Читать книгу The Tangled Tree: A Radical New History of Life - David Quammen, David Quammen - Страница 29

Bacteria are versatile and diverse. That’s an understatement. Widespread—another understatement. They are hard to categorize, hard to identify, hard to sort into related groups, as even Stanier and van Niel finally admitted. They are nearly ubiquitous across most parts of Earth’s expanse, in both natural and human-made environments, floating through the air, coating surfaces everywhere, awash in the oceans, even present in rocks deep underground. Your skin as I’ve said is covered with them. Your gut is teeming. Your human cells may be outnumbered by them at a three-to-one ratio in your body. Bacteria live also in mudholes and hot springs and puddles and deserts, atop mountains, deep in mines and caves, on the tabletops at your favorite restaurant, and in the mouths of you and your dog.

A species called Bacillus infernus has been cultured from core samples of Triassic siltstone, buried strata at least 140 million years old, drilled up from almost two miles beneath eastern Virginia. Under the Pacific Ocean, 35,755 feet deep in the Mariana Trench, lie sediments that have also yielded living bacteria. In Antarctica, a body of water known as Subglacial Lake Whillans, lidded by half a mile’s thickness of ice and supercooled to just below zero, harbors a robust community of bacteria. They thrive there in the darkness and cold, eating sulphur and iron compounds from crushed rock.

Then again, some like it hot. Those are called thermophiles. Among the most famous of thermophilic bacteria is Thermus aquaticus, first cultured from a sample collected in Yellowstone National Park by the microbiologist Thomas Brock and a student, Hudson Freeze, in 1966. Brock and Freeze had found it in a steaming, multicolored pool called Mushroom Spring, in Yellowstone’s Norris Geyser Basin, at a temperature of about 156 degrees Fahrenheit. Functioning in such heat, Thermus aquaticus contains a specialized enzyme for copying its DNA, one that performs well at high temperatures, which became a key element in the polymerase chain reaction technique for amplifying DNA. That technique, widely useful in many aspects of genetic research and biotech engineering, earned its chief developer (but not Thomas Brock) a Nobel Prize.

Other heat-loving bacteria can be found around hydrothermal vents on the sea bottom, where they help anchor the food chains, producing their own organic material from dissolved sulfur compounds vented out with the hot water, and being fed upon by little crustaceans and other animals. A giant tube worm, one of those gaudy red creatures that waggle around such vents, with no mouth, no digestive tract, gets its nutrition from bacteria growing within its tissues.

By one estimate, the total mass of bacteria exceeds the total mass of all plants and animals on Earth. They have been around, in one form or another, for at least three and a half billion years, strongly affecting the biochemical conditions in which most other living creatures have evolved. That we don’t see bacteria is simply because our eyes are not calibrated to the appropriate scale. There may be more than a billion bacterial cells in an average ounce of soil, and five million in a teaspoon of fresh water, but we can’t hear their crackle or their fizz. A single kind of marine bacteria known as Prochlorococcus marinus, which drifts free in the world’s tropical oceans and photosynthesizes like a plant, may be the most abundant creature on Earth. One source places its standing population at three octillion individuals, a number that looks like this: 3,000,000,000,000,000,000,000,000,000.

They vary in shape and in size—interestingly in shape, drastically in size. A bacterial cell, on average, is about one-tenth as big as an animal cell. At the upper end of the range is Thiomargarita namibiensis, an odd thing discovered on the sea floor near Namibia, its cells ballooning up to three-quarters of a millimeter in diameter, stuffed with pearly globules of sulfur. At the lower end of the range is Mycoplasma hominis, a tiny bacterium with a tiny genome and no cell wall, which manages nonetheless to invade human cells and cause urogenital infections.

