The Microbial World

The microbial world includes the kinds of cells that van Leeuwenhoek observed looking through his simple microscope (figure 1.4). Although he could not realize it at the time, the microbial world, in fact all living organisms, can be classified into one of three major groups called domains. Organisms in each domain share properties of their cells that distinguish them from members of the other domains. Many properties, however, are shared among members of different domains because genes were transferred between domains billions of years ago, and the three domains may have a common ancestor. The three domains are the Bacteria (formerly called Eubacteria), the Archaea (meaning ancient), and the Eucarya. Microscopically, members of the Bacteria and Archaea look identical. Both are single-celled organisms that do not contain a membrane-bound nucleus nor any other intracellular lipid-bound organelles. Their genetic information is stored in deoxyribonucleic acid (DNA) in a region called the nucleoid. These simple cell types

Specimen holder

Focus screw

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Figure 1.4 Model of Van Leeuwenhoek's Microscope The original made in 1673 could magnify the object being viewed almost 300 times.The object being viewed is brought into focus with the adjusting screws. Note the small size.

Specimen holder

Focus screw

- Handle

Figure 1.4 Model of Van Leeuwenhoek's Microscope The original made in 1673 could magnify the object being viewed almost 300 times.The object being viewed is brought into focus with the adjusting screws. Note the small size.

Lens have their cytoplasm surrounded by a rigid cell wall and are termed prokaryotes, which means "prenucleus." All bacteria and archaea are prokaryotes. Although members of these two domains are prokaryotes because of their structural similarities, they differ in their chemical composition and are unrelated.

Members of the Eucarya, termed eukaryotes, which means "true nucleus," are distinctly different from members of the Bacteria and Archaea. Eukaryotes may be single-celled or multicellular, but they always contain a true membrane-bound nucleus and other internal cell organelles, making them far more complex than the simple prokaryotes. These structures include mitochondria, organelles which harvest chemical energy, from foods and chloroplasts, which harvest light energy in plants. Eukaryotes also have an internal scaffolding, the cytoskeleton, which gives the cells their shape. All algae, fungi, protozoa, and multicellular parasites considered in this book are eukaryotes.

The Bacteria

Most of the prokaryotes covered in this text are members of the Domain Bacteria. Even within this group, much diversity is seen in the shape and properties of the organisms. Their most prominent features are:

■ They are all single-celled prokaryotes.

■ Most have specific shapes, most commonly cylindrical (rod-shaped), spherical (round), or spiral (figure 1.5). ■ bacterial shapes, p. 49

■ Most have rigid cell walls, which are responsible for the shape of the organism. The walls contain an unusual chemical compound called peptidoglycan, which is not found in organisms in the other domains (see figure 3.32).

■ They multiply by binary fission in which one cell divides into two cells, each generally identical to the original cell. ■ binary fission, p. 50

■ Many can move using appendages extending from the cell, called flagella (sing: flagellum). ■ flagella, p. 63

The Archaea

The Archaea have the same shape, size, and appearance as the Bacteria. Like the Bacteria, the Archaea multiply by binary fission and move primarily by means of flagella. They also have

1.3 The Microbial World 9

rigid cell walls. The chemical composition of their cell wall, however, differs from that in the Bacteria. The Archaea do not have peptidoglycan as part of their cell walls. Other chemical differences also exist between these two groups.

An interesting feature of many members of the Archaea is their ability to grow in extreme environments in which most organisms cannot survive. For example, some archaea can grow in salt concentrations 10 times as high as that found in seawa-ter. These organisms grow in such habitats as the Great Salt Lake and the Dead Sea. Other archaea grow best at extremely high temperatures. One member grows best at temperatures above 105°C (100°C is the temperature at which water boils at sea level). Some archaea can be found in the boiling hot springs at Yellowstone National Park. Members of the Archaea, however, are spread far beyond extreme environments. They are widely distributed in the oceans, and they are found in the cold surface waters of Antarctica and Alaska.

The Eucarya

All members of the living world except the prokaryotes are in the Domain Eucarya, and all members of this domain consist of eukaryotic cells. The microbial world is composed of single-celled members of the Eucarya as well as their close multicellular relatives. These members include algae (sing: alga), fungi (sing: fungus), and protozoa (sing: protozoan). Algae and protozoa are also referred to as protists. In addition, some multicellular organisms are considered in this text because they kill millions of people around the world, especially in developing nations. Since they derive nutrients from the host organism they are termed parasites. They are given the general name of helminths and include organisms such as roundworms and tapeworms.

The Bacteria, Archaea, and Eucarya are compared in table 1.2.


The algae are a diverse group of eukaryotes; some are single-celled and others are multicellular. Many different shapes and sizes are represented, but they all share some fundamental characteristics (figure 1.6). They all contain a green pigment, chlorophyll, and some also contain other pigments that give them characteristic colors. The pigments absorb light, which algae use as a source of energy. Algae are usually found near the

Figure 1.5 Bacteria As Viewed Through a Scanning Electron Microscope (a) Cylindrical or rod shaped; (b) spherical; (c) spiral shaped.

10 Chapter 1 Humans and the Microbial World

Table 1.2 Comparison of Bacteria, Archaea, and Eucarya

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