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executioner."
    "That's a lot to swallow all at once," Montoya said. "I'm skeptical about bacteria communicating and cooperating. Don't they just grow and eat randomly?"
    "What kind of toothbrush do you use?" I asked.
    Montoya shook his head, puzzled. "Does it matter?"
    "Just tell me."
    "A Sonodyne. I've got a big investment in the company."
    "It uses high-frequency vibrating bristles, right?"
    "Yeah."
    "There are over five hundred different kinds of bacteria in our mouths," I said. "Not all of them cause cavities. Some repel or destroy their disease-causing cousins. A healthy mouth is more like the Amazon jungle than a Listerine commercial."
    Montoya puffed into his palm and sniffed the result. "Do I offend?" he asked, smiling.
    I smiled back. "Not at all. But some of them stick to each other and cement themselves to your teeth. After a while, they build up layers of bacterial architecture on your enamel. Dentists call it plaque. It's a community of cooperating bacteria of many different kinds--a bio film The Sonodyne vibrates the bio film until it falls apart--breaks the cement the bacteria use to fasten to the teeth. In essence, you're demolishing their houses and shaking them up so bad they can't even talk."
    "Look, Ma, no cavities," Montoya said.
    "Other bacterial communities colonize your skin, your mucus membranes, and, of course, your gut, where they perform essential digestive services." I could sense myself overstepping the bounds of what my angel might want to hear. "There are so many bacteria in your intestines that even people who are starving excrete feces--made up mostly of bacteria."
    "Wow," Montoya said. "Gossip in the big germ city. But if we're so important to them, why try to bring us down?"
    "A herd of antelopes sheds the old and tired to make way for the young and fit. Lions prune the herd like a rosebush. The lions may act like killers, but actually they're partners with a big investment in the health of the herd. Bacteria are more than just important partners--they're the most successful predators of all. We're their herd. Aging and death is one way to keep the herd fresh and healthy."
    "So, how do bacteria cause aging?" Montoya asked, leaning forward and moving his tongue over his lips.
    "Bacteria in our gut produce quantities of a tiny protein I call hades" Now I was really sweating. "Our tissues open special receptors, coded for in genes I believe once came from mitochondrial chromosomes. Hades creeps in. It winds up a molecular clock days or weeks after we're born. With each tick of the clock, the bacteria increase the amount of hades they import into our tissues. Hades alters the way mitochondria work--jams them up, makes them convert ATP with less efficiency. We accumulate the resulting oxidants and free radicals, byproducts of respiration that damage our DNA. Our cells can't repair the damage. We start to lose our youthful resilience. We grow old."
    Montoya held up his hand and rubbed a few small, liver-colored patches on the back. "Age spots," he said. "And I'm not that old. So what's in it for the bacteria?"
    "There's a pot of gold waiting for them. Eventually, we get so weak, so full of genetic errors, that disease or cancer finishes us off. Then, the bacteria have an orgy. They feast like retainers eating a dead king."
    "Jesus," Montoya said, and clenched his hand into a fist.
    "That's the work I'll be publishing in a few months, communication between E. coli and mitochondria in human intestinal cells. I'm leaving out the news about hades for now."
    "We could just kill all our bacteria. Wipe them out with radiation or something. Live in a sterile environment."
    "They tried that in the nineteen twenties, and it didn't work," I said. "The fact is, we're designed to die. The molecular clock also acts like a de adman switch. Without bacteria, we go on aging anyway-only faster. A certain amount of hades may serve double duty--if we're active and productive, it may even reset the timer on the clock.