What are microbes?

Microbes are organisms that are smaller than can be seen by the human eye, generally less than 0.1 mm across. Most are single-celled organisms living independently, but some form groups of cells called colonies. In some cases, colonies may become so large you can see them. However, each cell in a colony is usually capable of living apart from the other cells and even of forming a whole new colony.

	Diagram of a typical prokaryotic cell, showing subcellular components. (1) Plasma membrane (2) DNA (nucleoid) (3) capsule (4) cell wall (5) mesosome (6) ribosomes (7) cytoplasm (8) bacterial flagellum. Image source: Wikimedia Commons (public domain).

What are the different types of microbes?

Bacteria

The bacteria are so diverse that few descriptions can be applied to every member of the group. However, all bacteria are prokaryotes (say pro-CARE-ee-oats), which means that their cells are organized very simply. They have a single DNA molecule that is not separated from the rest of the cell by a membrane. In other words, their cells lack a nucleus. In most cases, bacterial cells are enclosed in a flexible membrane that is surrounded by a more rigid cell wall. Differences in the chemical composition of the cell walls and membranes cause various reactions to certain stains or dyes. These reactions are useful in identifying and classifying species.

These rod-shaped bacteria are the largest known prokaryotes. Epulopiscium fishelsoni lives in the intestines of surgeonfish in the tropical Pacific. It can grow up to 0.6 mm in length -- large enough to be seen without a microscope! © Norman Pace and Esther Angert, authors. Licensed for use, ASM MicrobeLibrary.

Although the largest species can actually be seen without a microscope, bacteria are generally very small, single-celled organisms. They come in several shapes, but the most common are rods, spheres, or spirals. They may form pairs, chains, or clusters that are characteristic of their species. Many bacteria are able to propel themselves around with whiplike appendages called flagella, while others glide about on a slimy substance they produce. Bacteria reproduce by dividing into two equal cells, a process called binary fission. (To see a video of binary fission in action click here.)

Bacteria are also very diverse in the ways they meet their nutritional needs. Most rely on organic material from other organisms, either living or dead. Some are capable of photosynthesis, while others sustain themselves from the chemical energy stored in inorganic substances. (Organic substances are related to or derived from living things, while inorganic substances are not, such as metals and other mineral compounds.)

Staphylococcus aureus is an example of a spherical bacterium that often clusters together. It is normally found on human skin and mucous membranes and is generally harmless unless it enters a break in the skin, at which point it can cause an assortment of infections. This image shows the purple-stained bacteria against a reddish background of human cells sampled from an infected wound. © Gloria Delisle and Lewis Tomalty, authors. Licensed for use, ASM MicrobeLibrary.		Borrelia burgdorferi is the name of the spiral-shaped bacteria that cause Lyme disease. Humans contract the disease when the bacteria are passed along through a bite by an infected tick. © Jeffrey Nelson, author. Licensed for use, ASM MicrobeLibrary.

	Diagram of a typical eukaryotic cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome and (13) centrioles. Image source: Wikimedia Commons (Creative Commons license).

Protozoans

The protozoans are single-celled organisms that are generally larger and more complex than the bacteria. They include the familiar pond-water species, Paramecium and Amoeba, but they have been found in most aquatic and soil environments. They come in a variety of shapes and are grouped according to their means of locomotion. Protozoans are eukaryotes (say yoo-CARE-ee-oats) -- organisms whose cells have a distinct nucleus in which the DNA is enclosed by a nuclear membrane. They also have tiny structures within the cell called organelles, which perform specialized functions similar to the organs of a plant or animal. Protozoans are the hunters and grazers of the microbial world, feeding on other microorganisms and forming an important link in the food chain themselves.

	Amoeba proteus is commonly found in freshwater ponds. It is a predator that captures other microbes including bacteria, protozoa, and algae by extending its pseudopods (false feet) to surround and engulf the prey. Note the internal cell structures visible in this image that are typical of a eukaryote. © Stephen Durr, author. Licensed for use, ASM MicrobeLibrary.

