A microbe is any living organism that spends its life at a size too tiny to be seen with the naked eye. Microbes include bacteria and archaebacteria, protists, some fungi and even some very tiny animals that are too small to be seen without the aid of a microscope. Viruses and the recently discovered prions are also considered microbes.
The term microbe is short for microorganism, which means small organism. To help people understand the different types of microbes, they are grouped or classified in various ways. Microbes are extremely diverse and represent all the great kingdoms of life, including the animals, plants, fungi, protists and bacteria (see the tree below). In fact, in terms of numbers, most of the diversity of life on Earth is represented by microbes.
At the most basic level, all living things can be divided into two major groups of organisms depending on the type of cells they have. Those organisms which contain more complex cells and which have nuclei are called eukaryotes ("true kernal or nucleus"). These organisms include animals, plants, fungi and protists. The other group of organisms are less complex and lack nuclei and are called prokaryotes ("before nucleus"). The prokaryotes are also known as bacteria or as monerans. An amazing recent discovery has shown that the bacteria can be divided into two groups, one of which, the archaebacteria, is more closely related to humans than is the other group of bacteria, the eubacteria, which are discussed below.
All microbes are related to one another and to humans. To see how all forms of life are related, check out the Tree of Life World Wide Web site.
Prokaryotes? contain no nucleus or other cell organelles. Often simple in form, bacteria have a huge range of biochemical diversity, meaning that they can grow on many different compounds.
Prokaryotes are lumped into two groups, the Eubacteria and the Archaea. Although many Eubacteria and Archaea look similar, if you were to look at the molecules that make them, you would find that the Eubacteria differ more from the Archaea than the Archaea differ from the rest of the Eukaryotes, which include plants, animals, fungi and protists.
The archaea are similar to the eubacteria in size and in many forms, yet they differ in many ways from the eubacteria. Generally, this group of bacteria, also known as archea, thrive in extreme environments, such as hot thermal vents, under conditions with no oxygen or in highly acid environments.
Fungi are organisms that scientists once confused with plants because they are non-motile, like plants. However, thanks to new research tools, scientists have found that, at the molecular level, the fungi are more like animals than they are like plants. For one thing, fungi cannot synthesize their own food via photosynthesis, like plants do, but instead they feed off of other organisms as do animals.
Are animals microbes? If it is too tiny to be seen with a naked eye, then it is a microbe. Most animals, like cows, butterflies and earthworms are not microbes. Tiny animals like rotifers and nematodes are considered microbes.
Viruses are small bits of genetic code in a protective covering. Viruses are not "alive," that is, they cannot replicate, unless they are inside another organism. A virus is definitely too small to be seen without a microscope. Since viruses are so small (tinier than bacteria) they may be considered microbes. However, since they are not "alive" outside of a host organism, it is debatable whether they are really organisms at all. For convenience since they are neglected in other areas of biology, viruses are discussed in microbiology. Most viruses are known because they cause disease.
Like viruses, prions are "elements" that can replicate within the appropriate hosts, yet they cannot live on their own. Unlike all other microbes, which contain nucleic acids, prions appear to contain only proteins that allow for their replication. Prions are so named because of a nicer sounding variation of the contraction of proteins and infections.