Introductory Microbiology

Nutritional classification

• Microorganisms are grouped in relation to nutritional requriement into two groups;

  I. Autotrophs: which can utilize inorganic nutrients or light as a source of energy. Ex: Nitrifying bacteria, algae etc.
  II. Heterotrophs: Which requires organic compounds as a source of energy. Ex: Many bacteria, fungi and protozoa.
• Currently microorganisms are grouped into four major groups based on the above parameters.

  1. Photoautotrophs: which use light as energy source and CO2 as carbon source. Ex: Algae and many photosynthetic bacteria.
  2. Photoheterotrophs: Which use light as energy source and reduced organic compounds as C source EX: purple non-sulphur bacteria.
  3. Chemoautotrophs: Which use inorganic chemicals as energy source and CO2 as carbon source. Obtain their energy by the oxidation of reduced inorganic compounds such as NH3, NO2, H2, H2S, Fe2+. These organisms are also called as chemolithotrophs. Since these can grow in an inorganic medium in the absence of light.
  4. Chemoheterotrophs: Which use organic compounds as an energy source as well as principal carbon source. The clear distinction between the energy source and the carbon source which is the characteristic of the above 3 groups is lost since this group can desire both carbon and energy from a single organic compound.

Microorganisms	Energy	Carbon	Examples
Photolithotrophs	Light	Carbon dioxide	Algae, Purple sulphur bacteria, Green sulphur bacteria
Photo organotrophs	Light	Organic	Purple non-sulphur bacteria
Chemolithotrophs	Oxidation of inorganic compounds	Carbon dioxide	Nitrifying bacteria, Iron bacteria, Hydrogen bacteria
Chemo organotrophs	Oxidation of organic compounds	Organic compounds	Most bacteria fungi and protozoa