Without a microscope, the human eye can only see objects larger than one-tenth of a millimeter long. Given the right conditions, you might be able to see a human egg or a creature called a paramecium. Gazing down at these tiny objects, you are standing at the edge of an entire world of creatures invisible to the naked eye. In this strange land that is all around us, tiny microbes live out their tiny lives. Like us, they eat, move and reproduce. They interact with their world, obtaining food and energy from the environment, but also changing it in some very large ways.
With a light microscope, you could clearly see bacteria clumped together or floating freely. Next to the bacteria, a fungus would look like a giant pebble, but still just three millionths of a meter long. To see the even smaller viruses, however, you would need a special electron microscope. With millions of different kinds of microbes existing today, these organisms cover the Earth. They are so versatile that they live in just about every kind of habitat. Over the billions of years that they have existed on the planet, microbes have adapted to fit their environments, developing ways of gathering food and energy that are still used by most organisms today.
Microbes are remarkably diverse organisms. They live in habitats throughout the world: in fresh and salt water, on land, in the air, and on or inside other organisms. They are found in oceans, rivers, lakes and streams. They also exist in groundwater and in the sediment at the bottom of the ocean. Microbes even survive in extreme locations, like in hot springs and inside rocks deep beneath the Earth’s surface. The human skin is covered with microbes. Many others live within our digestive system, from the mouth to the intestines.
Microbes survive in many locations because each kind is adapted to the conditions of its environment. Like other organisms, all microbes need water to survive. Some microbes, however, can survive even when there is only a tiny amount of water, such as around a particle of soil. The microbes that live within the human stomach can withstand its extremely acidic environment. Those that live near thermal vents deep beneath the ocean are comfortable among the extreme temperatures and pressures found there.
Millions of species of microbes live on the planet. In order to understand them better, scientists organize them based upon physical and other characteristics. This is similar to creating a family tree. More closely-related microbes are placed nearer to each other on the tree. Microbes fall into two main categories — prokaryotes and eukaryotes — depending upon whether they have a cell nucleus. Prokaryotes lack this membrane around their genetic material. This group includes bacteria, and the similar archaea.
Microbes that have a cell nucleus are called eukaryotes. These include algae, microscopic animals, fungi and protists. Plants and animals are also eukaryotes, but they are much too large to be considered microbes. Most microbes are single-celled organisms, although some are found in colonies or clusters. Viruses are another group of microbes. They aren’t considered either eukaryotes or prokaryotes, but form their own separate category. Viruses are the smallest type of microbes. They cause disease and reproduce by infecting and using other cells.
Symbiotic microbes are not actually a separate grouping. These microbes have a special relationship with other organisms. In this relationship, called symbiosis, both organisms benefit. Symbiosis occurs throughout the world, even with plants and animals. One example is the oxpecker and the cape buffalo in Kenya. This bird feeds on the ticks that live on the buffalo. The bird gets a steady supply of food, and the buffalo benefits from the removal of ticks.
Many types of microbes have mutually-beneficial relationships with other organisms. The large number of bacteria that live inside the human digestive system are symbiotic microbes. The bacteria help break down the food that we eat, which enhances our digestion. This benefits not only the person, but also the bacteria, which have a place to live. Lichens are another example of symbiotic microbes, in this case between fungi and other organisms that can convert sunlight into energy. This relationship is even more close, with the two organisms forming what appears as a new plant.
Like all organisms, microbes must obtain energy and nutrients from their environment. In humans, we obtain nutrients from the food that we eat. The food also serves as a source of energy, with the body converting the food molecules into other forms of chemical energy needed to keep us functioning.
Microbes can be grouped by how they obtain their energy. Some convert sunlight into energy, in the same way as plants. Others eat other organisms or decaying matter. And there are those that use a combination of those two methods. There are even microbes that create their own energy from non-living compounds in their environment. There are also differences among microbes in how they react to oxygen. Certain bacteria, for example, cannot live in the presence of oxygen. Others, like humans, require it to survive.
Origins of Microbes
Microbes are the oldest form of life on Earth. Microbes were around — and evolving — two billion years before the first eukaryotes existed. Many of the genes and cellular functions found in modern organisms first appeared in the earliest microbes. Understanding how the early microbes developed and changed provides scientists with insights into how the evolution of microbes — and other organisms — continues today.
Some of the earliest microbes were single-celled bacteria. Amazingly, scientists have found fossils of these tiny creatures. The first microbes were prokaryotes and lacked a cell nucleus. They were also missing other specialized components inside their cells. Scientists believe that at some point, microbes with certain abilities were ingested by larger microbes to form a symbiotic relationship. The smaller microbes continued to live inside the larger cell, and shared their unique skills with their host. Eventually, the smaller microbes could no longer live on their own, and they became specialized components of the larger microbe.