Microorganisms
In this topic you will:
- learn about the different kinds of microorganism
- grow some microorganisms on agar jelly.
Key words
- agar jelly
- algae
- bacteria
- colony
- fungi
- microorganism
- mushroom
- Petri dish
- protozoa
- single-celled
- sterile
- toadstool
- yeast
What is a microorganism?
A microorganism is a living organism that is so small that you can only see it clearly by using a microscope.
Like all living organisms, microorganisms are made of cells. Many microorganisms are made of only one cell: they are single-celled.
There are several different kinds of microorganism. They include bacteria, microscopic fungi, protozoa and algae. Each of these microorganisms is described later in this topic.
Common Mistake
Don't confuse single-celled organisms with small multi-celled organisms. Being small doesn’t mean it is single-celled — what matters is the number of cells, not the size!
Bacteria
Bacteria are everywhere. (Bacteria is a plural word. The singular word, for just one of them, is bacterium.)
Each bacterium is made of a single cell. Cells of bacteria are much smaller than animal cells or plant cells. You could fit 1000 of the bacteria in the photograph, lined up end to end, between two of the millimetre marks on your ruler.
Most bacteria are harmless but there are a few kinds that can make you ill.
Common Mistake
Many people think “bacteria” is singular, but it’s actually plural. The correct singular form is bacterium. For example: “One bacterium is in the sample” — not “one bacteria”.
Fungi
Fungi (singular: fungus) are not always microorganisms. Many fungi, including mushrooms and toadstools, are large and easy to see.
Mushrooms and toadstools are only part of the fungus’s body, though, and they only grow at certain times of year. Most of the time, the fungus is just a tangle of very thin threads. The threads often grow under the ground, or inside a dead log. The threads are so thin that they are difficult to see without a microscope.
There are also some kinds of fungi that do not produce mushrooms or toadstools. They are made of single cells, not threads, so they are definitely microorganisms. The powdery substance that you sometimes see on the surface of grapes is made up of millions of cells of yeast, which is a microscopic fungus.
Common Mistake
Don’t assume all fungi are microorganisms. Only fungi made of single cells — like yeast — are classified as microorganisms. Fungi made of threads, like mushrooms and toadstools, are usually visible and not considered microorganisms.
Questions
Show Answer
Viruses are not considered true living organisms because they cannot carry out life processes on their own. They need a host cell to reproduce, so they are not classified as microorganisms.
Show Answer
Yeast is made of single cells and can only be seen when many are grouped together. Individually, they are microscopic — that’s why yeast is classified as a microorganism.
Growing microorganisms
A single microorganism is too small to see without a microscope, but when left to grow, a single cell of a bacterium or fungus divides repeatedly to make a collection of many cells. This collection of cells is called a colony. The colonies are big enough for you to see without a microscope.
This can be done safely in the laboratory. Scientists let microorganisms grow in a Petri dish containing a special kind of jelly, called agar jelly.
The dish and the jelly have to be sterile. This means that any living organisms on them have been killed.
Common Mistake
Don’t forget that agar jelly must be sterile before use. If the Petri dish or jelly contains any unwanted microbes, the results of the experiment could be misleading or contaminated.
Microscopic algae and protozoa
If you look at some pond water through a microscope, you will see many tiny living organisms in the water. Some of them are tiny plant-like organisms, called algae. Some of them are animal-like organisms, called protozoa. (The singular forms of these two words are alga and protozoan.)
Common Mistake
Don’t mix up algae and protozoa. Algae are more like tiny plants — they can make their own food using sunlight. Protozoa are more like tiny animals — they move and feed like animals but are still single-celled.
Questions
Show Answer
The cells may be arranged in chains or clusters. These arrangements help the microorganisms grow or move together but they are still considered microscopic.