• Bacteria (singular bacterium) are unicellular microorganism which are of microscopic size and cannot be seen with unaided eyes.
  • Constitute large domain-prokaryotes.
  • They are among the first life form evolve on the earth and present in most of the habitat.
  • Bacteria inhabit normal to the extreme habitat like air, water, soil, radioactive waste, hot springs, deep seas, even in human gut, etc.
  • Also live in symbiotic and parasitic relationship with plants and animals. Example rhizobium associate with leguminous plants.
  • Length of bacteria ranges in few micrometres. E.coli (1.0-2.0 micrometre long and 0.5 micrometre in radius), Mycobacterium tuberculosis (2-4 micrometre long and 0.2-0.5 micrometre width), V cholerae (1-3 micrometre long and 0.5-0.8 micrometre radius).
  • The study of this discipline of microbiology is called bacteriology.
  • Bacteria are beneficial for human and other animals in a way that they produce various kind of vitamins, enzyme and food products. Example- vitamin B12, lactic acid, alcohol.
  • They are key components of our biosphere playing important role in biogeochemical cycles, removal of toxic substance and decomposition of waste materials.
  • Involve in nitrogen fixation hence improve soil fertility.
  • With beneficial characteristics several bacteria are pathogenic and cause various kind of disease in human, plants and animals. Example- cholera, tuberculosis, syphilis, anthrax, and more.
  • In industries bacteria are useful in waste water treatment and industrial fermentation for cheese and yogurt production.


  • Bacteria is a prokaryotic organism their body lacks nucleus and cellular components.
  • Bacteria are covered by a membrane called cell wall chiefly made up peptidoglycan (murein layer).
  • Peptidoglycan layer mainly constitute of polysaccharide which are cross linked by peptide bonds.
  • Peptidoglycan layer is made up of two glucose derivative N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) chain. The chain is linked with tertapeptide bonds
  • Four protein in tetrapeptide bond are L-alanine, D-alanine, L-lysine or meso-diaminopimelic acid (DPA) and D-glutamine.
  • Cell wall contributes to the survival of the bacteria, protection from harsh environment and antibiotics.
  • Peptidoglycan layer in gram positive bacteria is 20-80 nm thick in contrast gran negative bacteria contain 2-7 nm thick layer.
  • Gram negative bacteria contain acidic substance known as teichoic acids which provide rigidity to the cell wall.

Outer Membrane

  • Over the cell wall gram negative bacterium contain an external layer called outer membrane it contain lipopolysaccharide, phospholipids, lipoprotein and proteins.


  • It’s the carbohydrate enriched layer that covers the outside of the bacteria.
  • It provides protection against host
  • This glycocalyx layer associate with the pathogenic property to the bacteria.
  • Glycoclayx in a tightly packed form is called as capsule, in contrast in loose packing it is called as slime layer.

Surface Auxiliary


  • Hairlike structure, attach on the surface of the cell, main function is to provide mobility to the bacteria
  • Both gram positive and gram negative bacteria contain flagella. Consist of three parts filament, hook and basal body.


  • Thin hairlike structure on the surface of gram negative bacteria
  • Play an Important role in conjugation process


  • Present on both kind of cells gram negative an gran positive
  • Helps in attachment to the surface


  • Shape
Bacillus (Rod-Shaped)Escherichia coli (E. coli)
Coccus (Sphere)Streptococcus pneumoniae
Vibrio (Comma Shaped)Vibrio cholerae
Spirilla or spirochete (Spiral)Spirillum volutans
  • Mode of Nutrition
Autotrophic BacteriaPurple bacteria
Heterotrophic BacteriaAll disease-causing bacteria
  • Cell Wall
Gram positiveStaphylococcus aureus
Gram negativeEnterobacteriaceae
  • Mode of Respiration
AerobicPsuedomonas aeruginosa


  • The mode of replication in bacterium is binary fission.
  • In this process the parent bacterial cell divided into two identical daughter cells.
  • Replication of DNA starts in the parent cell and each copy is transfer into the daughter cell.
  • Rate of reproduction is depend on the conditions like temperature, nutrient availability, moisture this is called favourable condition. E.coli generation rate is 2 million bacteria in 7 hrs.
  • In some rare cases they undergo sexual reproduction by conjugation, transduction and transformation. Helps in genetic modification in bacteria which results in the antibiotic resistant property.

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