The bacteria and fungi living on your skin play a surprising role in keeping us healthy. Is it possible to give them a boost?

Scratch the surface of the skin, and you'll find a bustling community of bacteria living there. This is a good thing, as research increasingly shows that having the "right kind" of microbes can help keep our skin young, supple, and smooth for longer. Simply by being there, "good bacteria" protect us from infection from pathogenic, harmful microbes. They also help to repair wounds and may even counteract some of the harmful effects of UV rays.

All-the-more reason to look after our skin microbiome. But how do we do this? One method that has become increasingly popular is the use of topical skin probiotics – which technically means applying live microorganisms to the skin to improve its health.

As far back as 1912, scientists were rubbing bacteria into people's faces in an attempt to improve conditions such as acne and seborrhea – a common form of dermatitis that causes a red, itchy rash and white or yellow scales.

Today there are dozens of skincare companies selling what they describe as probiotic products, from serums to cleansers and moisturisers. In each case, the balms offer to rebalance the delicate skin microbiome, leaving it "renewed" and "refreshed".

But while skincare products often claim to be "probiotic", very few, if any, actually contain live bacteria.

What's more, as probiotic skin treatments are classed as "cosmetics" and not medicines, their manufacturers don't have to share the results of any trials that have been conducted on their products, so it's difficult to know how effective they are.

"The rules around skin care products are very different to the rules around drugs, so claims can be made with less rigorous testing than would be made for a pharmaceutical agent," says Richard Gallo, a dermatologist at UC San Diego School of Medicine.

Most "probiotic" skincare products contain either prebiotics – nutrients that feed and encourage the growth of good bacteria on the skin, or postbiotics – proteins or other products made by beneficial bacteria.

"What we are seeing more and more are approaches that try to influence the microbiome," says Bernhard Paetzold, co-founder and chief scientific officer of S-Biomedic, a company that aims to treat conditions by restoring the skin's microbiome through bacterial "transplants".

According to Paetzold, the main reason for this is that live bacteria are extremely difficult to keep alive throughout the manufacturing, storing and distribution process. Once on the skin there's no guarantee that they'll even take hold, as they must fight to compete with the millions of other microbes already living there.

The idea of promoting a healthy skin microbiome is rooted in the theory that bacteria and fungi living on the outside of our body help to protect us from those that might do us harm.

Yet the science behind how certain "bad" bacteria can contribute to skin diseases isn't yet fully understood. What we do know is that people with eczema, rosacea, acne and psoriasis all have different types or levels of certain bacteria living on their skin.

"What exists at the moment is a lot of papers showing that nearly every skin disease imaginable is associated with a shift in the skin microbiome," says Paetzold. "But this observation just shows an association. It doesn't say that the shift in the microbiome causes that disease. It could also be that the disease changes the skin environment, and that's why the bacterial population changes."

To prove that a particular strain of bacteria is causing a disease, you would need to........

© BBC