Micro organism – Types and growth

Micro-organisms are very small living things which are normally not visible to the naked eye but can be seen with the help of a microscope.

Types of Micro-organisms

Micro-organisms include the following:

(i) All Viruses e.g. Polio virus, Smallpox virus, etc.

(ii) All bacteria e.g. Salmonella, Clostridium, Treponema, Escherichia coli, etc.

(iii) All protozoans e.g. Plasmodium, Trypanosoma, etc.

(iv) Some fungi e.g. Rhizopus (mould) and Yeast (e.g. Saccharomycetes).

(v) Some algae e.g. diatoms, dinoflagelletes, etc.

(vi) Blue-green algae e.g. Nostoc

Micro-organisms live everywhere, in water, air, soil, inside and outside of plants and animals including human beings. There are many more microorganisms than visible plants and animals in the world. They may have beneficial or harmful effects. Micro-organisms that cause disease are referred to as pathogens and are usually parasitic.

Growth of Micro-organisms

Culturing is the growing of micro-organisms in prepared media in the laboratory. The prepared medium is called the ‘culture medium’. Bacteria, fungi and algae grow easily in test-tubes, flasks or Petri dishes of culture media. Virus on the other hand, can only grow and multiply inside living cells, so they cannot be grown in a culture medium.

Micro-organisms are able to increase in size and multiply in number of cells. The growth of micro-organisms is measured based on increase in population size rather than increase in cell size. Under favourable conditions (food, adequate temperature and humidity) micro-organisms reproduce asexually by binary fission. Generation time varies from species to species e.g. rapidly growing species like Escherichia coli can divide every 30 minutes.

Measurement of the Growth of Micro-organisms

Two methods are used to measure the growth of micro-organisms:

(i) First Method: This involves inoculating a bacterial sample into a nutrient broth. As the bacterial population increases, the clear liquid medium becomes cloudy/turbid. Increase in turbidity indicates an increase in number of bacterial cells. Turbidity can be measured using a spectrophotometer. Thus by measuring the turbidity of a bacterial culture in nutrient broth at regular intervals, the growth of a bacterial population can be measured.

(ii) Second Method: This involves taking small samples of bacteria from a nutrient broth at regular intervals of time and diluting the samples several times. Each diluted sample is then inoculated onto a nutrient agar medium in a petri dish and incubated. The number of colonies formed in each petri dish is counted and this indicates the number of living bacterial cells in the diluted sample. From this, the actual number of bacteria in the original sample can be calculated.


  1. Mention five microorganisms.
  2. How is the growth of microorganisms measured?
  3. Describe two ways of growing microbes in the laboratory.

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