Bounteous and (Mostly) Beneficial: Bacteria

Bacteria are the invisible workhorses of the planet. They include commonly encountered microbes such as the beneficial strain of Escherichia coli present in the human gut and Staphylococcus aureus, which causes skin infections, as well as not so commonly encountered ones like the fluorescence-emitting bacteria living in the head of a sea squid. While all bacteria are prokaryotic, and most possess a cell wall, their genetic diversity translates into a wide variety of differences in nutrition, metabolism, structure, and lifestyle.

Like all organisms, bacteria can be categorized by what they eat. Some bacteria are autotrophs (literally, “self-feeders”): they are able to make their own food directly, using material from the nonliving environment, from carbon dioxide to rocks. Others are heterotrophs (literally, “other feeders”): they must rely on other living organisms to provide them with food.

One of the largest and most important groups of autotrophic bacteria is the cyanobacteria, which are found in oceans and freshwater, as well as on exposed rocks and soil—virtually everywhere sunlight can reach them. Cyanobacteria use the energy of sunlight to carry out photosynthesis in a manner similar to plants, taking in CO₂ and generating much of the oxygen that other organisms, including humans, rely on. Cyanobacteria are thought to be the oldest photosynthetic organisms on Earth, dating back roughly 2.5 billion years and playing a pivotal role in making the atmosphere breathable for the rest of us. Many cyanobacteria also perform the ecologically useful task of converting nitrogen from the atmosphere into a form that plants can use to grow. This process, called nitrogen fixation, is indispensable for the survival of life on Earth (see Chapter 23).

Not all autotrophic bacteria rely on sunlight for energy. Some, including those living at Lost City, can obtain energy directly from geological sources such as inorganic gases pouring out of hydrothermal vents—making them among the few organisms on Earth that do not rely on the sun’s energy to survive.

Heterotrophic bacteria include those that obtain food by consuming material from living or dead organisms. Such heterotrophic bacteria play an important role in decomposition, allowing carbon and other elements—which would otherwise be trapped in dead organisms, sewage, or landfills—to be recycled. They are also useful in bioremediation projects. For example, some types of bacteria metabolize droplets of oil, much the way humans digest butter, so they naturally help clean up oil spills.

Bacteria break down their food molecules through a variety of metabolic pathways, some of which require oxygen, some of which do not. For example, many bacteria employ the anaerobic process known as fermentation (see Chapter 6) to get energy from food. The products of fermentation can be valuable (and tasty) to humans. You may have seen “L. bulgaricus” listed as an ingredient of yogurt; live Lactobacillus bulgaricus bacteria are present and at work in the yogurt, fermenting sugars into lactic acid, which helps the milk solidify into yogurt and which gives yogurt its tangy taste. Other bacteria use oxygen to break down organic molecules, like the aerobic bacteria that feast on an oil spill.

Highly resourceful, bacteria have a diverse range of living arrangements with other creatures. Many live in close association, or symbiosis, with other organisms—often to the benefit of one or both partners. Naturally occurring lactobacilli in the female vaginal tract, for example, obtain nourishment by fermenting naturally occurring sugars to lactic acid. The resulting acidity of the vaginal tract suppresses the growth of yeast, preventing yeast infections. Antibiotics taken for a bacterial infection are likely to kill the resident lactobacilli as well as the invaders, and a yeast infection is often an unhappy side effect.

Another example of beneficial bacterial symbiosis is Vibrio fischeri, a bioluminescent bacterium that lives and feeds inside the light organs of certain species of squid. The glow-in-the-dark Vibrio produces light beneath the squid and helps obscure the shadow that the squid might cast on a moonlit night, making it less noticeable to its prey as it hunts.

Unfortunately, not all bacteria are beneficial to the host. While the vast majority of bacteria do not cause human disease, some do. Bacteria and other organisms that cause disease are known as pathogens. Many pathogenic bacteria cause disease by producing toxins that harm their hosts. Such toxins can either be part of the bacterial cell itself or secreted by the bacterium. For example, certain strains of the bacterium Escherichia coli secrete a potent toxin that causes bloody diarrhea and even sometimes kidney failure and death in its host (these cases are often associated with the O57:H7 strain of E. coli that has been implicated in several foodborne outbreaks). Keeping food refrigerated helps prevent food poisoning by slowing the growth of the bacteria and, therefore, production of its toxin.

Not all pathogens produce toxins. Some cause disease by living and reproducing in the body and interfering with its normal processes-an example is the bacterium Treponema pallidum, which causes syphilis, a sexually transmitted disease (STD).

Sometimes the line between harmless and harmful bacteria can be blurred. Organisms that can, but don’t always, cause disease are known as opportunistic pathogens. For example, most of us have Staphylococcus aureus on our skin at many times during our lives. Most of the time, S. aureus does not cause any harm, but if it penetrates the skin—through a wound, for example—it can cause a serious infection and even death, as discussed in Chapter 14.

In addition to nutritional and metabolic differences, bacteria display a variety of structural adaptations that suit their various lifestyles. They come in different shapes: spherical (in which case they are known as cocci), rod-shaped (bacilli), and spiral (spirochetes). Many bacteria are equipped with flagella, tiny whiplike structures that project from the cell and help it move. For example, the bacterium Helicobacter pylori, the most common cause of stomach ulcers, uses its flagella to propel itself through the gastric mucus of the stomach. Pili are shorter, hairlike appendages that enable bacteria to adhere to a surface. Neisseria gonorrhoeae, the bacterium that causes the STD gonorrhea, uses its pili to remain attached to the lining of the urinary tract. Without pili, the bacteria would be flushed out by the flow of urine.

Other bacteria are surrounded by a capsule, a sticky outer layer that helps the cell adhere to surfaces and to avoid the defenses of the host. Streptococcus mutans, for example, produces a capsule that allows it to adhere to teeth, where it forms the plaque that can lead to cavities (INFOGRAPHIC 18.6).

For all their impressive diversity and abundance, bacteria are far from the totality of prokaryotic life. Moreover, their lifestyles and adaptations may seem tame next to those of the other domain of prokaryotic life: the Archaea.