Fungi

	This scanning electron micrograph of a single cell of baker’s yeast, Saccharomyces cerevisiae, has been color-enhanced to show the scars left behind after bud formation, one of the yeast’s means of reproduction. Can you tell how many daughter cells this yeast cell has produced? © Alan Wheels, Anna Cosney, and John Forsdyke, authors. Licensed for use, ASM MicrobeLibrary.

If you think of mushrooms when you hear the word fungi, you might be surprised to find them listed as microbes. Fungi range in size from the single-celled yeast, to the molds that grow on food in your refrigerator, to mushrooms that are among the largest organisms on Earth, spreading across acres of forest floor. Even when you can see a fungus, however, there are important differences between the fungus and a true multicellular organism. Fungi are eukaryotes, but the cross-walls separating one cell from another are incomplete. In a sense, a fungus is one large, continuous cell with many nuclei. In addition, the most extensive part of many fungi can’t be easily seen. Beneath the surface of the soil or the rotting fruit, a tangled network of tiny, often microscopic filaments called hyphae is growing and absorbing nutrients. Fungi obtain nourishment from their environment by absorbing organic material from the soil, water, or a plant or animal host. They are important decomposers, working alongside bacteria to break down dead plants and animals and their wastes, returning nutrients to the soil, and preventing our planet from becoming a giant heap of debris.

Fungi reproduce by several means. Single-celled yeasts produce buds -- new cells that form on the side of a mature yeast cell and then break off. (Click here to view a video of budding yeast.) In fungi with hyphae, bits of the hyphae can break off and start a new fungus. Many fungi also produce spores. Under the right conditions, the hyphae form special stalks or branches that contain the seed-like spores. When they are "ripe," the spores are released. They may fall nearby or be carried by the wind to a new location where they either germinate or wait in a dormant state for the environment to become more favorable.

	Aspergillus unguis belongs to a genus of fungi that are commonly found on decaying vegetation as well as in soil and many other environments. In this image, magnified 400 times, you can see both the filamentous hyphae and the spore-producing bodies. © David Treves and Candace Martens, authors. Licensed for use, ASM MicrobeLibrary.

Algae

	Calcite plates, or coccoliths, provide structure for the microscopic algae known as coccolithophores. The fossil coccolith above, from the species Calcidiscus leptoporus, is about 6 million years old. © Amanda Waite, Bioimaging Laboratory, Delaware Biotechnology Institute.

Like fungi, algae are sometimes quite noticeable in our environment. Though some dwell in moist soil, unicellular algae live primarily in water, where they form the base of the aquatic food chain. In large numbers, they can give natural waters a greenish tinge or form a green scum on the surface of a pond or in damp places. That green color comes from chlorophyll, the light-absorbing pigment that enables algae to carry out photosynthesis. Through photosynthesis, algae produce their own food from light, carbon dioxide, and water and release oxygen as a by-product. Algae in the open ocean are thought to be responsible for more than half the oxygen in our atmosphere. Not all algae are photosynthetic, however. Some members of the group known as dinoflagellates are predators of bacteria and other algae. When conditions are right, a bloom of these dinoflagellates results in “red tides.” Some produce potent neurotoxins that are lethal to fish and even humans.

Only a handful of microscopic marine algae are known to produce toxins; most of these belong to the group called dinoflagellates. Since its discovery in 1991, Pfiesteria piscicida has been found to be responsible for large fish kills in the Mid-Atlantic region and nervous system damage in people who have come into contact with the toxin. Photo courtesty of Burkholder Laboratory.

Archaea

	Some archaea live in “extreme” environments such as this hydrothermal vent that spews out super-heated, chemical-rich water.

For quite a while, scientists thought the archaea were bacteria. On the surface, they look and act a lot like bacteria -- they have similar shapes and means of moving around. Like the bacteria, they are prokaryotes. Most have a rigid outer cell wall. However, once scientific tools became sophisticated enough to analyze their cellular chemistry and genetics thoroughly, scientists realized they were dealing with two distinct groups. For starters, archaeal cell walls and membranes have a different structure than bacterial cells. But even more importantly, their genetic make-up is quite different from bacteria.

