Bacteria are a wide group of unicellular organisms that are ubiquitous in nature.

When cultures on agar, bacteria grow as colonies that contain many individual cells.

Bacteria are placed under the kingdom Protista and are prokaryotic cells.

The unit of measurement used in bacteriology is the micron (μ) or also called micrometer (μm).

1 μm = 10-6 or 0.000001 m

Largest known bacteria in the world is – Thiomargarita namibiensis

Smallest known bacteria in the world is – Mycoplasma genitalium

Longest known bacteria in the world is – Epulopiscium fishelsoni

There are different criteria for the classification of bacteria –

  • Morphological
  • Anatomical
  • Staining
  • Cultural characteristics
  • Nutrition
  • Environmental factors
  • Biochemical reactions
  • Antigenic structure

Morphological classification of bacteria – 

⇒ Cocci – These are spherical or oval cells. On the basis of arrangement of individual organisms they can be described as

  • Monococci (Cocci in singles) – Monococcus
  • Diplococci (Cocci in pairs) – Streptococcus pneumoniae
  • Staphylococci (Cocci in grape-like clusters) – Staphylococcus aureus
  • Streptococci (Cocci in chains) – Streptococcus pyogenes
  • Tetrad (Cocci in group of four) – Micrococcus
  • Octard (Cocci in group of eight) – Sarcinae

⇒ Bacilli – These are rod-shaped bacteria. On the basis of arrangement of organisms, they can be described as

  • Diplobacilli
  • Streptobacilli
  • Palisades
  • Chinese-letter form
  • Coccobacilli
  • Comma-shaped

⇒ Actinomycetes (actin- ray, mykes-fungus)

  • These are rigid organisms like true bacteria but they resemble fungi in that they exhibit branching and tend to form filaments.
  • They are termed such because of their resemblance to sun rays when seen in tissue sections.

⇒ Spirochetes

  • These are relatively long, slender, non-branched microorganisms of the spiral shape having several coils.

⇒ Mycoplasma

  • These bacteria lack in the rigid cell wall (cell wall lacking) and are highly pleomorphic and of indefinite shape.
  • They occur in round or oval bodies and in interlacing filaments.

⇒ Rickettsiae and Chlamydiae

  • These are very small, obligate parasites, and at one time were considered closely related to the viruses. Now, these are regarded as bacteria.

Check out the Morphology & Culture Characteristics of Escherichia coli


Anatomical classification of bacteria –

  • Capsule
    • CapsulateStreptococcus pneumoniae
    • Non-capsulate – Viridans streptococci
  • Flagella
    • Flagellate
      • Monotrichous
      • Lophotrichous
      • Amphitrichous
      • Peritrichous
    • Aflagellate – Shigella spp.
  • Spore
    • Spore-formingBacillus spp.
    • Non-sporingEscherichia coli

Classification of bacteria based on staining reaction –

    • Gram-positive cocci Staphylococcus aureus
    • Gram-negative cocci Nesseria gonorrhoeae
    • Gram-positive rods Clostridium
    • Gram-negative rods coli
    • Acid-fast bacilliMycobacterium tuberculosis
    • Non-acid-fast bacilliSalmonella typhi

Classification of bacteria based on Cultural characteristics –

Extra growth factors requirements

  • FastidiousHemophilus influenzae
  • Non-fastidiousEscherichia coli

Hemolysis on Sheep Blood Agar

  • Alpha-hemolysisStreptococcus pneumoniae
  • Beta-hemolysis – Streptococcus pyogenes
  • Gammahemolysis –
  • Target-hemolysis –
  • Alpha-prime hemolysis –

Utilization of carbohydrates

  • OxidativeMicrococcus
  • FermentativeEscherichia coli

Growth rate

  • Rapid growersVibrio cholerae
  • Slow growersMycobacterium tuberculosis

Pigment production

  • Pigment producer Staphylococcus aureus
  • Pigment non-producerEscherichia col

Classification of bacteria based on Nutrition –

  • Phototrophs – Those organisms which get energy from photochemical reactions are called as Phototrophs or phototrophic organisms.
  • Chemotrophs – Those which get energy from chemical reactions are called as Chemotrophs or chemotropic organisms.
  • Autotrophs – Those organisms which can synthesize essential metabolites from inorganic sources are called as Autotrophs or Autotrophic organisms.
  • Heterotrophs – Those organisms which cannot synthesize their essential metabolites and depend upon external sources are Heterotrophs or heterotrophic organisms. Most of the pathogens are heterotrophic.

Check out the Morphology & Culture Characteristics of Salmonella typhi


Classification of bacteria based on environmental factors –

  • Temperature
  • Oxygen dependence
  • pH
  • Salt concentration

⇒ Temperature dependence –

  • Psychrophiles (15-200C)Pseudomonas fluorescens
  • Mesophiles (20-400C)Escherichia coli, Salmonella enterica, Staphylococcus aureus
  • Thermophiles (50-600C)Bacillus stearothermophillus
  • Extremely Thermophiles (as high as 2500C) –

⇒ Oxygen dependence –

  • AerobeThose organisms that use oxygen for the growth and metabolism are called aerobes or aerobic organisms. They obtain energy from oxidative processes. E.g. – Staphylococcus spp., Streptococcus spp.
  • Obligate aerobes Those organisms that strictly require O2 for their growth are called Obligate aerobes or Obligate aerobic organisms e.g. Pseudomonas aeruginosa.
  • MicroaerophilicThose organisms that grow under reduced O2, 5-10% and increased CO2, 8-10% are called Microaerophilic organisms or microaerophiles e.g. – Campylobacter jejuni, Helicobacter pylori.
  • Facultative anaerobe – Those organisms that are capable of growing either in presence or absence of O2 are called facultative anaerobes or facultatively anaerobic organisms. – coli.
  • Obligate anaerobe Those organisms that strict absence of oxygen for the growth is called obligate anaerobe or obligate anaerobic organisms. e.g. – Clostridium
  • Capnophilic – Those organisms that require increased concentration of CO2, i.e., 5-10% for the growth is called Capnophilic organism or capnophile e.g. influenzae, N. gonorrhoeae.
  • Aerotolerant – Those organisms which cannot use oxygen for growth, but tolerate its presence are called Aerotolerant organisms. e.g. – Streptococcus, Clostridium

⇒ pH dependence –

  • Acidophiles – pH less than 3 – e.g. – Lactobacillus acidophilus
  • Alkaliphiles – pH roughly 8.5–11 – e.g. – Vibrio
  • Neutralophiles (pH 6.5-8) – e.g. Streptococcus pneumoniae, Pseudomonas aeruginosa
  • Majority of the medically important bacteria grow best at neutral or slightly alkaline reaction (pH 7.2-7.6).

⇒ Salt concentration –

  • Non – Halophiles – Unable to grow in high salt concentration. e.g. – E.coli.
  • Halotolerant – Tolerate low level of salt concentrations i.e. up to 8% salt concentration e.g. –
  • Halophiles – those organisms that grow in high salt concentration are called Halophiles or halophilic (salt-loving) organisms. These are further classified as –

1.) Slightly halophilic – require 0.5-3% salt concentration for growth e.g. – Vibrio, Pseudomonas

2.) Moderately halophilic – requires 3-15% salt concentration for growth e.g. – Bacillus, Micrococcus

3.) Extremely halophilic – requires 15-30% salt concentration for growth e.g. – Halobacterium, Halococcus, Natranococcus spp.

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