10.16: The bacteria domain has tremendous biological diversity.

Morning breath is stinky. And learning the cause of the offensive smell may make the situation even worse. You see, when you wake up, your mouth contains huge amounts of bacterial waste products. Perhaps the only consolation is that it gives us a glimpse into just how diverse and resourceful bacteria are.

At any given time, there are several hundred species of bacteria in your mouth—mostly on your tongue—all competing for the resources you put there (FIGURE 10-29). Some of the bacteria are aerobic, requiring oxygen for their metabolism, and others are anaerobic. At night, because the flow of saliva slows down and the oxygen content of your mouth decreases, the anaerobic bacteria get the upper hand in terms of growth and reproduction. These bacteria metabolize food bits in your mouth, plaque on your teeth and gums, and dead cells from the lining of your mouth, breaking down proteins in these materials to use as their energy source. Because proteins are made from amino acids, some of which contain the smelly chemical sulfur, their breakdown leads to the odor in the accumulating waste products.

Once you wake up, you breathe more and produce more saliva, both of which increase the oxygen level in your mouth. This tips the battle for space and food back in favor of the aerobic bacteria. Because aerobic bacteria prefer carbohydrates as their energy source and because carbohydrates don’t contain sulfur, the sulfur smell goes away as the aerobic bacteria start to outcompete the anaerobic bacteria. The aerobic bacteria are, of course, also filling your mouth with waste products, it’s just that their waste products don’t smell as bad.

On a small scale, your mouth reveals some of the tremendous biological versatility of the bacteria: hundreds of species can live in a tiny area—a teaspoon of soil, for example, is home to more than a billion bacteria—they can thrive in a variety of unexpected habitats, they can utilize a variety of food sources, and they can survive and thrive with or without oxygen. Looking around the world, we find the clear dominance of bacteria. By any measure, this is their planet: the biomass of bacteria (if they were all collected, dried out, and weighed) exceeds that of all the plants and animals on earth (FIGURE 10-30). Bacteria live in soil, air, water, arctic ice, and volcanic vents. Many can even make their own food, utilizing light from the sun or harnessing energy from chemicals such as ammonia.

While the various species differ in many ways, the bacteria are a monophyletic group, sharing a common ancestor. For this reason, they all have a few features in common. All bacteria are single-celled organisms with no nucleus or organelles, with one or more circular molecules of DNA as their genetic material, and using several methods of exchanging genetic information. Because they are asexual—they reproduce without a partner, just by dividing—the biological species concept cannot be applied to bacteria when classifying them into narrower categories. As a consequence, bacteria are classified on the basis of physical appearance or, preferably, genetic sequences.

Many people think of bacteria as illness-causing organisms. However, although bacteria are responsible for many diseases, including strep throat, cholera, syphilis, pneumonia, botulism, anthrax, leprosy, and tuberculosis, disease-causing bacteria are only a small fraction of the domain, and bacteria seem to get less credit for their many positive effects on our lives. Consider that bacteria (E. coli) living in your gut help your body digest the food you eat and, in the process, make certain vitamins your body needs. Other bacteria produce antibiotics such as streptomycin. Still others live symbiotically with plants as small fertilizer factories, converting nitrogen into a form that is usable by the plant. Bacteria also give taste to many foods, from sour cream to cheese, yogurt, and sourdough bread. Increasingly, bacteria are used in biotechnology—from those that can metabolize crude oil and help in the cleanup of spills to transgenic bacteria used in the production of insulin and other medical products, as described in Chapter 5.

We explore the great diversity of the bacteria in more detail in Chapter 13.