The archaea got their name from the fact that they are often found in places like hot springs and hydrothermal vents -- places that resemble conditions on the ancient Earth more than 3.5 million years ago when the seas regularly reached boiling temperatures. Many scientists believe that the archaea may provide important clues to the origin and early evolution of life on the planet. Archaea species also thrive in many other extreme environments, including highly acidic, alkaline, or salty conditions, and frigid temperatures as well as hot. Scientists are actively investigating the ways in which these “extremophiles” defend themselves against conditions that would destroy other cells.

Viruses

This transmission electron micrograph shows the relative sizes of a bacteriophage virus (the stickpin-shaped object) just below a “gigantic” bacterium that is about to be infected. © Kui Wang, Bioimaging Laboratory, Delaware Biotechnology Institute.

Viruses occupy a unique place on the border between life and nonlife. They are not cells, only tiny protein capsules containing a small amount of genetic material. Viruses are extremely small, thousands of times smaller than the average bacterial cell and usually visible only with an electron microscope. On their own, they display no biological activity -- they don’t metabolize nutrients, excrete wastes, reproduce, or propel themselves around. They drift about at the mercy of air or water currents until they find a host cell.

Not just any host will do. The structure of a virus’s outer coating determines whether it will be able to latch on to the outer membrane or wall of a particular type of cell. Some viruses can find a host in more than one species; others are more specific. Once they’ve made contact with an acceptable host, they release their genetic material inside the cell. This small piece of either DNA or RNA contains genetic code that allows the virus to “hijack” the cell’s reproductive mechanisms. Instead of producing its own proteins, the cell begins cranking out copies of the virus. Often the cell is destroyed in the process, and the new virus particles are released to infect more cells. This is the mechanism of most viral diseases. (Click here to see an animated gif of a virus infecting a bacterial cell.)

Despite their small size, viruses have played an important role in the history of life on our planet by providing a gene transport service. Sometimes in the process of infecting a host cell, pieces of the viral DNA may break off and get mixed up with the host’s DNA or vice versa. Think of it as nature’s way of downloading and sharing music files, with countless little smash-ups over time resulting in genetic re-mixes in and among organisms.

Where do microbes live?

The simple answer is everywhere. In additional to “normal” habitats -- soil, water, household surfaces -- microbes have been able to exploit habitats that would be lethal to other organisms. For starters, some microbes prefer anaerobic, or oxygen-free, environments. Some microbes live at great depths in the ocean, under conditions of very high temperature, pressure, and toxicity adjacent to hydrothermal vents. It’s not too surprising then that they have also been found in terrestrial hot springs. Other microbes are at home in salt ponds where the concentration of salt far exceeds that of the ocean, while others live in caves filled with poisonous cyanide, carbon monoxide, or hydrogen sulfide gases.

Many microbes find a comfortable existence in or on other organisms. Some of these microbes cause disease, but many more are neutral or even beneficial to the plant or animal that serves as the host. For example, plants depend on certain bacteria that are capable of converting nitrogen in the air to a form that’s more usable as a nutrient. Bacteria also inhabit the digestive tracts of animals, manufacturing vitamins or helping to break down hard-to-digest foods. In fact, you have more microbes living in your intestines than you have cells in your body (and you have about 100 trillion cells)!

The geothermal features of Yellowstone National Park provide habitat for a range of heat-tolerant microbes. Runoff from geysers and hot springs creates a V-shaped pattern of microbial growth. The most heat-tolerant species, members of the archaea, occupy the hottest water in the center of the channel. Algae are limited to the coolest outer edges, while various species of bacteria occupy the zones in between. © Linda Fisher, author. Licensed for use, ASM MicrobeLibrary.		Escherichia coli is the most studied microbe. It comes in a number of slightly different varieties, or strains. One strain is a helpful, normal inhabitant of our digestive tracts. But the O157:H7 strain pictured here is a pathogen often implicated in outbreaks of food-borne illness or the closing of swimming areas. The difference is that E. coli O157 carries a gene for the production of a toxin that brings on severe diarrhea. Photo courtesy of CDC/Elizabeth H. White, M.S.

