Microorganisms typically face the world as single cells rather than as the multi-cellular assemblies of higher organisms. Each single cell must therefore contain all the structures necessary for managing its internal state and dealing with the outside environment. Not surprisingly, this evolutionary process results in the use of rather similar structures and processes to solve similar needs in different microorganisms. However, prokaryotes have been on this earth for a long period of time and this has allowed them to differentiate into a dizzying number of different species. Eukaryotic microbes are not quite so diverse, but they still display a remarkable range of properties. No one cell contains all the structures that we describe here, but we will explore the more common structures that have been observed by scientists in the past 150 years as show in Figure 3.1. A distinction in this discussion must be made between the two types of prokaryotes: the Archaea and their cousins, the Bacteria. We will initially focus on the Bacteria, since that is what we know the most about. Many of the structures we will examine are found in both the Bacteria and the Archaea, but there are some significant differences and these will be covered at the end of the chapter. Finally, we will talk about the features that are distinctive among the microbial eukaryotes.
Figure 3.1. The Generalized Bacteria. This cartoon displays many of the common structure found in prokaryotic microorganisms, though not every one will have every one of these structures.
So how did scientists find out so much about such very small organisms? As you might guess, many techniques come into play when tackling a subject as complex as bacterial structure. Microscopes help scientists to visualize where these structures are located and how they are arranged spatially in the microbe. Bacterial genetics and molecular biology identify and analyze the genes necessary for the synthesis and regulation of these structures. Biochemistry permits the detailed examination of each part separately, with implications for its role in the living bacterium. The powerful combination of these disciplines has provided a deep understanding of how a bacterium is put together, but there is still much to learn.