Bacteria fall into a category of life called the Prokaryotes. The prokaryotes' genetic material, or DNA, is not enclosed in a cellular compartment (the nucleus).

What Do They Look Like?

There are thousands of species of bacteria, but all of them basically fall into one of three different shapes.

  Animation—To learn about the sizes of microorganisms, bacteria in particular, go to the site

Some bacterial cells exist as individuals while others cluster together to form pairs, chains, squares or other groupings.

Some microbes live on our skin and protect us from harmful agents. The drier areas, like the back, have few microbes; moist areas, such as the armpit, have many more as shown in the video.

A single teaspoon of topsoil contains more than a billion (1,000,000,000) bacteria.

Some bacteria are photosynthetic—they can make their own food from sunlight, just like plants. Also like plants, they give off oxygen. Other bacteria absorb food from the material they live on or in. Some of these bacteria can live off unusual "foods" such as iron or sulfur. The microbes that live in your gut absorb nutrients from the food you've digested.

Bacteria are classified according to the single cell's shape and size, and the way in which the cells organized.

Some bacteria have unique additional structures:

Capsule—protects the bacterium from phagocytic cells, which are part of the host’s defense mechanism.

Pili—cilia that enable pathogenic bacteria to attach to the body and cause disease.

Flagellum—a structure used for locomotion.

Biochemical and physiological properties (stemming from the structure and chemical components)—the chemical components of the cell, the cell wall structure, its adaptation to environmental conditions, the ability to cause a specific (immune) response, proliferation, level of infectiousness.

Endospore creation—a thick envelope that surrounds the bacterium when the spore is hibernating, i.e. its metabolism is at a minimal level. The process of creating the spore takes place when the environmental conditions are not suitable for the bacterium's proliferation (extremely low humidity, high temperature, presence of antibacterial substances, radiation). bacteria can survive for long periods of time in the spore state. When the conditions are right for its development/proliferation, the envelope is dismantled.

Only a small number of bacteria have the ability to form spores: among them are bacteria that cause severe diseases.

Molecular properties: the properties of the genetic material and nucleic acid (nucleotide) sequence.

Some bacteria move about in their environment by means of long, whip-like structures called flagella. They rotate their flagella like tiny outboard motors to propel themselves through liquid environments. They may also reverse the direction of flagellar rotation so that they tumble about in one place. Other bacteria secrete a slime layer and ooze over surfaces like slugs. Yet others are fairly stationary.

You can see how bacteria move in the following clips. 

Click on the picture to learn more about bacterial growth.

Bacteria are single-celled organisms; they reproduce by what is known as vegetative/asexual reproduction. This means that each cell divides into two new, genetically identical cells. After the bacteria have finished dividing, each new cell functions as an independent unit.

We are used to thinking of bacteria as disease-causing agents. Indeed, bacteria are at the origin of various worrisome diseases. However, it is important to point out that, in contrast to popular opinion, only a small number of the thousands of bacterial species are pathogenic, meaning disease-causing. Most bacteria do not harm humans at all, and many species of bacteria are even beneficial and vital to our health.

The bodies of humans and animals serve as a natural environment for bacteria. The bacteria that live in our bodies produce and provide us with different vitamins.

For instance:

In the human digestive system, there are different species of friendly bacteria which take part in an important and inseparable way. These bacteria are vital to the metabolism of food, the production of enzymes and vitamins (e.g., bacteria that manufacture vitamin B or vitamin K), the demolition of disease-causing microorganisms including fungi, and the regulation of intestinal acidity. These bacteria play a decisive role as symbionts in most multicellular organisms.

Bacteria that live in ruminants' digestive system break down cellulose to its monosaccharide components, releasing usable energy in the process.

Not only do the bacteria located at the entrances to the body (for example the mouth, skin and woman's vagina) not cause damage, they also do not allow disease-causing microorganisms to settle there or to invade our bodies through these entrance points.  

The essential uses of bacteria for humans can be seen in the environmental protection arena:

Sewage-disposal facilities are aided by bacteria in the breakdown of many waste products (not only organic) and in the neutralization of some of the toxic substances that make their way into sewage.

Bacteria play an important role in the biological view of pest control—as the natural enemies of different organisms that damage food crops and the agricultural industry.

Several bacteria play a major role in modern genetic-engineering techniques: by inserting specific genes into the bacterial cell, the bacteria can be induced to produce the protein which these genes encode. These proteins can then be used to produce medicines and other vital substances, such as hormones.

Another use of bacteria is in the making of food, especially dairy products. Special fermentation bacteria convert milk to products such as cheese, cultured milk and yogurt, the manufacture of which would be impossible without them.                                         

Bacteria are part of the food chain

A food chain is a model that shows how energy is passed, in the form of food, from one organism to another. The arrows between the organisms show the direction of energy flow.

Bacteria and fungi are decomposers. They break down waste products and dead organisms for food. These broken-down materials are returned to the soil to be recycled and reused by plants. An example of this would be a fungus growing on a log. Although decomposers are very important to the ecosystem, they are not usually shown in food-chain models.

Connection to genetics

A part of the bacterial genome is independent (that is, it is not part of the chromosome). It consists of circular sequences of DNA, called plasmids. Plasmids are much smaller than the chromosome, unattached to it, and capable of independent self-replication, irrespective of bacterial replication. These plasmids can also pass from one bacterium to another, even across species. Plasmids carry a variety of properties that are not vital to the bacteria’s survival.

This property confers, among other things, resilience to toxic substances, antibiotics and more.

Plasmids are widely used in genetic engineering technology.