High-five! Uh, not until you wash your hands.
To get a sense of the flora residing on our hands, researchers scrutinized the palms of 51 undergraduate students for bacteria, just after the students completed their academic exams. A sampling of the DNA microbes revealed some 332,000 gene sequences, or about 100 times more than was found in previous studies of skin bacteria.
On average, each hand was home to about 150 different species of bacteria. In total, researchers identified more than 4,700 bacterial species on all hands and only five were common among the volunteers. Women had more germ diversity than men, possibly due to different acidities on the hands, different hand-washing regimens, differential production of sweat, variable hormones, and how often moisturizers or cosmetics were applied.
In general, handwashing didn’t affect the diversity of bacteria—although researchers stressed it’s still a good practice. Either the bacteria came back quickly after handwashing, or handwashing just doesn’t dislodge the bacteria.