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This book will focus on the biology of small things, but what is microbiology? Microbiology could be defined as the study of organisms too small to be seen with the naked eye. Figure 1.1 shows the relative size of microbes compared to other living things. However, the relatively recent discovery of bacteria of near 1 mm in size has made this definition somewhat inaccurate. In the grand tradition of science, a new definition is in order.
Figure 1.1. The relative size of microbes. Though microbes are small, they nevertheless span a large range of sizes from the smallest bacterial cells at ~0.15 µm to giant bacteria larger than 700 µm. The viruses depicted at the far left of the scale are even smaller.
We will consider microbiology to be the study of organisms that can exist as single cells, contain a nucleic acid genome for at least some part of their life cycle, and are capable of replicating that genome. This broad description encompasses an understandably large group of organisms, including fungi, algae, protozoa, and bacteria. Figure 1.2 has some examples of each. This definition would also include viruses, which microbiology texts traditionally discuss along with living organisms.
Figure 1.2. Some examples of the types of microbes present in the environment. Many different organisms fall under the definition of microorganisms. Shown here are: A, the bacterium Escherichia coli; B, a photosynthetic cyanobacterium; C, a fungus; D, Ebola virus; E, the protozoan malaria parasite. (Sources: B, Mike Clayton; C-E, CDC). Note that the scale on each of these pictures is different.
Microbiology also involves a collection of techniques to study and manipulate these small creatures. Because of their size, scientists use special instruments and methods to perform interpretable experiments on microorganisms. These methods are not restricted to microbes alone but have also found utility in working with populations of cells from higher organisms.
Microorganisms are everywhere, but why is it worth learning about them? The short answer is that they affect your life in many different ways. Before we begin our study of these creatures, we will first take a tour of some of their habitats and point out why your existence depends upon them. We will then briefly explore the history of microbiology.