Introduction to Bacteria, Viruses, Fungi, and Parasites
Written by Benjamin Tanner, Ph.D.
Welcome to Antimicrobial Test Laboratories' germs page. Here at ATL we love our germs and try to know everything we can about them. For our customers (and other interested folks), here is a quick primer on germs: bacteria, viruses, fungi, and parasites.
With the exception of newly discovered prions, viruses are the smallest agents of infectious disease. Most viruses are exceedingly small (about 20 - 200 nanometers in diameter) and effectively round in shape. They consist of little more than a small piece of genetic material surrounded by a thin protein coating. Some viruses are also surrounded by a thin, fatty envelope.
Viruses are different from all other infectious microorganisms because they are the only group of microorganisms that cannot replicate outside of a host cell. Because viruses do not eat food, but instead seize materials and energy from host cells by hijacking cellular machinery, some scientists argue that they are more like complex molecules than living creatures. Viruses are known to infect nearly every type of organism on Earth. Some viruses, called bacteriophages, even infect bacteria.
At Antimicrobial Test Labs, we have made use of the physical similarity between animal viruses and certain bacteriophages to do faster, more cost-effective antiviral testing. Simply put, certain bacteriophages are great surrogates for mammalian viruses when it comes to disinfectant testing.
Bacteria are ten to 100 times larger than viruses. Typically, they are typically 1 to 3 microns in length and take the shape of a sphere or rod. Most bacteria consist of a ring of DNA surrounded by cellular machinery, contained within a fatty membrane.
They acquire energy from the same essential sources as humans, including sugars, proteins, and fats. Some bacteria live and multiply in the environment while others are adapted to life within human hosts. Some bacteria can double in number every fifteen minutes while others may take weeks or months to divide and grow.
Bacteria cause many types of diseases, ranging in severity from mild skin irritation to lethal pneumonia.
Parasites are part of a large group of organisms called eukaryotes. Parasites are different from bacteria or viruses because their cells share many features with human cells including a defined nucleus.
Parasites are usually larger than bacteria, although some environmentally resistant forms are nearly as small. Some parasites only replicate within a host organism, but many can multiply freely in the environment. Parasites can be made of one cell, as is the case with Giardia, or many cells, as is the case with parasitic worms.
In developing countries, parasitic worms are a major source of disease. Waterborne, non-worm parasites, including Giardia and Cryptosporidium are most common sources of parasitic disease in the United States.
Fungi are diverse in terms of their shape, size and means of infecting humans. Fungi are eukaryotes, meaning that like parasites, their cells have a true nucleus and complex internal structures.
They are most commonly found as environmentally resistant spores and molds, but can cause disease in humans in the form of yeasts. Fungi most often cause skin infections and pneumonia. Fungal diseases are particularly dangerous to immunocompromised people, such as those suffering from AIDS.