Classification is the organization of things into groups. One way scientists classifyglossary term (opens in a new window) prokaryotes is by the substances from which they are built. The two domains of prokaryotes, archaea and bacteriaglossary term (opens in a new window), have substantially different chemistries. These differences are numerous, but the main ones are that their cellglossary term (opens in a new window) walls are formed from different substances, and their cell membranes have a different structureglossary term (opens in a new window). In fact, archaea resemble eukaryotes more than they do bacteria. Archaea can also live in different places than bacteria. Some are called extremophiles because they like extreme environments. For example, some species live in boiling springs, while others thrive in toxic chemicals. Some organisms in the domain archaea live deep underwater by hydrothermal vents. Others can survive under icepacks, in the world’s saltiest conditions, or even in the radiationglossary term (opens in a new window) of space! Of course, not all archaea live in extreme habitats; many are found in regular soil and water or the guts of animals; most live in the ocean.

The domain bacteria is probably the one you are more familiar with. Bacteria all have cell walls, but some bacteria have protective outer membranes, and others don’t. Bacteria with the protective membraneglossary term (opens in a new window) are called “gram negative” because they will not take up a certain dye. “Gram positive” bacteria will soak up the dye into their cell walls. Gram stains helps physicians determine what antibioticglossary term (opens in a new window) should be used to fight the bacteria. For example, penicillin works very well against gram-positive bacteria, but poorly against gram negative. Tetracycline works well against gram negative, but not against gram positive.

Bacteria are often identified based on their shapes and the ways they are arranged. The main shapes of prokaryotes are bacillus (rod shaped), coccus (round), and spirillum (spiral). Other shapes include coccobacillus (oval), spirochetes (tighter spirals), and vibrio (curved rods). When a prokaryoteglossary term (opens in a new window) is arranged as a single cell, it is referred to as its shape (for example, a bacillus bacterium). If there is a pair of cells, the prefix diplo- is added, as in diplococcus.

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