Bacteria are all around us. They are in our food, on our skin, inside our bodies, in the air and in water. Most bacteria are harmless and many are extremely useful. For example, bacteria in our large intestine make vitamin K and we use others to make cheese and yogurt. Some bacteria are harmful and can cause diseases like tuberculosis, meningitis and cholera.
Activity To show the presence of micro-organisms in air and in soil
Apparatus Three sterile Petri dishes containing nutrient agar (agar plates), sample of fresh soil
Nutrient agar
Petri dish
Fresh soil sprinkled on the agar
Method 1. Sprinkle some fresh soil on one agar plate. Replace the lid as quickly as possible to allow minimum exposure to air.
Petri dish exposed to air
Petri dish not exposed to air
2. Open a second agar plate for three minutes to expose it to the air. Replace the lid. 3. Do not open the third agar plate. This plate is a control to show that any bacteria that grows on the other plates has come from either the soil or the air.
4. Leave the three plates in a warm place in the laboratory for one week. 5. Examine the three plates and record your results. 6. Follow the teacher’s instructions to ensure the safe disposal of the bacteria-covered plates.
Effects of bacteria Negative effects of bacteria
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Cause diseases, e.g. cholera and food poisoning
Cause tooth decay
Cause body odour due to the build-up of bacteria if a person does not wash regularly.
l Spoil food, e.g. souring milk Positive effects of bacteria
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Prevent the growth of harmful bacteria in our large intestines.
Produce vitamins in the human digestive system.
Used in the making of cheeses and yoghurt.
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Recycling nutrients in the soil by decomposing (breaking down) dead plants and animals to release their nutrients back into the soil.
