During the Golden Age of Microbiology, bacteria were viewed as unrelated individualists. Pasteur studied the bacteria that made lactic acid by fermenting sugar. Joseph Lister focused on germs causing infections in hospital patients. Robert Koch discovered the anthrax pathogen, Bacillus anthracis, and delved into the processes of bacterial disease. He would develop a set of criteria (Koch's postulates) that gave birth to today's methods for diagnosing infectious disease. Not until microbial ecology developed did biologists recognize the interrelated world of bacteria as well as the relationship between environmental bacteria and humans.
Staphylococcus epidermidis contributes to body odor, a bacteria-human connection easily detected. But thousands of hidden bacterial activities shape the very ecology of the planet. In soil, Azotobacter pulls nitrogen from the air, chemically rearranges it, and hands it off to Nitrosomonas, which changes the nitrogen again and shuttles it to Nitrobacter. Nitrobacter then secretes the nitrogen in the form of nitrate, which disseminates throughout soils. Some of the nitrate reaches the roots of legumes such as clover or soybeans. Inside the plant roots anaerobic Rhizobium absorbs the nitrate and converts it to a form the plant can use. This process is vital in replenishing nitrogen that higher organisms need.
For carbon to make a similar cycle through the Earth's organic and inorganic matter, the bacteria of decay must help decompose the planet's fallen trees, plants, and animals. The common soil inhabitant Bacillus breaks down proteins, fats, and carbohydrates by excreting the enzymes protease, lipase, and amylase, respectively. Thousands of other species break down organic matter in similar ways. For example, Cellulomonas bacteria produce the enzyme cellulase—rare for bacteria—that digests plant cellulose fibers. Bacteria emit carbon dioxide as an end product, which enters the atmosphere. A massive population of photosynthetic bacteria in the Earth's surface waters then captures this gas and inserts the carbon into a new food chain of bacterial cells, protozoa, invertebrates, and so on until the carbon ends up in tuna sashimi on a restaurant menu.
If clouds begin to form while a person lunches on sashimi, bacteria have a part in that, too. Photosynthetic marine bacteria and algae produce dimethyl sulfide gas as a waste product of their normal metabolism; they emit 50 million tons annually. When the gas rises and enters the atmosphere, it chemically rearranges into sulfate, which attracts water vapor. The vapor turns to droplets and forms clouds. On a global scale clouds inhibit the photosynthetic bacteria and less dimethyl sulfide forms. When the clouds thin, the cycle begins again.
Albert Kluyver of the Technical School of Delft—the town where van Leeuwenhoek discovered bacteria in 1677—praised the wonderful "unity and diversity" of microorganisms, a perfect description for dissimilar organisms that share more than 95 percent of their genes. The human body possesses its own unity and diversity of microbes that in most situations keep the body's metabolism working at its best. Pathogens more than good bacteria gain the attention of researchers and doctors. For this reason, epidemics have expanded our knowledge of bacteria. Many of the discoveries in microbiology came about from a blend of genius and serendipity, a fair description of all science.