• Classification based on morphology of bacteria
  • Anatomical Based Classification
  • Classification Based On Staining
  • Classification Based Of Cultural Characteristics
  • Classification Based On Environmental Factors
  • Largest bacteria- Thiomargarita namibiensis
  • Longest bacteria- Epulopiscium fishelsoni
  • smallest bacteria- Mycoplasma genitalium

Classification Based On Morphology Of Bacteria:

  • Cocci: spherical shape, classified on the basis of arrangement
DiplococciStreptococcus pneumonia
StaphylococciStaphylococcus aureus
StreptococciStreptococcus pyogenes
  • Bacilli: rod shaped, classified on the basis of arrangement
DiplobacilliMoraxella bovis
StreptobacilliStreptobacillus moniliformis
PalisadesCorynebacterium diphtheria
Chinese-letter formCorynebacterium glutamicum
CoccobacilliGardnerella vaginalis
  • Actinomycetes: they are bacteria but resemble fungi by exhibiting branching

Example: Actinomyces israeli

  • Spirochetes: these are spiral shaped long, slender, non- branched microorganism.

Example: Borrelia burgdorferi

  • Mycoplasma: lack rigid cell wall and are highly pleomorphic.

Example:Mycoplasma pneumonia

  • Rickettsiae and Chlamydiae: they are small and obligate parasite.

Example: Chlamydiae psittaci, Rickettsia rickettsii

Anatomical Based Classification:

  • Capsule:
CapsulateStreptococcus pneumonia
Non-capsulateViridans streptococci
  • Flagellate:
MonotrichousVibrio cholerae
LophotrichousPseudomonas fluorescens
Amphitrichousrhodospirillum rubrum
PeritrichousE. coli
  • Aflagellate: shigella spp.
  • Spore:
Spore-formingBacillus spp.
Non-sporingEscherichia coli

Classification Based On Staining:

  • Gram’s stain:
Gram-positive cocciStaphylococcus aureus
Gram-negative cocciNesseria gonorrhoeae
Gram-positive rodsClostridium
Gram-negative rodsEnterobacteriaceae
  • Acid-fast stain:
Acid-fast bacilliMycobacterium tuberculosis
Non acid-fast bacilliSalmonella typhi

Classification Based Of Cultural Characteristics:

  • Extra growth factors requirements:
FastidiousHemophilus influenza
Non-fastidiousEsherichia coli
  • Hemolysis on sheep blood agar:
Alpha-hemolysisStreptococcus pneumonia
Beta-hemolysisStreptococcus pyogenes
Gamma-hemolysisStaphylococcus saprophyticus
Alpha-prime hemolysisStreptococcus pyogenes
  • Utilization of carbohydrates:
FermentativeEscherichia coli
  • Growth rate:
Rapid growersVibrio cholera
Slow growersMycobacterium tuberculosis
  • Pigment production:
Pigment producerStaphylococcus aureus
Pigment non-producerEscherichia coli

Classification Based On Environmental Factors:

  • Temperature:
Psychrophiles(15-20o C)Pseudomonas fluorescens
Mesophiles (20-40o C)Salmonella enterica
Thermopiles (50-60o C)Bacillus stearothermophillus
Extremely thermophiles (as high as 250o C)Pyrococcus furiosus
  • Oxygen:
AerobeStreptococcus spp.
Obligate aerobesPseudomonas aeruginosa
MicroaerophilicCampylobacter jejuni
Facultative anaerobeE. coli
Obligate anaerobeClostridium
CapnophilicN. gonorrhoeae
  • pH:
Acidophiles (pH less than 3)Lactobacillus acidophilus
Alkaliphiles (pH roughly 8.5-11)Vibrio
Neutralophiles (pH 6.5-8)Pseudomonas aeruginosa
  • Salt concentration:

Non-halophiles: cannot grow in high salt concentration

Example: E. coli

Halotolerant: Can tolerate low level upto 8% of salt concentration


Halophiles: can grow in high salt concentration

Slightly halophilic (require 0.5-3% concentration)Vibrio, pseudomonas
Moderately halophilic (requires 3-15% conc)Bacillus
Extremely halophilic (requires 15-30% conc)halococcus



  • 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.