Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Bacteria are among the most ancient forms of life on Earth. Our Earth formed about 4.6 billion years ago. Life is thought to have begun on Earth about 3.8 billion years ago and there is fossil evidence for the existence of bacteria 3.5 billion ago. Bacteria, whose cells do not have nuclei, were the only life forms on Earth for over two billion years as nucleated cells like those of fungi, plants and animals did not evolve until about 1.4 billion years ago.
So what are bacteria?
Bacteria are Prokaryotic organisms meaning they do not have a nucleus or any other membrane-bound organelles. Some bacteria are autotrophic i.e. they make their own food. These bacteria can either be photosynthetic (use light to make food) orchemosynthetic (use energy produced in reactions involving sulphur and iron compound and ammonia). Other bacteria areheterotrophic i.e. they cannot make their own food. Most bacteria are saprophytic, feeding on dead organic matter and acting as decomposers. Some heterotrophic bacteria are parasitecausing harm to their host organism while others form symbioticrelationships with their host.
Where are bacteria found?
Millions of species of bacteria have successfully evolved since their origin on the planet, exploiting every possible habitat of the biosphere including other living organisms. Our skin and gut are colonised by very different but specialised communities of bacteria, much to their and our advantage. We have evolved in tandem with gut bacteria, resulting in mutual benefit – mutualism. However, some of the bacteria naturally found in the human body can have negative effects. Therefore, it is important that the balance of bacteria be maintained to favour the beneficial over that potentially harmful ones.
Our gut bacteria carry out many crucial functions including:
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