Micro Organisms Around Us – SS1 Biology Lesson Note (Third Term)

Micro-organisms, also called microbes or germs, are living things too small to be seen with unaided eyes, so a microscope is needed to see them. They live in water, air, soil, on surfaces and on or inside living organisms. The main groups are bacteria, viruses, protists, fungi and algae. Some cause diseases and spoil food.

Behavioural objectives

By the end of the lesson, students should be able to:

• define micro-organisms and state where they are found;
• name the groups of micro-organisms and give examples of each;
• describe the shapes of bacteria and name diseases that some of them cause;
• explain what a culture medium, a pure culture and a mixed culture are;
• state how pathogens enter the body and how carriers spread microbes;
• match common vectors with the micro-organisms and diseases they carry.

What are micro-organisms?

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

They exist almost everywhere: in water, air, soil, on the surface of objects, and 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.

The invention of the microscope opened the way to the world of these minute living organisms. The first person to observe microbes was a Dutchman called Anthony Leeuwenhoek (1632 to 1723). Using a simple microscope, he was astonished to find that rainwater collected from pools was full of little organisms.

What are the groups of micro-organisms?

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

Bacteria

Bacteria are minute unicellular organisms or simple associations of similar cells which multiply by binary fission. Most bacteria cells range between 0.2 µm and 2 µm in diameter and between 0.0005 mm and 0.002 mm in length. Each bacterium cell has a cell wall with cytoplasm. There is no well-defined nucleus, so bacteria are prokaryotic organisms.

Bacteria show a range of shapes. Certain kinds have long thread-like structures called flagella, which assist in locomotion. Bacteria with spherical shapes are called cocci (singular: coccus). The main forms are:

• Streptococci: arranged in chains. They cause sore throat.
• Staphylococci: stick together to form irregular bunches. They cause boils.
• Diplococci: occur in pairs, for example pneumococci, which cause pneumonia.
• Bacilli: rod-shaped. They cause typhoid fever.
• Spirilla (singular: spirillum): rod-shaped bacteria twisted into a spiral shape.
• Spirochaetes: also spiral in shape but more flexible and slender, with helically coiled structures, for example Treponema pallidum, which causes syphilis.
• Vibrios: comma-shaped bacteria, for example Vibrio cholerae, which causes cholera.

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 0.1 µm and 0.25 µm in diameter. The largest virus is less than one-fourth the size of a typhoid bacterium.

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

Protists

Protists are single-celled animals, most of which are only visible using a microscope. They are common in fresh water and moist soils. Examples include Euglena, Paramecium, Trypanosoma and Plasmodium.

Fungi

Fungi are diversified in form. The blue and green growth on oranges, lemons and cheese, and the white or grey growth on bread, are usually signs of fungal infections. Fungi feed saprophytically. Examples include Mucor, Rhizopus, Penicillium and Aspergillus.

Algae

Most algae are unicellular and very small. They have chlorophyll. They occur abundantly in water, moist soils, bark of trees, stones and similar places. Free-floating microscopic algae are called phytoplankton, and they form the major food of aquatic animals. Examples of unicellular algae include Chlamydomonas and Protococcus.

What is culturing?

Studying microbes requires that they first be cultivated under laboratory conditions. It is therefore important to know the nutrients and physical conditions the organisms need.

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 a 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 called a pure or axenic culture. A culture which contains two or more species growing together is called a mixed culture.

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

How are micro-organisms identified?

There are many ways of identifying micro-organisms. These include the use of different types of microscope stains, the types of colonies the microbes form, their food requirements and their oxygen requirements.

To show the presence of microbes around us, suitable media are used to culture them in petri dishes. The dishes are first sterilised by heating them in a pressure cooker, an autoclave or an oven.

Microbes in the air

Micro-organisms cannot grow in the air itself. Exposing nutrient agar to the air, however, shows the growth of colonies of different bacteria. Microbes commonly found in the air include viruses, bacteria and fungi.

Microbes in water, soil and sewage

Microbes in aquatic habitats may be grouped into natural water, soil and sewage microbes.

• Natural water microbes include protists, algae, some fungi and bacteria.
• Soil microbes include Rhizobium, Nitrosomonas and Nitrobacter.
• Sewage microbes include Entamoeba histolytica and Escherichia coli.

Microbes in and on the body

Microbes living in our bodies form normal populations without causing any harm. Under certain conditions, however, they may become dangerous. Pathogenic organisms cause diseases when body resistance is low or when the normal microflora is disturbed 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 in four main ways:

• through the air;
• through food and water;
• through contact (direct or indirect);
• through insect bites and cuts.

What are carriers of micro-organisms?

Any agent that carries microbes from one place to another is called a carrier. Carriers can be living or non-living. Non-living carriers include air, water and food. Animals such as houseflies, mosquitoes, rats and cats are the living carriers.

Animal vectors carry pathogens either mechanically or biologically. In the mechanical method, animals carry pathogens on their bodies, where the pathogens cannot grow or multiply. In the biological method, the vector becomes infected by feeding on the body fluid of infected persons or animals.

Evaluation

1. Define micro-organisms and state three places where they are found.
2. List the five groups of micro-organisms and give one example of each.
3. Name three shapes of bacteria and state a disease caused by each.
4. Distinguish between a pure culture and a mixed culture.
5. Explain the difference between mechanical and biological carriage of pathogens by vectors.

Assignment

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
6. Differentiate between pathogens and vectors.
7. Describe the structure of a virus.