The Nail-biting Truth About Your Hands’ Bacteria

And the bacteria that stays there even after washing.

If you’ve bought a new smartphone in the past couple of years, you’ll perhaps unlock it using your fingertip.

Touch ID, as fingerprint recognition is known in Apple circles, was introduced in September 2013.

Gaining access to your phone in this way is just one of a multitude of tasks we use hands for, and of course some of these result in us picking up bacteria, making our hands fascinating in terms of their microbiome.

We have Dr Philip B. Price, of the long-gone Cheeloo University in China, to thank for his 1938 discovery that bacteria on the hands can be divided into two categories, which he defined as Resident or Transient.

Pretty much independent of how much hand washing you do, you have Resident bacteria that live under the outermost cells of the skin on the surface of your palm, which is known as the stratum cornea, Latin for “horny layer.”

(No jokes at the back of the class, please.)

The dominant species among the hand’s resident bacteria is Staphylococcus epidermis, and one of your Resident flora’s main protective functions is what scientists term “microbial antagonism.”

Basically this is your good guys fighting off the bad ones.

Your Transient flora, on the other hand (sorry, it had to be done) colonise the superficial layers of your hand and are more amenable to hygiene.

The Center for Disease Control and Prevention reminds us, of course, just how important it is to keep hands clean.

It says good hygiene can prevent about 30% of diarrhea-related illnesses, and around 20% of respiratory infections (colds, for example).

It’s particularly crucial to wash hands after using the toilet, for we’re reminded that a single gram of human feces—about the weight of a paper clip—can contain a trillion bacterial cells.

You never know what’s on an eight-year-old boy’s hands, but some stunning photos did the rounds last year of a large Petri dish bearing a handprint made of cultured bacteria from the hand of lab tech Tasha Sturm’s son after he’d played outside and petted the family dog.

So which of your hands do you think has the biggest bacterial variety?

Right or left?

Well, it depends on which is your dominant one.

In 2008, researchers at Washington University School of Medicine examined the dominant and nondominant hands of 51 healthy young adult volunteers, finding that a typical palm harbored more than 150 unique species of bacteria, but also that hands from any one individual shared only 17% of their species.

Their theory was that your dominant hand is the one that comes into contact with many more surfaces.

Just think—door-handles, pets, cleaning… not to mention, uh, wiping.

The same research showed a gender divide in diversity, and the findings might surprise you.

You see, the scientists discovered a significantly higher bacterial diversity on women’s hands than they did on men’s.

They weren’t entirely certain why, but suggested that one reason could be that men’s skin tends to be more acidic than women’s—acidity being a condition that’s not generally conducive to bacterial growth.

So far we’ve mainly focused on the palms of your hands, but flip them over and you’ll see ten places in which bacteria can really flourish: under your fingernails.

Basically, there can be a ton of microbes lurking in what is technically known as the subungual region—the space under your nails.

Those nails, by the way, consist of a substance called keratin—the same material that rhino horns are made of, actually.

The species of bacteria under the nails tend to be the same as those on the rest of the hands, but there are many, many more cells hanging out there.

That’s mainly because these little crevices provide perfect conditions for growth, including protection and moisture.

In 1989 a group at the VA Medical Center in Sioux Falls, South Dakota looked into bacteria beneath artificial nails, finding that nurses wearing them had considerably more bacteria beneath them than were found under natural nails.

Their recommendation?

Short, clean nails are better, even if the salon industry might disagree.

Of course it’s reasonably unlikely that wearers of artificial nails will be inclined to chomp into them, but fingernail biting can be an unfortunate way of transferring bacteria into the gut.

Nail-biting even has a technical term: onychophagia.

“Onycho” means “pertaining to the nails.”

And in 2012, the American Psychiatric Association decided to re-classify it as a form of OCD.

This in turn led to an article in the journal Behaviour Research and Therapy, arguing that nail-biting in young adults is almost always related to stress and boredom, and is in no way a psychiatric disorder.

Be that as it may, however, knowing your hands and fingernails are often home to 150 or more species of bacteria may be a good reason to steer clear of living hand-to-mouth.

Further reading

5 Little-Known Dangers of Nail Biting

The influence of sex, handedness, and washing on the diversity of hand surface bacteria

Bacterial carriage by artificial versus natural nails

What lives under your fingernails

Epidemiologic Background of Hand Hygiene and Evaluation of the Most Important Agents for Scrubs and Rubs

Hand print

Human Hands and Fingernails

Normal bacterial flora on hands – WHO Guidelines

Show Me the Science – Why Wash Your Hands

Striking photo shows all the bacteria on an 8-year-old’s hand

The Bacteriology of Normal Skin