The Canon (33 page)

Whenever I'm feeling glum or lazy and start thinking, Maybe tonight I'll skip a step or two, I rouse myself by recalling the vile day at age ten when my dentist told me that I had twenty-two new cavities and that he'd be spending every Saturday for the next six months with his distressingly furry forearms in my mouth; or my more recent realization, while gazing at my dental X-rays, that I'd had—Holy Novocain! can this be right?—nine root canals to date. Or I think of what I learned from Bonnie Bassler, the microbiologist at Princeton University, whose own thick pelt happens to reside on her head, about the floral story of tooth decay.

You probably know that cavities are caused by bacteria, she said. But what you may not appreciate is how sophisticated, resourceful, and relentlessly disciplined those bacteria can be. It turns out that, my tragic dental history notwithstanding, it isn't easy for the agents of decay to sink their teeth into ours, to remain in place long enough to drill holes
through the tooth's protective enamel and thence to dine on the soft tissue beneath. For one thing, the mouth salivates constantly and purposefully: saliva is part of the body's defense system, a mildly antiseptic fluid designed to help sweep bacteria from the teeth and down into the gastric mulcher below. For another, tooth enamel is the hardest substance in the body. It is harder than bone, harder than an unclipped toenail on a camping trip. Enamel has enabled many a tooth to last posthumously into posterity; teeth are so abundantly represented in the fossil record, Michael Novacek of the American Museum of Natural History joked to me, that one might think the history of life on Earth consisted of teeth mating with teeth to beget other teeth.

So how do mouth bacteria manage to hang on and hammer through the enamel of live teeth, and in less than a single lifetime? We give them a head start by making poor dietary choices—chewing sugary bubblegum, for instance, or inexplicably reaching for one of those cellophane-wrapped hard candies that your grandmother has kept on her coffee table since the Ford administration. Not only does sugar attract bacteria, it helps them to cling to your teeth and begin their attack on your personal Pearly Harbor. The military analogy is a fitting one. Just as in a full-scale assault you bring out your bombers, your helicopters, your tanks, your Seabees and SEALs, so it is that six hundred distinct species of bacteria take part in the chop op. I'm not talking about six hundred individual bacteria. I'm talking about six hundred different species—or strains, as some microbiologists call them—each of them as genetically different from the other, said Bassler, "as Martians might be from humans." Hundreds of species, and hundreds of thousands or millions of members of those species, all cooperating to beat your teeth to a pulp. One species might be able to metabolize the sugar residues on the teeth, Bassler said, while another might be good at clinging to the enamel, and the next might release abrasive chemicals to begin scraping at the enamel. You can't see any of these wretched little grunts, of course. Like most bacterial cells, our oral flora are ridiculously small, a fraction the size of our body cells. Our pinhead, recall, could carry 3 million. But you can feel your cariel bacteria, oh yes, you can feel the thin coat of slime they leave on your teeth, the slime we call plaque. This plaque is Rasputin, or Mr. Johnson's cat. Do what you will, the plaque will always come back. "You can brush your teeth at night, but the bacteria will be back again by morning," said Bassler. "They'll be back, and not willy-nilly either. They will be back with the same highly structured order every time."

And so every day I hack right back, with floss and brush and minty
rinse. I know the enemy. I admire the enemy. I may not keep the enemy away, but by systematically minimizing its impact on my teeth, I at least have a shot with the dentist.

You can't sterilize your mouth, or your hands or face, no matter how many bottles of Purell sanitizing gel you go through in a week. You are covered with bacteria. Maybe a half billion of them blanket your skin, a teeming microtropolis of several thousand different strains. Billions more happily fill the moist orifices of your body—the mouth of course, and nose, ears, vagina, urethra, anus, and lower intestines. When you breathe, you breathe in happenstance vortices of airborne bacteria, the great majority of which are harmless, are incapable of colonizing your lungs and making you sick. When you walk, you walk through and upon a drifting tulle of bacteria, like a Christo confabulation in Central Park but less saffrony. Rub your index finger across this page, and, poof, a million microbes ruffled or displaced. We galumph through all this life heedlessly, like giants in a Gary Larson cartoon, attending to it only when we seek to kill it—kill the plaque, the strep, the bearers of your tuba-toned bronchitis. Yet most bacteria are benign, want no more from us than we do from them, and many of them are quite useful, and some of them are essential to our survival. They feed us, they cook for us, they clean up our messes. By "fixing" nitrogen into a form fit for plants, root-dwelling bacteria help give plants leave to grow, and plants in turn give us all that we eat—our daily bread, our lettuce and tomato, our sliced roast beast. Once ingested, our meals are digested with the help of intestinal bacteria. Something like ninety-nine out of a hundred cells in our small intestine are bacterial cells, which flourish in the warmth and plenty of our plumbing and in return synthesize vitamins for us and help extract from our food essential nutrients that otherwise would pass through unclaimed.

