Bacteria are a:

  • Prokaryotic microorganism
  • Single-celled
  • Microscopic organisms
  • Lack nuclei and other organized cell structure
  • Several species are pathogenic (capable of causing disease)
  • Most are non-infectious
  • Have a role in decay, fermentation, nutrient recycling, and nitrogen fixation.

Bacteria are usually classified as gram-positive or gram-negative based on a basic microbiological staining procedure called the gram strain.

They come in a variety of shapes and sizes.

Shapes and sizes

  • Cocci

Small, spherical, or oval (0.75-1.25µm). They are unicellular. Either they may remain as a single cell or may aggregate together for various configurations.

  1. Monococcus or Micrococcus: Single, discrete round. Ex. Micrococcus flavus.
  2. Diplococcus: Cell of diplococcus divides ones in a particular plane and after division; cells remain attached to each other. Ex. Diplococcus pneumonia.
  3. Streptococcus: The cells divide repeatedly in one plane to form a chain of cells. Ex. Streptococcus pyogens.
  4. Tetracoccus: Consists of 4 round cells, which defied in 2 planes at a right angle to one another. Ex. Goffkya tetragena.
  5. Staphylococcus: Cells are divided into 3 planes forming a structured likes bunches of grapes giving an irregular configuration. Ex. Staphylococcus aureus.
  6. Sarcina: Cells divide into 3 planes but form a cube-like configuration consisting of 8 or 17 cells but they have a regular shape.

  • Bacilli

Bacillium meaning Stick. Rod-shaped or cylindrical. Either remains singly or in pairs. Its length is 2-10 times its width. For some bacilli, the length of a cell may be equal to the width. Those are called Coccobacilli (short bacilli). Ex. Bacillus cereus.

  1. Chain of bacilli: ex. Bacillus anthracis
  2. Flagellate rods: ex. Salmonella typhi
  3. Spore-former: ex. Clostridium botulinum

  • Vibrios

The comma in shape or curved rods. They have a single genus. Ex. Vibrio cholerae, Vibrio comma.

  • Spirals/Spirilla

Spiral or spring-like. Have multiple curvature and terminal flagella. These are longer rigid rods with several curves or coils. Helical in shape. Ex. Spirillum ruprem, Spirillum volutans, Helicobacter pyloria.

On mode on nutrition

  • Phototrophs:

Those bacteria gain energy from light.

  1. Photolithotrophs: gain energy from light and uses reduced inorganic compounds such as H2S as an electron source. Ex. Chromatium okenii.
  2. Photoorganotrophs: gain energy from light and uses organic compounds such as succinate as an electron source.

  • Chemotrophs:

Bacteria gain energy from chemical compounds. It cannot carry out photosynthesis.

  1. Chemolithotrophs: gain energy from the oxidation of chemical compounds and reduces inorganic compounds such as NH3 as an electron source. Ex. Nitrosomonas.
  2. Chemoorganotrophs: gain energy from the chemical compound and uses organic compounds such as glucose and amino acids as a source of an electron. Ex. Pseudomonas pseudoflava.

  • Autotrophs:

Bacteria use CO2 as a sole source of carbon to prepare their food. There are 2 types on basis of energy utilized to assimilate CO2.

  1. Photoautotrophs: utilize light to assimilate CO2 basis of the electron source.
  2. Chemoautotroph’s: utilize chemical energy for the assimilation of CO2.

  • Heterotrophs:

Bacteria use the organic compound as a carbon source. They cannot fix CO2. Most human pathogenic bacteria are heterotrophic.

  1. Simple heterotrophs have simple nutritional requirements.
  2. Fastidious heterotrophs: require special nutrients for their growth.

Basis of temperature requirements.

  • Psychrophiles:
  1. Bacteria that can grow at 00C(min.) – 150C (opt.) – 200C (max.).
  2.  they have polyunsaturated fatty acids in their cell membrane which gives fluid nature to cell membranes at a lower temperature.
  3. Ex. Virio psychroerythrus, Vibrio Marinus, Polaromonas vascular, Psychroflexus.

  • Psychrotrophs / Facultative psychrophiles:
  1. Bacteria that can grow even at 00C but optimum temperature are 20-30oc.
  • Mesophiles:
  1. Bacteria grow at 250C (min.) – 370C (opt.) – 400C.
  2. Most human pathogens are mesophilic.
  3. Ex. E.coli, salmonella, staphylococcus.
  • Thermophiles:
  1. Bacteria best grow above 45oC.
  2. Contain saturated fatty acids in their cell membrane so their cell membrane does not become too fluid even at a higher temperature.
  3. Capable of growing in mesophilic range (400C) are Facultative thermophiles.
  4. True thermophiles (<450C) are Stenothermophiles
  5. Ex. Streptococcus thermophiles, Thermus aquaticus, Bacillus stenothermophiles.
  • Hypethermophiles:
  1. Best above 800C.
  2. Mostly Archeobacteria is this type.
  3. The monolayer cell membrane of archeobacteria is more resistant to heat and they adapt to grow in higher temperatures.
  4. Ex. Thermodesulfobacterium, aquifex, Pyrolobus fumari, thermotoga.

On basis of oxygen requirement.

  • Obligate aerobes:
  1. Requires O2 to live.
  2. Ex. Pseudomonas.
  • Facultative anaerobes:
  1. can use O2, but can grow in its absence.
  2.  Have a complex set of enzymes.
  3.  Ex. E.coli, yeasts, and many intestinal bacteria.
  • Obligate anaerobes:
  1. Cannot use O2, but tolerate the presence of O2.
  2.  Can break down toxic forms of O2.
  3. Ex. Lactobacillus carries out fermentation regardless of O2 presence.
  • Microaerophiles:
  1. Requires O2, but at low concentration.
  2. Sensitive to toxic forms of O2.
  3. Ex. Campylobacter.

On basis of pH of growth.

  • Acidophiles:
  1. Grow best at an acidic pH. (below 7)
  2. The cytoplasm of these bacteria is acidic.
  3. Some acidophiles are thermophilic, called Thermoacidophiles.
  4. Ex. Thiobacillus thioxidans, Thiobacillus, ferroxidase.
  • Alkaliphiles:
  1. Grow best at an alkaline pH. (above 7)
  2. The optimum pH of growth is 8.2
  3. Ex. Vibrio cholera
  • Neutrophiles:
  1. Grow best at neutral pH. (6.5 – 7.5)
  2. Most of the bacteria grow at neutral pH.
  3. Ex. E.coli.