Humans have a love–hate relationship with the prokaryotes. They cause terrible diseases, but by recycling the indispensable materials that all organisms need, make life possible on Earth.
Among the other accomplishments of the prokaryotes is their ability to parasitize all manner of plants and animals. Human bacterial diseases include tuberculosis, Lyme disease, black plague, cholera, botulism, pneumonia, and hundreds of others. Bacteria cause particularly vicious disease in plants of all sorts. The wilt diseases are produced by bacteria that live within the xylem vessels, plugging them and preventing water from reaching the upper part of the plant, which causes wilting. After filling the vessel openings, the bacteria next attack the cell walls, rupturing the cells, causing the plant to collapse. Crown‐gall, a cancerous tumor on stems, is caused by bacteria that inject DNA plasmids into the host cell nuclei, thereby taking control over hormone synthesis—and acquiring a home in the process.
Within the number of pathogens are the mycoplasmas—bacteria that lack cell walls. They are the smallest organisms able to live independently. They are 0.2–0.3 μm in diameter, live in the sugar solutions carried in the phloem of vascular plants, cause over 200 extremely destructive diseases of plants, and were completely unknown for decades because they passed through standard laboratory filters and were invisible in ordinary light microscopes. Researchers grew old and gray trying to determine what was happening in their supposedly cell‐free solutions before the existence of mycoplasmas was known.
Genetic engineers use recombinant DNA techniques learned from manipulating bacterial genomes to cut and splice packets of genes. Bacteriophages, viruses that infect bacteria, often are used to carry genes between organisms. Plasmids with genes of interest can be isolated and the genes duplicated by the billions in industrial applications to produce vaccines and hormones or, in agricultural applications, to develop plants and animals with desirable characteristics. Opinions vary concerning the ethics of doing so. Proponents minimize the risks to the environment, but the potential to wreak havoc remains a disturbing possibility.
The prokaryotes are a mish‐mash of contradictions. Some are the smallest of living organisms, but they outnumber all others on Earth. They are the most abundant organisms, but can't be seen with the naked eye, and even light microscopes reveal only gross details; they are structurally the simplest of organisms, but they alone live and thrive in the most extreme environments imaginable. They have been on Earth the longest of all organisms, but we have only begun to identify the kinds extant around us. They cause the deadliest diseases, and the most efficacious antibiotic drugs. We probably could survive without cheese and yogurt or pickles and sauerkraut, which are possible because bacteria produce lactic acid and vinegar, but life itself could not survive without the bacteria.