13.1–13.2: There are microbes in all three domains.

Dental plaque, a film that forms on teeth, is caused by the growth of bacterial colonies (shown here). It can generally be removed by brushing, but, if left, can lead to tooth decay.

Microbe is an appropriately descriptive name, but it’s sloppy. It is simply a combination of Greek words that means “small life.” We could use “microbe” to point to any one of many different types of organisms too small to see without magnification. In Chapters 3 and 10 we introduced two very different kinds of microbes: bacteria and viruses. In this chapter we explore bacteria and viruses in more detail, and also include two other kinds of microbes—the protists and the archaea. The amoeba, a kind of protist, may be familiar to you, but the archaea are probably unfamiliar to you. That’s not surprising, because archaea were only recently recognized as a distinct group of microbes. The group doesn’t even have a common name (unless you like “the group formerly known as archaebacteria”). We’ll just call them archaea.

It is hard to make generalizations about microbes, because they are grouped together simply because they are small, not because they all share a recent common ancestor. In fact, microbes occur in all three domains of life—bacteria, archaea, and eukarya—and so the various types of microbes could not be more widely separated (FIGURE 13-1). In this chapter we focus on the tiny organisms from each of these domains. The microbes in the domains bacteria and archaea are prokaryotic, although archaea have some characteristics similar to those of bacteria and some similar to those of eukaryotes. Protists are the mostly microbial members of the domain eukarya. And viruses, another type of microbe, are not classified into any domain at all because they are only at the borderline of life.

Figure 13.1: Microbes are highly diverse. Microbes are found in every domain of life.