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Micro-organisms otherwise called microbes or germs can be defined as living things which cannot be seen with unaided eye but by the use of microscopes.


They exist almost everywhere, in water, air, soil, surface of objects, as well as on and within living organisms.  They are carried by air currents from the earth’s surface to the upper atmosphere. They occur most abundantly where there is food, moisture and adequate temperature for their growth.


It was the invention of microscope that opened the gateway to the world of these minute living organisms. The first person to discover microbes was a Dutch man called Anthony Leeuwenhoek (1632-1723). Using a simple microscope, he was astonished to discover that rain water that had been collected from pools was full of little organisms.



Micro-organisms include all viruses, bacteria and the protists. Others are the cyanobacteria, certain fungi and algae.

  1. BACTERIA: These are minute unicellular organisms or simple association of similar cells which multiply by binary fission. Most bacteria cells range between 0.2 µ-2µ in diameter and 0.0005mm-0.002mm long. Each bacterium cell has a cell wall with cytoplasm. There is no well defined nucleus. Consequently, they are prokaryotic organisms.


There are different kinds of bacteria showing a range of shapes. Certain kinds of bacteria have long thread-like structures called flagella which assist in locomotion. Bacteria with spherical shape are referred to as cocci (singular-coccus). There are several forms as shown on the next page.

Streplococci– These are arranged in chains. They cause sore throat.

Staphylococci- These stick together to form irregular bunches. They cause boils.

Diplococci– They occurs in pairs. e.g. pneumococci which causes pneumonia.

Bacilli– They is rod-shaped. They cause typhoid fever.

Spirilla (singular = spirillum)- These are rod-shaped bacteria twisted into a spiral shape.

Spirochaetes- These are also spiral in shape but are more flexible and slender with helically coiled structure e.gTreponemapallidum which causes syphilis.

Vibrios- These are comma-shaped bacteria e.gVibriocholera which causes cholera.


  1. VIRUSES: Viruses are a large group of pathogens whose presence is felt only when they are in contact with living cells. They are very small and vary between 0.1µ-0.25 µ in diameter. The largest virus is less than one-fourth the size of typhoid bacterium.


A virus consists of a nuclear material either DNA or RNA, enclosed within a protein coat. Outside living organisms they are like complex chemicals.

  1. PROTISTS: These are single-celled animals, mostofwhich are only visible by means of microscope. They are common in fresh water and moist soils. Examples include Euglena, Paramecium, Trypanosoma, Plasmodium,
  2. FUNGI: They are diversified in form. The blue and green growth on oranges, lemons, cheese and the white/grey growth on bread are usually signs of fungal infections. Fungi feed saprophytically. Examples of fungi include Mucor, Rhizopus, Penicillium, Aspergillus, etc.
  3. ALGAE: Most algae are unicellular and very small. They have chlorophyll. They occur abundantly in water, moist soils, bark of trees, stones, etc. Free floating microscopic algae are referred to as phytoplanktonsand they form the major food of aquatic animals. Examples of unicellular algae include Chlamydomonas and Protococcus.



  1. What are microorganisms
  2. List five groups of microorganisms with two examples each



A pre-requisite to studying microbes is their cultivation under laboratory conditions. Hence, it is important to know the nutrients and physical conditions needed by the organisms.


It is easier to grow bacteria, fungi, and algae in appropriate media. The material on or in which microbes grow in the laboratory is called culture medium. Some media are prepared from complex extracts of plant or animal tissues. A culture is the population of organisms cultivated in a medium.


If a culture contains only one living species of organism regardless of the number of individuals, it is said to be a pure or axenic culture. A culture which contains two or more species growing together is called a mixed culture.


An important medium used for growing microbes is agar. It is a dried polysaccharide extract of red algae which is used as a solidifying agent. It is not broken down by microbes.


  1. What is a culture medium?
  2. Differentiate between axenic and mixed culture



There are many ways of identifying micro-organisms around us. These include the use of microscopes stains of different types, types of colonies formed by the microbes, their food requirement and oxygen requirement of the organisms.


To determine the presence of microbes around us, suitable media are used to culture them in petri-dishes which are first sterilized by heating them in a pressure cooker, autoclave or oven.

Micro-organisms are not capable of growing in the air. The exposure of nutrient agar to the air will show the growth of different bacteria colonies in the air. Microbes commonly found in the air include viruses, bacteria, fungi, etc.


Microbes in aquatic habitat may be grouped into natural water, soil and sewage microbes. Examples of the first category include protists, algae, some fungi,bacteria, etc. Examples of the second group include Rhizobium, Nitrosomonas, and Nitrobacter.  Examples of sewage microbes are Entamoebahistolytica, Escherichia coli, etc.


Microbes living in our bodies form normal population without causing any harm. However, under certain conditions, they may become dangerous. Pathogenic organisms cause diseases when body resistance is low or when normal microflora is de-established by the use of antibiotics. Any food item left unpreserved for a long time will be spoilt by the activities of microbes.

Pathogens enter the body through four main ways, namely: air, food and water, contact (direct or indirect), and insect bites/cuts.



  1. Give two examples each of soil and sewage microorganisms
  2. List four ways through which microorganisms enter the body



Any agent that carries microbes from one place to another is called a carrier. Carriers can be living or non-living things. Non-living carriers include air, water, and food while animals (e.g. houseflies, mosquitoes, rats, cats, etc) are the living carriers. Animal vectors carry pathogens either mechanically or biologically. In mechanical method, animals carry the pathogens on their bodies where they cannot grow or multiply. In biological method, the vector becomes infected by feeding on the body fluid of infected persons or animals.


Vector or Carrier Micro-organisms Disease caused
(i) Anopheles (female) mosquito Plasmodium Malaria fever
(ii) Tse-tse fly Trypanosome Sleeping sickness
(iii) Housefly Vibro cholera Cholera and typhoid fever
(iv) Aedes mosquito Virus Yellow fever



  1. What are vectors
  2. Mention two ways by which animal vectors carry pathogens.
  3. Explain each of the method described above



  1. Give two examples each of the following microorganisms (i) fungi   (ii) Bacteria     (iii) Algae    (iv) Protozoa
  2. What do you understand by the word ‘agar’
  3. Describe ways by which microorganisms can be transmitted
  4. State the vectors and the diseases caused by the following organisms (i) plasmodium (ii) trypanosome     (iii) vibro-cholerea



  1. When bacteria are arranged in chains, they are called (a) spirilla(b) staphylococci (c) streptococci     (d) bacilli
  2. Viruses are considered to be living organisms because they (a) possess transmittable characters       (b) move from one place to another      (c) respond to stimulation      (d) ingest food materials
  3. Which of the following is not a protozoan? (a) paramecium     (b) plasmodium       (c) penicillium      (d) Amoeba
  4. Which of the following best describes a culture solution?  (a) A population of micro-organisms cultivated in a medium      (b) A population of weeds cultivated in a medium       (c) Solution containing different chemicals    (d) Solution containing dead organisms
  5. Which of the following organisms is not a fungus? (a) Rhizopus    (b) Plasmodium (c) Mucor      (d) Aspergillus


  1. Differentiate between pathogens and vectors
  2. Describe the structure of a virus


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