nitrites & ammonia and use the released energy for ATP production.
- They help in recycling nutrients like nitrogen, phosphorous, iron and sulphur.
c. Heterotrophic bacteria:
- They are the most abundant in nature.
- The majority are important decomposers.
Impacts of Heterotrophic bacteria on human affairs:
· They are used to make curd from milk.
· Production of antibiotics.
· Fixing nitrogen in legume roots etc.
· Some are pathogens causing diseases. E.g. Cholera, typhoid, tetanus and citrus canker.
Reproduction in Bacteria:
· Bacteria reproduce mainly by fission.
· Under unfavourable conditions, they produce spores.
· They also reproduce by a sort of sexual reproduction (DNA transfer from one bacterium to other).
Mycoplasmas are organisms without a cell wall. They are the smallest living cells. They can survive without oxygen. Many are pathogenic in animals and plants.