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Introduction to Bacterial Structure

©2001 Timothy Paustian, University of Wisconsin-Madison

The importance of structure to understanding microbes

Bacteria are very small. Yet despite their size they show a surprising degree of structural complexity. In this section we will look at the various compounds that make up a microbe and how these are put together. Learning the structure of a microbe helps in understanding how a microbe functions. To drive this point home, here are a few examples.

• Disease causing bacteria (pathogens) have various structures that enhance their ability to cause illness. One important property is the ability to attach to the intended victim. pili, a proteinaceous surface structure on the bacteria, are critical in this process.

• Many bacteria are capable of movement in their environment either by flagella or gliding motility. In the case of flagella, the bacteria have a long, flexible, spiral shaped structure, the flagellum, that helps to push the microbe through solution. Flagella also help in the detection of favorable or unfavorable conditions and move the bacteria in an appropriate direction.

• As a microbe grows it has to synthesize more of itself. Knowing what it is made of and how it is put together is critical to gain an understanding of the growth process.

• Microbes are also capable of exchanging genetic information by mating. This process involves another type of surface structure, the F-pilus.

• Bacteria will take steps to insure their survival. This can take the form of creating resting structures that allow the microbe to "sleep" during bad times. During abundant times, many microbes will store excess carbon, nitrogen, sulfur or phosphorous in inclusions in the cell.

Not only is structure important to understand functional relationships, it's also fascinating to observe what these little architects come up with.

The "general" bacteria

Figure 1 - A cartoon of the general bacteria.

In this part of the course, we will be examining a "general" bacteria. Realize that no one bacteria contains all these structures and that this survey is not exhaustive. We will be exploring the more common structures that have been observed by scientists in the past 100 years. A distinction in this discussion must be made between the Archaea, the most ancient of organisms on this planet, and their younger cousins the Bacteria. Most of this treatment will focus on the Bacteria, since that is what we know the most about. Archaea and Bacteria share many structures, but significant differences. These will be covered once you have a handle on general bacterial structure.

Universal structures in bacteria

There are some universal structures that all bacteria have. The basic building blocks of life, DNA, RNA, and protein, are common to all organisms not just microbes. Also, all microbes have a cell membrane. Much of what we know about these structures was obtained by studying bacteria, yet another reason to study them. Finally, most bacteria have a cell wall, but not all.

How was this investigated?

So how did scientists find out all this stuff? As you might guess, many techniques come into play when tackling a subject as complex as bacterial structure. Light, fluorescence, and electron microscopes are important of course. But so is bacterial genetics, molecular biology and biochemistry. Microscopes help scientists to visualize where these structures are located and how they are arranged spacially on the microbe. Bacterial genetics and molecular biology identify and analyze the genes necessary for the synthesis and regulation of these structures. Biochemistry permits the examination of each part separately and in the presence of the whole bacteria. The powerful combination of these disciplines has presented a deep understanding of how a bacteria is put together, but there is still much to learn.

Organization of this chapter.

This module on bacteria structure is separated into units, each one focusing on a group of structures in the cell. Explore them any way that makes sense to you, but the chapter uses an inside out organization. The basic building blocks of a cell and the main polymers that they are formed into (nucleic acids and proteins) are covered first. Ribosomes role in generating protein from DNA is covered next. Other cytoplasmic constituents are then examined. The cell membrane and cell wall are then discussed along with some of the important functions that each carries out. Finally flagella and other surface structures are touched upon. In this discussion I assume that the student has had an introductory chemistry course (at least in high school) and is somewhat familiar with chemical notation.

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