<em>Clostridium tetani</em> b/w <em>Clostridium tetani</em>

Clostridium tetani

These bacteria are usually found in soil they enter the body through breaks in the skin. Clostridium tetani causes tetanus (lockjaw). This is a serious human disease that causes painful tightening of the muscles, usually all over the body. It can lead to "locking" of the jaw so the victim cannot open his mouth or swallow.

Archaea

Archaea can be spherical, rod, spiral, lobed, rectangular or irregular in shape. An unusual flat, square-shaped species that lives in salty pools has also been discovered. Some exist as single cells, others form filaments or clusters. Until the 1970s this group of microbes was classified as bacteria.

Many archaea have been found living in extreme environments, for example at high pressures, salt concentrations or temperatures, and have been nicknamed extremophiles. Their cell wall differs in structure from that of bacteria and is thought to be more stable in extreme conditions, helping to explain why some archaea can live in many of the most hostile environments on Earth.

Examples of archaea habitats are boiling hot springs and geysers such as those found in Yellow Stone Park, USA and ice such as the Artic and Antarctic oceans which remain frozen for most of the year.