Bacterial shapes, as I’ve mentioned, range through rods, spheres, filaments, and spirals, with variations that in some cases represent adaptations for movement or penetration. It turns out that their geometries, notwithstanding the efforts and convictions of Ferdinand Cohn, are unreliable guides to their phylogeny. Shape can be adaptive, but adaptations can be convergent as well as ancestral. Roundness may be good as a hedge against desiccation. Elongation as a rod or filament seems to help with swimming, and a flagella definitely does. Filamentous bacteria that are star shaped in cross section, recently discovered in a wonderfully named substance called “mine-slime,” deep in a South African platinum mine, may profit from all their surface area by way of enhanced absorption in nutrient-poor environments. The twisting motion of spirochetes, such as the ones that cause syphilis and Lyme disease, evidently allows them to wiggle through obstacles that other bacteria can’t easily cross, such as human organ linings, mucous membranes, and the barrier between our circulatory system and our central nervous system—a fateful degree of access. Even the less dynamic shapes, the short rods known as bacilli, the spheres known as cocci, and the rods slightly curved like commas, serve well enough the bacteria responsible for a long list of diseases: anthrax, pneumonia, cholera, dysentery, hemoglobinuria, blepharitis, strep throat, scarlet fever, and acne, among others.

Although many bacteria live as solitary cells, taking their chances and meeting their needs independently, others aggregate into pairs, clusters, little scrums, chains, and colonies. The coccoid cells of Neisseria gonorrhoeae, which cause gonorrhea, lump together by twos, forming bilobed units resembling coffee beans. The genus Staphylococcus gets its name from Greek words for “granule” (kókkos, the spheroid aspect) and “a bunch of grapes” (staphylè), because staph cells tend to bunch. Most of the forty staph species are harmless, but Staphylococcus aureus can inflict skin infections, sinus infections, wound infections, blood infections, meningitis, toxic shock syndrome, plus other nasty conditions, and if you’re so unlucky as to pick up a dose of those little grapes in one of their antibiotic-resistant forms, such as MRSA, a monstrous product of horizontal gene transfer (as I’ve mentioned, and to which I’ll return), you could be in a world of hurt. Cells of Streptococcus species, including those that cause impetigo and rheumatic fever, stick together like beads on a chain.

Bacteria can also form stubborn, complex films on certain surfaces—the rocks of a sea floor, the glass wall of an aquarium, the metal ball of your new artificial hip—where they may cooperate together in exuding a slimy extracellular substance that helps nurture them collectively, maintain the stability of their little environment, serve as a sort of communications matrix among them, and even protect them from antibiotics. These living slicks, known as biofilms, can be thinner than tissue paper or as thick as a good dump of snow, and may incorporate multiple species. The little rods of Acinetobacter baumannii are infamous for their ability to lay down persistent biofilms on dry, seemingly clean surfaces in hospitals.

Cyanobacteria, including that monumentally abundant Prochlorococcus, convert light to energy and deliver, as byproduct, a large share of Earth’s free atmospheric oxygen. Purple bacteria photosynthesize too, but do it by drawing upon sulfur or hydrogen instead of water as fuel for the process, and they don’t produce oxygen. Lithotrophic bacteria, the rock eaters, deriving their energy from iron, sulfur, and other inorganic compounds, exist in more ingenious variants than you care to know. Japanese researchers have recently discovered a new bacterium, Ideonella sakaiensis, that digests plastic. Certain enterprising ocean bacteria, such as Marinobacter salarius, have risen to the challenge of degrading hydrocarbons from the Deepwater Horizon oil spill. Other bacteria are quite capable, in the presence of oxygen or without it, of feasting on garbage, sewage, various inorganic compounds, plants, fungi, and animal tissue, including human flesh. Lactic acid bacteria, which may be rod shaped or spherical, turn up in milk products, busy at their task of carbohydrate fermentation and resistant to the acid they create. Many of them also like beer.

Not all such particulars were known to Carl Woese in 1977 as he examined the fingerprints from his first few methanogens. But the vast scope, ubiquity, and multifariousness of bacteria certainly were. The terrain of bacteriology was known even better to Ralph Wolfe, who had trained in the classic fundamentals under van Niel and others. Woese’s reaction to his own preliminary results must have seemed all the more radical, then, all the more shocking, as he shared it not just with George Fox and members of his own lab but also with Wolfe, just after they repeated the rRNA analysis of delta H, the first methanogen. “Carl’s voice was full of disbelief,” Wolfe wrote in a memoir, “when he said, ‘Wolfe, these things aren’t even bacteria.’”