In 1907 a French scientist ran an unusual experiment. He approached bearded and beardless men on the streets of Paris and asked each of them to kiss his female assistant. Before each kiss, the women’s lips were cleaned with antiseptic. After each kiss her lips were swabbed and the cultures preserved. At the end of the experiment, the swabs were examined and the scientist published his findings. He claimed that bearded men’s kisses contained considerably more bacteria than those from clean-shaven men, and the bacteria was far more varied.
Moustaches, Whiskers & Beards by Lucinda Hawksley, National Portrait Gallery