Micro-organisms, like all living things, do increase in size and multiply in number of cells using either the culture medium provided or any suitable surface such as moist bread as source of food. Microorganisms can also increase in mass. Such an increase in size, mass or number of cells is regarded as growth in micro-organisms.

If environmental conditions of growth such as food, adequate temperature and humidity are favourable, such an increase in mass, size and number of cells of the colony is an index of growth of micro-organisms.


Ways of Measuring Growth in Microorganisms

There are two major ways of measuring growth in micro-organisms. These are:

  1. First method: A bacterial sample is innoculated into a nutrient agar (a clear liquid culture medium). As the bacterial population increases, the clear liquid medium becomes cloudy or turbid. Progressive increase in turbidity indicates a related increase in the number of bacterial cells. This property is used to measure bacterial growth. In the laboratory, turbidity can be measured with a spectrophotometer. This instrument measures the amount of light that can pass through a liquid medium. As turbidity increases, the amount of light that can pass through the liquid medium decreases. Thus, by measuring the turbidity of a bacterial culture in a nutrient agar at regular intervals, one can measure the growth of a bacterial population.
  2. Second method: In this method, small samples of bacteria are taken from the nutrient agar at regular intervals of time. Each sample is diluted several times. Each diluted sample is innoculated on to a nutrient agar medium in a petri-dish and incubated. The number of colonies formed in each petri-dish is counted. As each colony is formed by the multiplication of a single bacterium, the number of colonies indicate the number of living bacterial cells in the diluted sample. From this, the actual number of bacteria in the original sample can be calculated.



Some micro-organisms especially bacteria and fungi are beneficial to man in three major ways: in nature, in medicine and in industries.

In Nature

  1. Compost formation: Microorganisms especially bacteria aid compost formation through the decay of dead organisms and humus.
  2. Nitrogen fixation: Certain bacteria (Rhizobium Leguminosarium) aid nitrogen fixation into plants through the root nodules of leguminous plants.
  3. Maintenance of soil fertility: Most saprophytic bacteria due to their decomposition activities release nutrients into the soil which aid its fertility through nitrogen cycle. Digestion of cellulose: Some bacteria living in the rumen of ruminant animals like cattle, sheep and goat help such animals to digest cellulose in the rumen.
  4. Decomposition: Micro-organisms especially saprophytic bacteria also aid the decomposition of dead plants and animals thereby releasing nutrients to the soil.
  5. Silage making: Some bacteria are also useful in silage making which involves the preservation of pasture crops for future use.
  6. Sewage treatment: Bacteria are also involved in the decomposition of sewage into harmless substance.
See also  PRIMATES


In Medicine

  1. Manufacturing of drugs / vaccines: Micro-organisms are used in medicine for the manufacturing of drugs, vaccines and antibiotics which help in curing many human diseases, e.g. penicillin (an antibiotic) is obtained from Penicilium notatun (a fungus) and streptomycin, another antibiotic, is obtained from a bacterium called Streptomyces griseus.
  2. Sources of vitamin B: Yeast and certain bacteria are rich sources of vitamin B complex.
  3. Sources of enzymes: Certain yeast and bacteria are also rich sources of enzymes like amylase and invertase.


In Industries

  1. For baking: Yeast (fungus) is useful in baking industries as it causes dough to rise in bread and other allied food processing.
  2. Preparation of alcoholic drinks: Yeast is also useful in brewing industries as it is used for the fermentation of sugar to produce alcohol.
  3. Making of cheese / yoghurt: Certain bacteria are used in food processing such as cheese, vinegar and yoghurt.
  4. Retting of jute: Certain bacterial action are used in the retting of jute to obtain sack fibres from flax plants.
  5. Curing of tobacco: Bacteria are useful in the curing or ripening of tobacco leaves.
  6. Tanning of hides and skins: The action of bacteria is useful in the conversion of hides and skin to leather during the tanning process.



  1. Cause of disease: Micro-orga-nisms are responsible for various diseases in plants and animals.
  2. Spoilage of food: Micro-organisms are generally known to cause food spoilage.
  3. Deterioration of materials: Micro-organisms are also responsible for the deterioration of materials like wood, cotton, paper and leather.
  4. Causes of death: Micro-organisms can also cause the death of plants and animals.



1a) Animal disease caused by viruses: Animal diseases caused by viruses include poliomyelitis, infective hepatitis, measles, common cold, German measles, influenza, small pox, chicken pox, yellow fever, rabies, Aquired Immune Deficiency Syndrome (AIDS), food and mouth disease, rinder pest, and Newcastle diseases.

b) Plant diseases caused by viruses: Plant diseases caused by viruses include rosette disease, cassava mosaic disease, maize streak, yam mosaic disease, cowpea mosaic disease, leaf curl, tristeza, bunchy top disease and swollen shoot disease.


2a)Animal diseases caused by bacteria: Animal diseases caused by bacteria include tuberculosis, leprosy, tetanus, typhoid, dysentery, cholera, pneumonia, anthrax, diphtheria, gonorrhea, syphillis, meningitis, yaws, contagious abortion and whooping cough.

b) Plant diseases caused by bacteria: Plant diseases caused by bacteria include leaf blight of cassava, black arm, cassava wilt, banana wilt, tomato rot, onion rot, galadima, bacterial wilt, bacterial fire blight and yam rot.


3a) Animal diseases caused by fungi: Animal diseases caused by fungi include ringworm, aspergilosis, athletic foot and thrush.

b) Plant diseases caused by fungi: Some plant diseases caused by fungi are maize smut, rice blight, maize rust, leaf spot, cocoa black pod, coffee leaf rust, okra damping off, onion twister disease, rice smut, Gummosis, citrus scab, Panama disease, sigatoka disease, black spot disease, blast disease, anthracnose and freckle disease.


4) Animal diseases caused by protozoa: Some animal diseases caused by protozoa include malaria, trypanosomiasis, coccidiosis, trichomoniasis and red water disease.

  • Malaria is caused by Plasmodium.
  • Trypanosomiasis is caused by Trypanosome.
  • Coccidiosis is caused by Eimeria spp.
  • Red water disease is caused by Babesia spp.


Spread of Disease-Causing Microorganisms

Micro-organisms are spread or transmitted through the following:

  • Air, i.e ., air-borne pathogens,
  • Water, i.e ., water-borne pathogens, Food, i.e ., food-borne pathogens,
  • Animal vectors or carriers,
  • Personal or direct skin contact.

Certain diseases known as contagious diseases can be contracted by direct contact with an infected person,e.g. leprosy, measles, tuberculosis and sexually transmitted disease (STD) such as gonorrhea, syphilis and AIDS.

Names of some diseases, causative micro-organisms, mode of transmission, host and major symptoms are stated in the tables below.





Diseases Spread by Vectors

A vector is an animal which transmits a disease-causing organism (pathogen) from the person suffering from the disease to another person. The table below shows the vectors, micro-organisms, and the diseases they transmit.




Diseases Spread Through Sexual Contact

Diseases which are transmitted by sexual contact are known as venereal diseases (VD) or sexually transmitted diseases (STD). Examples of venereal diseases are gonorrhea, syphilis and Acquired Immune Deficiency Sydrome (AIDS). Others are urethritis and herpes which are caused by viruses, trichomoniasis caused by a protozoan, thrush caused by a fungus and staphilococcus caused by a bacterium.

Some sexually transmitted diseases, causative organisms, mode of transmission, their hosts and symptoms are outlined in the table below.


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