'Bacteria' is the common name of a very large group of one-celled microscopic organism that, we believe, may be the smallest, simplest and perhaps even the very first form of cellular life that evolved on earth. That is why they are observable only under a microscope. There are three main types of bacteria, which are classified according to their shape.

The bacilli are a group of bacteria that occur in the soil and air. They are shaped like rods. If we look at them under a microscope, we find them in motion, they always seem to be rolling or tumbling under the microscope. These bacilli are largely responsible for food spoilage. There is another group of bacteria who tend to grow in chains. They are referred to as the cocci group. A common example of this type is streptococci that causes strep throat. Finally, there is the spiral shaped bacteria called spirilla. They look a little like corkscrews, and they are responsible for a number of diseases in humans. Some species of bacteria cause diseases, but mostly bacteria live harmlessly on the skin, in mouth, and intestines. In fact bacteria are very helpful to researchers. Bacterial cells resemble the cells of other life forms in many ways, and may be studies to give us insights.

According to the text, which characteristic is common in bacteria?