What are Bacteria? Protists? Viruses?
Bacteria are one of the simplest and oldest forms of life on earth. All bacteria are unicellular; their bodies always consist of a single cell (though they can occur in colonies). Bacteria are also all prokaryotic, meaning that they don’t have membrane-bound organelles and their genetic information is not contained in a nucleus.
Despite their simple makeup, bacteria are found in the air, earth, and water of almost every environment on earth. Bacteria can even be found inside glaciers, in boiling hot springs, and around high-pressure hydrothermal vents on the bottom of the ocean.
Bacteria are able to capture energy from light (like plants and algae) or from breaking down other living organisms (like animals and fungi). However, some bacteria can also collect energy through something called “chemosynthesis”, which is the breakdown of non-living chemicals like iron or sulfur.
Unlike bacteria, protists are always eukaryotic. Examples of protists include algae and amoebas. In simple terms, “protist” is a name for any eukaryotic organism that is not a plant, fungi, or animal.
Most protists are unicellular (single-celled) or form very small, simple colonies, but there are a few that are multicellular. One example of a multicellular protist is a type of algae called sea kelp, which can grow strands up to 150 feet long and forms massive underwater forests.
Some protists obtain energy from sunlight, others from consuming other living things, and still others meet their energy needs through a mix of both methods. Protists do not undergo chemosynthesis like some bacteria do and usually cannot survive in as extreme of environments as bacteria.
A virus is a microscopic infectious agent that lacks the ability to reproduce its genetic material without the help of a host cell. The inability to reproduce on its own is a very important facet of viruses, because it means that they are not technically considered alive based on the widely accepted characteristics of living organisms.
Viruses are also not technically considered to be living organisms because they are not made up of one or more cells. A virus is essentially just a protein capsule containing a bundle of genetic information; there is no cell membrane or cytoplasm.
In order to reproduce, a virus must infiltrate a host cell and “trick” that cell into reading and reproducing the virus’s genetic code. This is how viruses spread within the body.
Although a tiny, non-living bundle of genetic information doesn’t sound very threatening, viruses can still be incredibly dangerous. Deadly diseases like Ebola, swine flu, rabies, and HIV are all caused by lowly viruses.
Socha, Arek (artist). (2016). Virus Microscope Infection [digital art]. Retrieved from