Prokaryotes live in and exploit all sorts of environments and are part of all ecosystems. In the following pages, we'll examine the roles of prokaryotes that live in soils, in water, and even in other living organisms, where they may exist in a neutral, benevolent, or parasitic relationship with their host's tissues.
Prokaryotes are important players in element cycling
Animals depend on photosynthetic plants and microorganisms for their food, directly or indirectly. But plants depend on other organisms—prokaryotes—for their own nutrition. The extent and diversity of life on Earth would not be possible without nitrogen fixation by prokaryotes. Nitrifiers are crucial to the biosphere because they convert the products of nitrogen fixation into nitrate ions, the form of nitrogen most easily used by many plants (see Figure 37.8). Plants, in turn, are the source of nitrogen compounds for animals and fungi. Denitrifiers also play a key role in keeping the nitrogen cycle going. Without denitrifiers, which convert nitrate ions back into nitrogen gas, all forms of nitrogen would leach from the soil and end up in lakes and oceans, making life on land impossible. Other prokaryotes contribute to a similar cycle of sulfur. Prokaryotes, along with fungi, return tremendous quantities of organic carbon to the atmosphere as carbon dioxide.
In the ancient past, the cyanobacteria had an equally dramatic effect on life: Their photosynthesis generated oxygen, converting Earth from an anaerobic to an aerobic environment. The result was the wholesale loss of obligate anaerobic species that could not tolerate the O2 generated by the cyanobacteria. Only those anaerobes that were able to colonize environments that remained anaerobic survived. However, this transformation to aerobic environments made possible the evolution of cellular respiration and the subsequent explosion of eukaryotic life. What other roles do prokaryotes play in the biosphere?
A time bomb lies deep under the ocean floor. Some ten trillion tons of methane, potentially an overwhelming source of "greenhouse gas," are located there. Will this methane escape to the atmosphere, hastening global warming?
What will prevent such an escape is the presence of legions of archaea, also lying below the bottom of the seas. As methane rises from its deposits, it is metabolized by these archaea, with the result that virtually none of the methane even gets as far as the deepest waters of the ocean. Thus, these archaea play a crucial role in stabilizing the planetary environment.
Prokaryotes work together with eukaryotes in many ways. In fact, mitochondria and chloroplasts are descended from what were once free-living bacteria. Much later in evolutionary history, some plants became associated with bacteria to form cooperative nitrogen-fixing nodules on their roots (see Figure 37.5).
The tsetse fly, which transmits sleeping sickness by transferring trypanosomes (microscopic protists described in the next chapter) from one person to another, enjoys a profitable association with the bacterium Wigglesworthia glossinidia. Biologists who decoded the genome of W. glossinidia in 2002 were surprised to learn that the bacterium's tiny genome contains almost nothing but the genes needed for basic metabolism and DNA replication—and 62 genes for making ten B vitamins and other nutritional factors. Without the vitamins provided by the bacterium, the tsetse fly cannot reproduce. The bacteria, living inside the fly's cells, are in effect vitamin pills. Researchers are now trying to determine whether an attack on W. glossinidia may succeed in combating sleeping sickness where more obvious direct attacks on tsetse flies or the trypanosomes have failed.
Many animals, including humans, harbor a variety of bacteria and archaea in their digestive tracts. Cows depend on prokaryotes to perform important steps in digestion. Like most animals, cows cannot produce cellulase, the enzyme needed to start the digestion of the cellulose that makes up the bulk of their plant food. However, bacteria living in a special section of the gut, called the rumen, produce enough cel-lulase to process the cow's daily diet. Humans use some of the metabolic products—especially vitamins B12 and K—of bacteria living in the large intestine.
We are heavily populated, inside and out, by bacteria. Although very few of them are agents of disease, popular notions of bacteria as "germs" arouse our curiosity about those few. Let's briefly consider the roles of some bacteria as pathogens.