How long have microbes been around?

The history of life on Earth is overwhelmingly microbial. As you can see from the timeline below, if the history of the Earth were condensed into a single year, microbes could be said to have originated around late February, while human beings didn’t make their appearance until late in the evening on December 31!

Fossil evidence of microbes has been found in some of the oldest rocks known to exist. The Earth is estimated to be about 4.5 billion years old. Microbes arose about 3.8 billion years ago, other animals about 700 million years ago, and modern humans less than 1 million years ago.

How many microbes are there?

	This transmission electron micrograph shows a diverse microbial population living together in a biofilm. Biofilms are complex micro-ecosystems that can form on almost any moist surface. Microbes can be free-floating or mobile, but they seem to prefer adhering to surfaces where they have access to nutrients and “safety in numbers.” © Ralph Robinson, author. Licensed for use, ASM MicrobeLibrary.

Microbes are the most abundant organisms on the planet -- way more numerous than human beings! The current human population is estimated to be about 6.33 billion. Let’s put that in scientific notation, because we’ll soon be talking about some very large numbers. Another way of writing 6,330,000,000 is 6.33 x 10⁹.

There’s no way to count exactly how many microbes exist, but scientists estimate that there are approximately 55,000,000,000,000,000,000,000,000,000,000 microbes sharing the planet with us. That’s 5.5 x 10³¹.

Let’s try to put that in perspective. That’s more microbes than there are stars in the Milky Way (which is only about 4 x 10¹¹). That’s 9 sextillion, or 9 x 10²¹, microbes for every human. In fact, a dense microbial culture in the laboratory contains about 5 billion microbes per milliliter. So a test tube can easily hold about a dozen times the world’s human population in microbes.

Admittedly, a test tube is not exactly a microbe’s natural habitat. But microbes are pretty thick in the natural environment as well. A mere teaspoon of topsoil harbors about a billion bacterial cells, not to mention some 120,000 fungal cells and 25,000 algal cells. And direct microscopic counts of viruses indicate they are 10 times more abundant than bacteria in natural waters, with up to 100 million viruses in every milliliter.

Okay, so there are a lot of microbes out there. But they are very tiny. You may wonder if the number really matters when the organism is so small. Another way to look at it is to compare biomass. This is a way of comparing “apples to apples,” of looking at the relative contribution of a species or group of organisms to the total amount of living matter in the area being considered, in this case, the planet. On Earth, measuring biomass is somewhat analogous to measuring weight (although on the moon, the same biomass would have a different weight, due to the difference in the force of gravity on the moon).

Biomass is measured in units of grams carbon. Here’s how we would estimate the biomass of the human population. Let’s say the average human weighs 150 pounds, which is equivalent to 68,000 grams. We multiply this by the number of people on Earth.

(6.33 x 10⁹)(68,000 g) = 4.30 x 10¹⁴ g

This is the total mass of the human population. We multiply this by a conversion factor to give us grams carbon, the unit of biomass:

(4.30 x 10¹⁴ g)(0.18 g C/g) = 7.7 x 10¹³ g C

By comparison, scientists have estimated that the microbe biomass is approximately 4.5 x 10¹⁷ g C. Therefore, microbes “outweigh” humans more than 5,000 to 1!

Additional resources

The American Society for Microbiology (ASM) offers a wealth of resources, such as the following, for kids, educators, and the general public on its Web site:

• Microbeworld is a fascinating site for the general public. It describes what microbes are and what microbiologists do, explores current issues involving microbes, provides weekly news items, and offers resources and hands-on activities.


• Intimate Strangers: Unseen Life on Earth is the title of a video series and companion book produced by ASM in 1999. The series also recently became available as a series of free podcasts.


• MicrobeLibrary is an extensive collection of microbe images and other teaching resources.

Whitman, W. B., Coleman, D. C., and Wiebe, W. J. 1998. Perspective Prokaryotes: The unseen majority. Proc. Natl. Acad. Sci. Vol. 95, 12:6578-6583.