13.6–13.10: In humans, bacteria can have harmful or beneficial health effects.

Escherichia coli (E. coli) bacteria (pink) on the surface of human skin and hair follicle.

Do you like yogurt? You can thank bacteria for it—Lactobacillus acidophilus and several other species of bacteria are added to milk to create yogurt. As the bacterial cells use the lactose (milk sugar) for energy, the by-product is lactic acid, which reacts with the milk proteins to produce the characteristic taste and texture of yogurt. If you buy a brand of yogurt that is labeled as containing “live cultures,” you are consuming living bacterial cells as you eat the yogurt (FIGURE 13-10). Bacteria are also used to produce many other foods, such as cheeses, and they (along with yeasts) are used in the production of beer, wine, and vinegar. Industrial microbiology is a multi-billion-dollar industry.

Figure 13.10: Yogurt contains beneficial bacteria.

But you can thank bacteria for more than just tasty snacks—you owe your life to them. Hundreds of species of bacteria grow in and on your body; these microbes are called your “normal flora.” The normal flora take up every spot on your body that a disease-causing bacterium could adhere to, and they consume every potential source of nutrition, making it difficult for a disease-causing bacterium to gain a foothold. Thus, maintaining a robust population of these benign bacteria is your first line of defense against infection by harmful bacteria.

Probiotic therapy is a method of treating infections by deliberately introducing benign bacteria in numbers large enough to swamp the harmful forms. Lactobacillus acidophilus, which is a normal inhabitant of the human body, is used to treat gastrointestinal upsets, such as traveler’s diarrhea, and urinary tract infections. In addition to replicating so vigorously that it crowds out harmful bacteria, L. acidophilus releases lactic acid, which interferes with the growth of other bacteria and prevents them from adhering to the walls of the urinary tract and bladder.