Scientists classify all living things into six kingdoms, which is the broadest of the seven levels of classification. Of the six kingdoms, eubacteria and archaebacteria are probably the least well known. Each describes a very different kind of bacteria, but both affect our lives in different ways.

Archaebacteria are the oldest living organisms on earth. They are prokaryotes and unicellular. Archaebacteria are found in very harsh conditions (such as at the bottom of the sea or in volcanic vents). This is thought to be because the early Earth's atmosphere was filled with poisonous gases and was very hot - nothing could survive, except the archaebacteria. These slowly gave way to modern organisms when the Earth's conditions settled down and oxygen was introduced to the atmosphere.

There are three phyla, or groups, of archaebacteria. The methanogens are characterized by their ability to harvest energy by converting H2 and CO2 into methane gas. They (and all archaebacteria) are obligate anaerobes, meaning they cannot live in the presence of oxygen. They are found in marshes and in the intestinal tracts of humans and some animals (cows, for example).

The second phylum is the extreme halophiles. These organisms are names because they are salt-loving. Though salt kills most bacteria, it helps the extreme halophiles to thrive. They are found in the Dead Sea, the Great Salt Lake, and other areas with a high salt content.

The third phylum is the thermoacidophiles. These bacteria are found in extremely acidic conditions and in areas with very high temperatures. They can survive in areas with temperatures as high as 230 degrees Fahrenheit and with pHs below 2 (hydrochloric acid, which is incredibly strong, has a pH of 1). These locations include volcanic vents and hydrothermal vents (cracks in the ocean floor where scalding water leaks out).

The eubacteria are more complex, and common, than the archaebacteria. It is the eubacteria that most people are talking about when they say bacteria, because they live in more neutral conditions. They can be found everywhere around us - in our bodies, our food, etc. There are four phyla of bacteria, some of which are divided into subgroups.

Cyanobacteria are photosynthetic, like plants, which means that they use the sun's energy to make food for themselves. They are thought to be the second organisms in existence, because they give off oxygen as a byproduct. This allowed aerobic organisms (ones that need oxygen to survive) to develop. They are found in the water and were once thought to be blue-green algae (however, they lack a membrane-bound nucleus and chloroplasts).

Spirochetes are gram-negative (a gram stain is a technique for classifying bacteria which will be explained later), spiral-shaped, and heterotrophic. Some of them live in the presence of oxygen, others don't. They may be parasitic, living symbiotically (where two organisms live off each other), or free-living. One type of spirochete causes syphilis.

Gram-positive bacteria are not all gram-positive, despite the same. They are grouped together due to other similarities. This phylum includes the strain of streptococcus bacteria that causes strep throat. It also includes the bacteria that produces yogurt, by growing and fermenting in milk (producing lactic acid). These bacteria also produce many of our antibiotics.

Proteobacteria is one of the largest phyla of all the bacteria. Many are gram-negative. They are divided into several subgroups, such as enteric bacteria, chemoautotrophs, and nitrogen-fixing bacteria. The enteric bacteria live mainly in intestinal tracts, like E. Coli. The chemoautotrophs oxidize chemicals in minerals to obtain energy. The nitrogen-fixing bacteria are essential to many ecosystems - one type converts the unusable nitrogen in the atmosphere to ammonia, the form plants can use most easily.

Beyond these classifications, all eubacteria are classified by gram staining. This is a process that determines many things - resistance to antibiotics, for one thing. It is also one of the first things scientists will do when trying to identify an unknown bacteria. It involves staining a group of bacteria with four different liquids. First, crystal violet is added. Then it's stained with iodine, and finally with safranin. Then it goes through an alcohol wash. The bacteria that retain the purple stain from the crystal violet are gram-positive, and those that take on the pink stain from the safranin are gram-negative.

This has to do with the outer layer of the cell - gram positive bacteria have a thick layer of peptidoglycan, which absorbs the gram stain. Gram-negative bacteria have a thick lipid bilayer on the outside, which is selectively permeable - not everything can pass through it, and one of those things is the gram stain. The gram-positive bacteria (since things can pass through it easily) are much more susceptible to antibiotics than.

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