• Bacteria are the sole members of the Kingdom Monera.

  • They are the most abundant microorganisms.

  • Bacteria occur almost everywhere.

• Hundreds of bacteria are present in a handful of soil.

  • They also occur in extreme habitats such as hot springs, deserts, snow, and deep oceans where very few other life forms can survive.

• Bacteria as a group show the most extensive metabolic diversity.

  • Bacteria are grouped under four categories based on their shape:

    • Coccus: spherical

    • Bacillus: rod-shaped

    • Vibrio: comma shaped

    • Spirillum: spiral.

Some of the bacteria are:

• Autotrophic, i.e., they synthesize their own food from inorganic substrates. They may be photosynthetic autotrophic or chemosynthetic autotrophic.

• Heterotrophic, i.e., they depend on other organisms or on dead organic matter for food.

Archaebacteria:

• They live in some of the harshest habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles), and marshy areas (methanogens).

• They differ from other bacteria in having a different cell wall structure which helps them to survive in extreme conditions.

  • Methanogens are present in the gut of several ruminant animals such as cows and buffaloes

• These are responsible for the production of methane (biogas) from the dung of these animals.

Eubacteria:

• They are also referred to as ‘true bacteria’.

  • They are characterized by the presence of a rigid cell wall, and if motile, a flagellum.

Photosynthetic Autotrophs:

The cyanobacteria (also referred to as blue-green algae) have chlorophyll similar to green plants and are photosynthetic autotrophs.

• The cyanobacteria are unicellular, colonial, or filamentous, and freshwater/marine or terrestrial algae.

  • The colonies are generally surrounded by a gelatinous sheath.

  • They often form a bloom in polluted water bodies.

  • Some of these organisms can fix atmospheric nitrogen in specialized cells called heterocysts.

• Examples: Anabaena and Nostoc.

Chemosynthetic Autotrophs:

• These bacteria oxidize 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 sulfur.

Heterotrophic Bacteria:

• The majority of these are important decomposers.

• They are helpful in making curd from milk, production of antibiotics, fixing nitrogen in legumes, etc.

• Some are pathogens causing damage to human beings, crops, farm animals, and pets.

  • Cholera, typhoid, tetanus, and citrus canker are well-known diseases caused by different bacteria.

Mycoplasm:

• These are organisms that completely lack a cell wall.

  • They are the smallest living cells known and can survive without oxygen (anaerobic).

  • Many mycoplasmas are pathogenic in animals and plants.

Reproduction:

• Bacteria reproduce mainly by fission.

  • Sometimes, under unfavorable conditions, they produce spores.

  • They also reproduce by a sort of sexual reproduction by adopting a primitive type of DNA transfer from one bacterium to the other.