Free Gulp: Adventures on the Alimentary Canal by Mary Roach Page A

clean with our mouths and then spit it out. But we would never consider this with other kids.”
    A similar psychology applies to breast milk. It’s considered natural for a child to consume a mother’s milk, or even for a lover to do so, but not a stranger. (Hence the 2010 hullaballoo over the New York City restaurateur who invited diners to try cheese made from his wife’s breast milk.) So reliable is breast-milk consumption as a delimiter of intimate family that Islam recognizes a category called “breast milk son,” which confers an exemption to the rules on segregation of the sexes. A man can be alone with a woman if she’s immediate family or if she breastfed him as a child. * (Sisters sometimes breastfeed each other’s infants, thereby creating breast-milk relatives.) Milk is thicker than blood, or about the same consistency.
    S ILLETTI HANDS ME a plastic cup and sets a timer. We are moving on to unstimulated saliva. This is background saliva, the kind that’s always flowing, though much more slowly. A minute passes. We turn away from each other and quietly spit in our cups.
    “Look at the difference, compared to stimulated.” Silletti tilts her cup. “You can’t pour it easily. It’s so viscous. Look!” She dips the end of a glass pipette into her sample and pulls it away. Filament is a nice word, Silletti’s word, for the mucoid strand that trails behind.
    Relatively little is known about unstimulated saliva. Partly, Silletti says, because no one wants to work with it.
    “Because it’s so gross?”
    “Because it’s harder to collect. And you can’t filtrate it. It clogs the filter, like hair in the drain. And you cannot be precise, because it’s so slimy.”
    “Right, it’s gross.”
    Silletti tucks a strand of her glossy black hair behind her ear. “It’s difficult to work with.”
    Unstimulated saliva’s trademark ropiness is due to mucins, long chains of amino acids repeating to form vast webs. Mucins are responsible for saliva’s least endearing traits—its viscosity, elasticity, stickiness. * They are also responsible for some of its more heroic attributes. Unstimulated saliva forms a protective film that clings to the surfaces of the teeth. Proteins in this film bind to calcium and phosphate and serve to remineralize the enamel. Webs of mucins trap bacteria, which are then swallowed and destroyed by stomach acids. This is good, because there are a lot of bacteria in your mouth. Every time you eat something, every time you stick your finger in your mouth, you’re delivering more.
    Picture one of those little silver * balls that cake decorators use. Strip away the metallic coating and soften the texture. You are now picturing the amassed bacteria in one milliliter of unstimulated saliva. Silletti put our samples in the centrifuge and spun cellular from noncellular. Some of what we are looking at is shed mouth cells, but most is bacteria—about a hundred million of them. More than forty species.
    Yet never in my life has a cut or sore in my bacteria-crazy mouth become infected. As much as saliva is a bacterial cesspool, it is also an antimicrobial miracle—the former necessitating the latter. As a germ killer, saliva puts mouthwash to shame. † Saliva has anti-clumping properties, which discourage bacteria from forming colonies on the teeth and gums. There are salivary proteins that retain their antimicrobial abilities even when they themselves are broken down. “And they may be even more effective than the whole protein of origin,” Silletti is saying. “It’s incredible!”
    Saliva’s antimicrobial talents explain some of the folk medicine remedies that have been making the rounds since the 1600s. One 1763 treatise advocates applying “the fasting saliva of a man or woman turn’d of seventy or eighty years of age” to syphilitic chancres of the glans penis. As with the ancient Chinese Materia Medica prescription of saliva “applied below arms to counteract fetid