From Germ tom Genome: WhatIs a Microbe?

From early childhood, we hear that we are surrounded by microscopic organisms, or "germs," that we can not see. What are microbes? Our modern concept of a microbe has deepened through two major research tools: advanced microscopy and the sequencing of gnomic DNA. Microscopy is covered in Chapter 2, and microbial genetics and genomics are presented in Chapters 7-12.

A Microbe Is a Microscopic Organism

A microbe is commonly defined as a living organism that requires a microscope to be seen. Microbial cells range in size from millimeters (mm) down to 0.2 micrometer (pm),

and viruses may be tenfold smaller (Table 1.1). Some microbes consist of a single cell, the smallest unit of life, a membrane-enclosed compartment of water solution containing molecules that carry out metabolism. Each microbe contains in its genome the capacity to reproduce its own kind. Our simple definition of a microbe, however, leaves us with contradictions.

■ Super-size microbial cells. Most single-celled organisms require a microscope to render them visible and thus fit the definition of a microbe. Nevertheless, some species of protists and algae, and even some bacterial cells, are large enough to see with the naked eye. The marine sulfur bacterium Thiomargarita namibiensis, called the sulfur pearl of Namibia, grows as large as the head of a fruit fly (Fig.1.4). Even more surprising, a single-celled plant, the "killer alga" Caulerpa taxifolia, spreads through the coastal waters of California. The single cell covers many acres with its leaflike cell parts.

 ■ Microbial communities. Many microbes form complex multi cellular assemblages, such as mushrooms, kelp's, and boffins. In these structures, cells are differentiated into distinct types that complement each others function, as in multi cellular organisms. And yet, some multi cellular worms and arthropods
require a microscope to see but are not considered microbes.

 ■ Viruses. A virus consists of a non cellular particle containing genetic material that takes over the metabolism of a cell to generate more virus particles. Some viruses consist of only a few molecular parts, whereas others, such as the Mimi virus infecting amebas (also spelled "amoebae"), show the size and complexity of a cell. Although viruses are not fully functional cells, the Mimi virus genome shows that it evolved from a cell.

In practice, our definition of a microbe derives from tradition as well as genetic considerations. In this book, we consider microbes to include prokaryotes (cells lacking a nucleus, including bacteria and archaea) as well as certain classes of eukaryotes (cells with a nucleus) that include simple multi cellular forms: algae, fungi, and protists (Fig. 1.5). The bacteria, archaea, and eukaryotes known as the three domains diverged from a common ancestral cell. We also discuss viruses and related infectious particles (Chapters 6 and 11).