Bacteria

Virus Bacterium Yeast

Above are pictures (relatively to scale) of three different kinds of microbes: viruses, bacteria and yeast. Though they are all microscopic single-celled organisms, they are quite different from each other, varying not inly in size, but in lifestyle and cellular growth-regulating mechanisms.

To the right is a diagram of a bacterium cut open. Like all cells it is enclosed by a lipid membrane, but in Escherichia coli, there is a double membrane covered by a polysaccharide capsule, which is the part often recognized by your immune system.Inside the bacterium there are a lot of proteins and a very long, but relatively thin, DNA genome. Bacteria have many fascinating observable qualities. Though they have no nervous system, they have a memory. They can be stressed in the presence of an antibiotic or temperature change. They enjoy glucose and can scurry across a plate to a food source. The inside of the cell is quite a crowded place. It is a real wonder how the proteins can find their right place on the DNA or elsewhere in the cell. This is what I call the blind fish problem.

Illustration by David S. Goodsell, the Scripps Research Institute.