What are bacteria? Bacteria are single-cell organisms that are neither plants nor animals. They usually measure a few micrometers in length and exist together in communities of millions. A gram of soil typically contains about 40 million bacterial cells. A milliliter of fresh water usually holds about one million bacterial cells. The earth is estimated to hold at least 5 nonillion bacteria, and much of the earth’s biomass is thought to be made up of bacteria. Types There are many different types of bacteria. One way of classifying them is by shape. There are three basic shapes. Spherical: Bacteria shaped like a ball are called cocci, and a single bacterium is a coccus. Examples include the streptococcus group, responsible for “strep throat.” Rod-shaped: These are known as bacilli (singular bacillus). Some rod-shaped bacteria are curved. These are known as vibrio. Examples of rod-shaped bacteria include Bacillus anthracis (B. anthracis), or anthrax. Spiral: These are known as spi...
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