Wherever you go, there they are, doughtily doing the world's dirty work. Dig up a gram of soil, a loamy pinch that can fit easily in a thimble, and you're looking at thousands of different species of bacteria, many of them detritus recyclers, breaking up the dumped and the dead and making them fit for new life. Or consider termites, the primary groundskeepers of tropical rainforests. They gnaw through dead or rotting trees and return much of the woody wealth back to the forest floor. What is a termite but a set of jaws joined to a petri dish, its gut a dense microecosystem of many hundreds of strains of microbes. Bacteria allow termites to wrest sustenance from sawdust and, like Geppetto, give dead wood a voice.

Some bacteria glitter, graced with the same incandescent chemicals
that make a firefly glow; and just as a firefly flashes for love, so these luminiferous microbes will light up only when surrounded by others of their kind. Some bacteria play Jackson Pollock, spattering the stacked calcium outflows of Yellowstone National Park in streaks of pink, blue, green, amber, and brick—each color the signature of a distinct bacterial clan afeast on chalk soufflé.

Bacteria live everywhere, and in the most hellforsaken nowheres. They live on the summit of Mount Everest, and at the bottom of the sea; they live in polar icecaps and by boiling hydrothermal vents. They survive deep within rocks buried deep underground; they suck up heavy metals and oil spills and do laps in Love Canal. One bacterial species aptly named
Deinococcus radiodurans
can withstand a blast of radiation 1,500 times greater than the dose that would kill us, and 15 times greater than what would stir-fry that canonical survivor, the cockroach.

Yet as admirable as bacteria may be for their panplanetary powers and boundless vim, that brilliance ultimately redounds to a brilliance even grander, handier, and more foundational than theirs, the supreme brilliance of the entity of which bacteria and every other being on Earth is built: the cell. The cell is surely the greatest invention in the history of life on this planet, and ever since the first cell arose, as Gunter Blobel said, it has been all cell, all the time, a never-ending splitting of cells to make more cells, to keep life alive in the only way it knows how: in the context of the cell, by the bauplan of the cell. Bacteria exemplify the cellular nature of life because they are single-celled organisms. Each bacterium is a living being. It holds within itself the chemicals, components, and conditions required to sustain life, and it encapsulates the tremendous success story that is the cellular calling, the permanent nonpareil cellular vocation, which has never taken a vacation, never been out to lunch or out of line, or outgrown, outdone, outfoxed, outmoded, or rubbed out since the first cell arose some 3 billion years ago. This is the amazing thing, one of the most profound basic principles that biology offers: that once the first cell had pulled itself together, had assembled itself into a serviceable self-serving self, there was no turning back, and there has never been a cell-free moment since. Through ungodly long temporal caravans, stultifying passages of duffel bags stuffed with epochs, through ice ages and asteroid crashes, volcanic revolts, oceanic tantrums, and mass extinctions that destroyed 90 percent of life on Earth, still, not for a single day, a single nano- or pico- or atto-girl, yocto-boy second, has the world been without some scrim of cells somewhere, some thread of life, however threadbare, defiantly clinging to life. For the substrate stone, the stubborn cells may be so much
plaque, the scum of the earth, and who knows but that the stone doesn't pine for a foolproof decellerator, a perfect Purell. Happily, it was not to be, in the history of our Earth; and aren't you glad the plaque always came back?

We know that all cells on Earth are monophyletic, are all descendants of a single founder cell, rather than being polyphyletic, of multiple, independent origins, because the unity of the genetic code tells us so. We also can see it in the structure of the cell, any cell, the cell of a bacterium, a corn plant, a fruit fly, a barfly. The cell, wherever it is stationed, has an unmistakable geography, a set of shared features that explain why it is the universal unit of life, and why it works so outrageously well. Think again about the Putumayo catalogue of bacteria we discussed: the ones in your mouth, the ones in your gut, the mountaineers, the thermophiles. In one sense, they're all very different from one another, every strain endowed with a subset of specialty genes that allows it to exploit weird resources like benzene or mercury and to weather the specific withering conditions of its niche. On the other hand, if you were to crack open any of these bacterial cells, you'd realize that they all look and feel very like-minded inside: similar chemical conditions, similar balance of acid and base. And the internal milieu of a bacterial cell is much like that of one of our liver or heart cells, or of any other cell of any other organism on Earth. This is the beauty and power of the cell, and one of the core insights to emerge from modern biology: A cell confronts the harshness and instability of the outside world by making itself a haven. A cell contains all the tools it needs to preserve order and stability within its borders, to keep its interior recesses warm and wet and chemically balanced. In this equilibrated, levelheaded setting, the cell's vast labor force of proteins and enzymes will operate at peak performance, and so sustain the cell in its state of mild grace. There is nothing more natural than a cell; the natural world, after all, is full of them. At the same time, a cell is the ultimate act of artifice, a climate-controlled limousine with cushioned seats and a private bar, cruising through a mad desert storm.

