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Hold your tongue? A kiss transfers 80 million bacteria

The Los Angeles Times
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REUTERS
U.S. Navy sailor Glenn Edward McDuffie kisses a nurse in Times Square in an impromptu moment at the close of World War Two, after the surrender of Japan was announced in New York August 14, 1945.

Scientists have demonstrated boys (well, men) really do have cooties (a term for bacteria) that can spread to girls (OK, women) by kissing them.

In fact, a 10-second “intimate kiss” can transfer a whopping 80,000,000 bacteria from one mouth to another, according to a new report in the journal Microbiome.

A team of Dutch researchers recruited 21 couples who happened to be visiting the Artis Royal Zoo in Amsterdam on a summer day. All 42 volunteers (whose ages ranged from 17 to 45) allowed the researchers to wipe their tongues with a cotton swab several times. They also agreed to spit into sterile tubes and answer questions about their kissing habits.

The researchers found that the particular community of bacteria living on a volunteer’s tongue was more similar to the bacteria on his or her kissing partner’s tongue than to a stranger’s tongue. They quantified this using a measure called the Morisita-Horn index, where 0 indicates complete overlap and 1 means no overlap at all. The MH index value for kissing couples was 0.37, significantly lower than the 0.55 for strangers.

Then the volunteers engaged in some public displays of affection (a 10-second kiss “involving full tongue contact and saliva exchange”) and were swabbed again. According to the bacterial analysis, a fresh kiss barely budged the similarity index value. That suggests that the overlap in tongue bacteria is probably “a long-term effect of couples living together” — sharing meals, toothpaste and other items from daily life.

In a further test, some of the volunteers were given a probiotic yogurt drink spiked with a marker bacteria. Researchers swabbed their tongues and asked them to kiss their partners. Then the partners had their tongues swabbed. Comparing the contents of the yogurt-drinkers’ swabs and their partners’ swabs, the researchers calculated that a single kiss can deposit 80,000,000 bacteria from one tongue to another.

A single kiss had no immediate effect on the composition of bacteria in the saliva samples, the researchers found. But swapping spit certainly entails swapping bacteria as well. The more often a couple kissed, the more similar the bacteria in their saliva samples. The scientists calculated that couples had to kiss at least nine times a day to achieve an MH value below 0.5.

And here’s a bonus finding: The men said they kissed their partners an average of 10 times a day, while the women recalled kissing only five times a day. The researchers concluded that the men were most likely exaggerating the frequency of their romantic encounters.

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