13.3–13.5: Bacteria may be the most diverse of all organisms.

The bacterium Staphylococcus epidermidis is part of the normal flora on human skin and many mucous membranes.

A bacterium is the simplest organism you can imagine—and in many ways, it is the most efficient. A bacterium has a cell envelope consisting of a plasma membrane and, in most cases, a cell wall—that’s what it needs to maintain conditions inside that are different from conditions outside. Inside the cell envelope it has cytoplasm—the substance that fills all kinds of cells (including your own). Because bacteria are prokaryotes, they have no organelles. Proteins in the cytoplasm carry out essential functions, such as digesting molecules of food and transferring the energy gained to ATP. DNA in the cytoplasm carries the instructions for making those proteins. And messenger RNAs carry this information to ribosomes, where the proteins are synthesized. That’s it—everything an organism needs, with no extra baggage.

Bacteria may be classified by their shape: some are spherical cells (known as the cocci), some are rod-shaped (the bacilli), and others are spiral-shaped (the spirilli) (FIGURE 13-5). Bacteria usually reproduce by binary fission, and in a few hours, a single cell can form a culture containing thousands of cells.

Figure 13.5: Bacteria basics. Bacteria are single-celled organisms that lack a nucleus.

As a bacterial cell divides, the number of cells doubles every generation, producing a colony of cells, each of which is a clone of the original cell. Colonies of different species of bacteria look different. The familiar human intestinal bacterium Escherichia coli, for example, forms beige or gray colonies that have smooth margins and a shiny, mucus-like covering. Species of Proteus, which are often responsible for spoiling food because they can grow at refrigerator temperatures, form colonies with a surface that looks like a contour map.

Microbiologists can often identify bacteria simply by looking at the colors and shapes of their colonies. They can get additional information by examining a single cell under a microscope, but living bacterial cells are transparent, so you can’t see them with an ordinary microscope unless they have been dyed. In 1884, Hans Christian Gram, a Danish microbiologist, described a method of staining the cell walls of bacteria to make them visible under a microscope, and a Gram stain is still the first test microbiologists use when they are identifying an unknown bacterium (FIGURE 13-6).

Figure 13.6: Bacterial IDs. Bacteria are often identified by the appearance of an entire colony or by the cells’ response to Gram staining.

Gram-positive bacteria are colored purple by the stain, because their cell wall has a thick layer of a glycoprotein called peptidoglycan. In Gram-negative bacteria, the layer of peptidoglycan is thinner and lies beneath an additional membrane, and it is not stained by the dye. Penicillin is effective in treating infections by Gram-positive bacteria because it interferes with the formation of peptidoglycan cross-links. Penicillin is less effective on Gram-negative bacteria because it does not pass through the outer membrane that covers the peptidoglycan layer.

Being able to distinguish these two groups of bacteria is a big help to microbiologists trying to identify a bacterium, but the peptidoglycan is there because it is important for the bacterial cells—not to make life easier for microbiologists. The extensive interlocking bonds of the long peptidoglycan molecules provide strength to the cell wall. Many bacteria also have a capsule that lies outside the cell wall. This capsule can restrict the movement of water out of the cell and allow bacteria to live in dry places, such as on the surface of your skin. In other cases, the capsule is important in allowing the bacteria to bind to solid surfaces such as rocks or to attach to human cells.