A cell is the basic unit of life, and the smallest unit of matter that can, by anyone's book, be considered alive. A virus is also a unit of matter that displays a few lifelike properties, most notably a zealous drive to replicate itself and the capacity to mutate and evolve; and a virus, being nothing more than a packet of genes wrapped in a jacket of protein and sugar molecules, is much smaller than even the smallest cells—bacterial cells. Nevertheless, most scientists argue that because a virus doesn't engage in such essential rituals of life as eating and excreting, and is entirely reliant on the apparatus of the host cell it infects to create new viral particles for it, a virus isn't a true life but a protolife, a wannabeing, a parasitic paralife as told on Post-it notes. They reserve their certificate of animate authenticity for the cell, as the smallest package of life on Earth and bearer of all the best gifts.

The cell lives, breathes, tastes, and makes waste, and when called upon will replicate. The cell is self-sufficient, and that is its conceptual beauty and power. But what, in a more practical, biomechanical, predilectional sense, is a cell? How does a cell work, what are its prime parts, and why is all life built on its spine? What does a cell look like, and why does it insist on being too small for the naked eye to see? First, I must point out that not every cell is microscopic. A cell has three basic parts to it: a greasy, waterproof outer membrane, the plasma membrane, which serves as the border between cell and setting, self and nonself; a gooey inner part, the cytoplasm, where most of the work of the cell is performed; and a cache of DNA, the cell's genetic content, its operating manual and ticket to tomorrow. In our cells, and the cells of any multicellular being and quite a few unicellular ones as well, the DNA is enclosed in a nucleus, a snug compartment surrounded by a smaller but double-layered version of the plasma membrane that moats the whole cell. In bacterial cells, the DNA floats free in the cytoplasm. Not surprisingly, the labels we assign to the two basic cell types lionize the DNA housing option we happen to possess. Cells with a nucleus are called eukaryotic cells, "eu" meaning "good" or "true," and "karyote" meaning "kernel" or "nucleus." Bacterial cells and other single-celled organisms lacking a nucleus we deride as "prokaryotic"—that's "pro" as in "pre," not "pro" as in "fan of" and certainly not as in "professional." Prokaryotic cells are "prenuclear" cells, the poor sods that kept the world busy for a billion years or so before the "good" cells, the ones with a nucleus, showed up. Bacteria have repaid the compliment through occasional displays of very professional pathogenicity—
viz.,
bacterial diseases such as bubonic plague, anthrax, syphilis, childbed fever, and, of course, tooth decay.

Nucleated or not, cells consist of these three defining ingredients, and it so happens that one type of bioentity that fulfills the criteria of cellhood is the egg. An egg has an outer membrane, a viscous cytoplasm that in the edible egg we call the yolk, and a set of genes—only half the number of genes needed to spawn an offspring, and half the number of genes found in other body cells of the egg bearer, but a gene set nonetheless. An egg, then, before it merges its DNA with a gene set supplied by a sperm and starts developing into an embryo, is a single cell, and
that goes for the egg you can see well enough to scramble. Yes, believe it or not, an unfertilized chicken egg of the kind you buy at the grocery store is a single cell, although strictly speaking it's the cheery, marmalade-colored yolk of the egg that is bounded by the plasma membrane and thus qualifies as the cell proper. The translucent, whippable, protein-rich "egg white," the hard outer shell of calcium carbonate, and the thin, slippery membrane lining the shell are all bonus coats added on later, as the yolk makes its way down the mother's cloaca. Still, chicken yolks are no joke, and they keep getting ever more jumbo even as we fret over the wisdom of eating any eggs at all. The largest egg in the world, and thus the largest cell in the world, is the ostrich egg, which measures about eight by five inches and weighs three pounds with its extracellular shell, two pounds without. (Interestingly, the ostrich egg is also the smallest bird egg relative to the size of its mother, amounting to only 1 percent of the female ostrich's body mass. The she-birds most deserving of every mother's pity are the kiwis and hummingbirds, which lay eggs that are 25 percent as big as they are—the equivalent of a woman giving birth to a thirty-pound baby.)