Microoraganisms in Action – SS1 Biology Lesson Note (Third Term)

Micro-organisms such as bacteria reproduce by binary fission, in which one cell divides into two. Their growth goes through four phases and can be measured by three methods. Micro-organisms are useful in food, medicine and industry, but they also spoil materials and cause diseases such as cholera, measles and malaria.

Behavioural objectives

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

• describe binary fission and explain generation time;
• list the four phases of bacterial growth;
• name the methods used to measure the growth of micro-organisms;
• state the beneficial and harmful effects of micro-organisms;
• name the causative agent, mode of transmission and control of common diseases caused by micro-organisms.

How do micro-organisms grow?

Bacteria reproduce by binary fission, in which a single cell divides into two. This process is called asexual reproduction. The time interval required for the cell to divide into two is called generation time. This time varies from one organism to another.

Generation time depends strongly on nutrient availability, temperature, gaseous requirement and pH.

What are the phases of bacterial growth?

There are four phases in the growth of bacteria:

1. Lag phase
2. Logarithmic or exponential phase
3. Stationary phase
4. Decline or death phase

How is the growth of micro-organisms measured?

The growth of micro-organisms can be measured by using any of these methods:

1. Turbidity method
2. Serial dilution method
3. Squared transparent paper or cellophane method

What are the beneficial and harmful effects of micro-organisms?

Beneficial effects

1. Bacteria help to digest cellulose in herbivores.
2. In humans, they synthesise vitamin K and B12.
3. Bacteria and fungi are widely used in the synthesis of antibiotics.
4. They are used to manufacture amino acids and vinegar.
5. Bacteria are used to process milk into different tastes and flavours.
6. They are used to decompose sewage into harmless inorganic compounds.
7. Microbial cultures are used to produce enzymes.
8. Yeast is used as a leavening agent in baking industries.
9. Algae play an important role in fertilising the soil.
10. Bacteria are used to produce single-cell protein (SCP).

Harmful effects

1. Bacteria cause decay and spoilage of food items.
2. Materials like wood, paper, textiles, rubber and metals are destroyed by microbes.
3. They cause diseases of different types.

Which diseases are caused by micro-organisms?

Evaluation

1. What is generation time?
2. Name the four phases in the growth of bacteria.
3. List the three methods of measuring the growth of micro-organisms given in this lesson.
4. State two harmful effects of micro-organisms.
5. Name the causative agent of tuberculosis and state how it is transmitted.

Assignment

1. The growth phase in bacteria in which cells divide steadily at a constant rate is called
   (a) exponential phase (b) lag phase (c) stationary phase (d) decline phase
2. Which of the following microbes causes cholera?
   (a) Virus (b) Bacterium (c) Protozoan (d) Fungus
3. Growth of micro-organisms can be measured by the following methods except ___________ method.
   (a) serial dilution (b) turbidity (c) squared transparent paper (d) dry weight
4. The following practices contribute to the control of the spread of diseases except
   (a) sewage treatment with chemicals (b) proper sewage disposal (c) disinfecting the surrounding (d) using human faeces as manure
5. The vector of the trypanosome parasite is
   (a) housefly (b) tsetse fly (c) mosquito (d) black fly
6. Define the following phases in micro-organisms’ growth:
   (i) lag
   (ii) exponential
   (iii) stationary
7. State four ways in which each of the following organisms is beneficial to humans:
   (a) Bacteria
   (b) Fungi