2015 Elsevier Inc. All rights reserved. This chapter introduces students to all classes of microorganisms found in the environment as opposed to clinical microbes found within the human body. Specific major domains consist of Archaea, Eukarya and Bacteria. Of these, the Bacteria and the Archaea are termed prokaryotes and the Eukarya are known as eukaryotes. Microorganisms within the Eukarya include fungi, protozoa and algae. In addition to the major domains, the other important biological entities are a unique group known as viruses that can infect eukaryotic or prokaryotic organisms. Viruses have no metabolic capacity and rely on infected hosts to produce viral parts that self-assemble. Similar to viruses are the viroids, which are infectious single-stranded RNAs that can infect plants. Finally, we discuss prions, which are infectious proteins that can destroy brain tissue-the transmissible spongiform encephalopathy (TSE) diseases. For all entities, structures, diversity and important environmental aspects are presented.
