A microscopic image of bacteria, a type of decomposer.

There are no dead dinosaur bodies around in part because bacteria are decomposers that feed on dead plants or animals. The tissues of dead organisms are broken down into nitrogen- and carbon-containing molecules.

A decomposing animal can be pretty gross! But the next time you see a dead bird or squirrel, take a good look at it. Then check on it again about a month later. There will be nothing left but bones. And that's what happened to the dinosaurs.

The Good ...

Rhizobia bacteria convert nitrogen gas into other substances that help plants grow.

Most bacteria are harmless and offer beneficial functions to living things and humanity. Some bacteria, such as E. coli, live in the intestines of animals and people, helping them digest food as well as producing vitamins. Other animals (including cows, goats, deer, and giraffes) depend even more than humans on bacteria to digest their food. Billions of them live in the animal's rumens (a special type of stomach) where they break down grass and hay into nutrients the animal can absorb into its bloodstream. A sample from a cow's rumen, for example, contains all kinds of eubacteria, including spirochetes (spiral-shaped bacteria) and several species of rod-shaped bacteria.

This cross-section of a legume root shows nitrogen-fixing bacteria.

Plants also depend on bacteria to help them absorb nutrients into their roots. Plants need nitrogen to build protein, but atmospheric nitrogen (which is 78% of the air) cannot be used because of its chemical properties. In the soil, and even inside the plant's roots themselves, bacteria help turn atmospheric nitrogen into protein.

For example, rhizobia live in the roots of legume plants such as peas, clover, and peanuts. The rhizobia change the nitrogen gas into compounds of nitrogen (such as nitrates and ammonia) that the plants then convert into proteins. The rhizobia bacteria and the legume plant live in a symbiotic relationship, meaning that each benefits from living together. The plant uses the nitrogen compounds from bacteria to produce protein, and the bacteria gets food from the plants. Collectively, these bacteria are called NITROGEN-FIXING BACTERIA. They play an enormously important role in ecosystems and in agriculture.

Most bacteria are beneficial to living things. This gutless oligochaete (a type of marine worm) is white because of sulfur globules in its symbiotic bacteria. The worm relies on bacteria to reduce toxins in its body.

Bacteria are also used in many commercial and industrial applications. Bacteria are used all the time in the food industry. Yogurt wouldn't be tangy without the help of a bacterium called Lactobacillus acidophillus.

Yogurt is made from milk. The lactobacilli devour the milk, rich in a type of sugar called lactose, and in the process release acid that makes the yogurt sour. The acid in the yogurt can also help preserve it because the acidity makes yogurt inhospitable to other bacteria.

Did you know ... Bacteria are used in Australia to extract gold from rock!

The benefits of bacteria are endless. One type of bacteria eats oil and is used to help clean up oil spills. Sewage treatment plants use bacteria to purify water. And still other bacteria are sources of life-saving medicines, such as antibiotics.

The Bad ...

Microscopic image of anthrax (Bacillus anthracis)

Anthrax is a deadly type of bacteria that infects animals, especially cattle and sheep. Plants can be infected as well with a number of dangerous bacteria. Fire blight is plant disease that occurs in apple trees and pear trees.

... and the Ugly

Say "Ahhh" When you're sick, sometimes doctors will prescribe antibiotics. What are they? And how do they make you feel better?

Some bacteria enter the body through the intestines. This occurs by eating or drinking contaminated food or liquids. Cholera, for example, is a disease caused by bacteria in unsanitary drinking water. Food poisoning is brought on by eating food contaminated with bacteria such as salmonella and E. coli. Other bacteria enter the body through the lungs. Tuberculosis and pneumonia are caused by breathing in airborne bacteria. Bacteria can also get inside the body from puncture wounds (such as tetanus) or insect or animal bites and cause a variety of diseases.

One particularly ugly episode in human history was due to a rod-shaped bacterium called Yersinia pestis, otherwise known as the bubonic plague or the Black Death (so named because of the dark discoloration in the victim's face after death).

The Black Death, transmitted by bacteria-carrying fleas, caused millions of deaths in 14th-century Europe.

During the 14th century, this tiny organism killed millions of people — more than half the population of Europe. Transmitted by flea bites, the plague was spread across the continent by flea-carrying animals such as mice and rats.

Even now, the bubonic plague still occurs in parts of the world. But it is treatable with antibiotics and does not cause the mass epidemics it once did.

However, something even more sinister lurks within the environment. Today they cause some of the deadliest diseases known to humans, including AIDS. They are viruses!

Yet scientists aren't even sure they are alive. How do we stop them? Life on Earth needs bacteria, but do we need viruses?