Microscopic algae and mould are sometimes thought of as kinds of plants, or as part of a separate group called protoctista . But blue-green algae are now thought of as bacteria. The protoctista are also part of a much bigger group of organisms called protists , which are dominated by protozoa . While bacteria are prokaryotic , which means they have no nucleus, protozoa are eukaryotic. Some protozoa are parasites of fish and land animals.
Amoebas can cause disease called amoebic dysentery .
Genetic engineering Micro-organisms have played a vital role in the development of genetic engineering techniques. The idea of genetic engineering is to change a creature's genetic code to eliminate faults such as inherited diseases, or to create a particular effect. So far, the most common use of genetic engineering has been to produce medicines cheaply in large quantities. If scientists can identify the gene responsible for making a certain substance, they can insert it into bacteria. Because bacteria multiply rapidly they will manufacture large quantities of the substance very quickly.