The late nineteenth century was a productive era in the history of medicine—a time during which bacteriologists, chemists, and physicians proved that many diseases are caused by mi-crobial agents. During this time the German physician Robert Koch laid down a set of four rules for establishing that a particular microorganism causes a particular disease:
1. The microorganism is always found in individuals with the disease.
2. The microorganism can be taken from the host and grown in pure culture.
3. A sample of the culture produces the disease when injected into a new, healthy host.
4. The newly infected host yields a new, pure culture of microorganisms identical to those obtained in the second step.
These rules, called Koch's postulates, were very important in a time when it was not widely accepted that microorganisms cause disease. Today medical science makes use of other, more powerful diagnostic tools. However, one important step in establishing that a coronavirus was the causal agent of SARS (Severe Acute Respiratory Syndrome), a disease that first appeared in 2003, was the satisfaction of Koch's postulates.
Only a tiny percentage of all prokaryotes are pathogens (disease-producing organisms), and of those that are known, all are in the domain Bacteria. For an organism to be a successful pathogen, it must overcome several hurdles:
► It must arrive at the body surface of a potential host.
► It must evade the host's defenses.
► It must multiply inside the host.
► It must damage the host (to meet the definition of a "pathogen").
Failure to overcome any of these hurdles ends the reproductive career of a pathogenic organism. However, in spite of the many defenses available to potential hosts that we considered in Chapter 18, some bacteria are very successful pathogens.
For the host, the consequences of a bacterial infection depend on several factors. One is the invasiveness of the pathogen—its ability to multiply within the body of the host. Another is its toxigenicity—its ability to produce chemical substances (toxins) that are harmful to the tissues of the host. Corynebacterium diphtheriae, the agent that causes diphtheria, has low invasiveness and multiplies only in the throat, but its toxigenicity is so great that the entire body is affected. In contrast, Bacillus anthracis, which causes anthrax (a disease primarily of cattle and sheep, but also sometimes fatal in humans, as we saw in Chapter 13), has low toxigenicity but an invasiveness so great that the entire bloodstream ultimately teems with the bacteria.
There are two general types of bacterial toxins: exotoxins and endotoxins. Endotoxins are released when certain Gramnegative bacteria grow or lyse (burst). These toxins are lipopolysaccharides (complexes consisting of a polysaccha-ride and a lipid component) that form part of the outer bacterial membrane (see Figure 27.6). Endotoxins are rarely fatal; they normally cause fever, vomiting, and diarrhea.
Among the endotoxin producers are some strains of Salmonella and Escherichia.
Exotoxins are usually soluble proteins released by living, multiplying bacteria, and they may travel throughout the host's body. They are highly toxic—often fatal—to the host, but do not produce fevers. Exotoxin-induced human diseases include tetanus (from Clostridium tetani), botulism (from Clostridium botulinum), cholera (from Vibrio cholerae), and plague (from Yersinia pestis). Anthrax results from three exotoxins produced by Bacillus anthracis.
Remember that in spite of our frequent mention of human pathogens, only a small minority of the known prokaryotic species are pathogenic. Many more species play positive roles in our lives and in the biosphere. We make direct use of many bacteria and a few archaea in such diverse applications as cheese production, sewage treatment, and the industrial production of an amazing variety of antibiotics, vitamins, organic solvents, and other chemicals.
Pathogenic bacteria are often surprisingly difficult to combat, even with today's arsenal of antibiotics. One source of difficulty is the ability of prokaryotes to form resistant films.
Many unicellular microorganisms, prokaryotes in particular, tend to form dense films called biofilms rather than existing as clouds of individual cells. Upon contacting a solid surface, the cells lay down a gel-like polysaccharide matrix that then traps other bacteria, forming a biofilm. Once a biofilm forms, it is difficult to kill the cells. Pathogenic bacteria are hard for the immune system—and modern medicine—to combat once they form a biofilm. For example, the film may be impermeable to antibiotics. Biofilms often include a mixture of bacterial species.
The biofilm with which you are most likely to be familiar is dental plaque, the coating of bacteria and hard matrix that forms between and on your teeth unless you do a good job of flossing and brushing. Biofilms form on contact lenses, on hip replacements, and on just about any available surface. Other biofilms foul metal pipes and cause corrosion, a major problem in steam-driven electricity generation plants. Biofilms are the object of much current research. For example, some biologists are studying the chemical signals used by bacteria in biofilms to communicate with one another. By blocking the signals that lead to the production of the matrix polysaccha-rides, they may be able to prevent biofilms from forming.
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