Alga (plural: algae) A plant-like protist found in water or moist ground. Algae make food by photosynthesis. They range from single-celled organisms to large, multicellular forms, such as seaweed.

Amoeba A protozoan with no definite shape. Amoebae live in water and move by flowing like a bag full of jelly. They feed on other micro-organisms, such as bacteria, and reproduce by splitting in two.

Apicomplexa A group of parasitic protozoans, including the species that cause malaria in humans. Most acicomplexans invade and live inside the cells of other organisms.

Archaea A group of single-celled micro-organisms with no cell nucleus. They differ from bacteria in their chemical make-up. Typically, they live in harsh environments such as hot springs or salt lakes, but they are also found in great abundance in the soil, the oceans, and inside plants and animals, including humans.

Aspergillus A microscopic fungus that grows on decaying matter and soil. Some species can cause disease in humans.

Bacillus A rod-shaped bacterium.

Bacterium (plural: bacteria) A simple single-celled micro-organism. Bacteria are found nearly everywhere, including in air, water, ice, rocks and inside other organisms. Some bacteria cause disease in humans.

Cell A tiny “building block” which makes up the tissues in all living things. It is typically a bag of jelly containing a nucleus and other working parts called organelles.

Ciliate A large protozoan covered with hair-like cilia. These beat back and forth to make the organism move.

Coccus A spherical, or ball-shaped, bacterium.

Cyanobacterium A bacterium, also known as blue-green algae, that produces its own food using photosynthesis.

Diatoms Single-celled algae that live in ponds, rivers and oceans. Diatoms are protected by a hard case.

Didinium A freshwater ciliate that feeds on protists much larger than itself.

Dinoflagellate A marine or freshwater alga with a strong, rigid cell wall and pointed horns that help it to float upright.

Euglena A protist that makes its food by photosynthesis and moves by flicking its tiny, whip-like flagellum.

Flagella Tail-like projections that enable some micro-organisms to swim. Flagella are used in a whipping motion to propel organisms through the water.

Foraminiferan A small protozoan, with a rigid, case-like cell wall, like a beautifully shaped shell.

Heliozoan A spherical, freshwater protozoan, surrounded by long, stiff projections that are used to catch prey.

Micro-organism A microscopic organism, also called a microbe or germ, when it causes disease. It may consist of a single cell or be made up of many cells. Micro-organisms include prokaryotes (bacteria and archaea), protozoa, fungi, algae and animals such as rotifers and planarians. Viruses are not strictly considered to be micro-organisms, because they cannot grow or reproduce by themselves.

Microbe Another word for a micro-organism, but usually used when describing pathogens (germs), those micro-organisms that cause diseases in plants and animals, including humans.

Multicellular Consisting of more than one cell.

Nucleus A large structure in some cells, that contains a cell’s genes and therefore controls all cell activities.

Organelle A tiny structure inside some cells. Each type of organelle has its own specific function. The word organelle means “little organ”.

Prokaryote A single-celled organism, such as a bacterium, that lacks a nucleus and organelles.

Protist An organism made up of a single cell containing a nucleus. Protists live mainly in water and damp places. Some (algae) make energy by photosynthesis but others (protozoans) take in food.

Protozoan (plural: protozoa) A protist that obtains energy by eating food. Some protozoa sift food particles from passing water. Others actively hunt prey.

Spirillum A corkscrew-shaped bacterium.

Virus A microscopic package of chemicals that causes disease and can only reproduce by invading the cells of a living organism. Some human diseases, such as colds and flu, are caused by viruses.

Vorticella A transparent protozoan with a bell-shaped body attached to a stalk. Hair-like cilia waft food into its mouth.

Yeast A microscopic, single-celled fungus found in soil and on plants. Yeasts contain chemicals that change sugar into alcohol and carbon dioxide gas.

Consultant: Chris Jarvis