Looking to find the most effective probiotics? You may need to look no further than your own body.

Scientists could rid eczema patients’ arms of disease-spurring Staphylococcus aureus simply by picking out rare but helpful bacteria also on their skin , growing it up to large quantities, and mixing it with off-the-shelf lotion that the patients slathered on. The finding, reported this week in Science Translational Medicine, is another example of harnessing the protective and disease-fighting potential of the human microbiome. Researchers are optimistic that in future clinical trials, the personal bacteria boosts will prove useful in longterm treatment for eczema, without the risks that come with antibiotics.

“This approach is inherently superior to current pharmaceutically derived antibiotics,” the authors conclude. Unlike bottled antibiotics that may kill microbes indiscriminately—friends or foes—the patient’s skin bacteria selectively killed off harmful S. aureus and left the protective community intact.

The researchers, led by dermatologist Richard Gallo of the University of California, San Diego, developed the treatment approach by first noting that people with healthy skin have a bunch of normal skin bacteria that seemed to keep S. aureus in check. But on people with atopic dermatitis (AD)—a type of eczema that results in dry, itchy patches of skin—those helpful skin bacteria are less abundant. This squares with what the dermatologists already knew: people with AD are more likely to carry around S. aureus, which can spur and exacerbate those itchy, dry skin patches.

When the researchers took a closer look at those beneficial bacteria— S. epidermidis and S. hominis—they found that the microbes were oozing out antimicrobial compounds that selectively kill S. aureus. The chemicals were strong enough to fight off even the dastardly Methicillin-resistant S. aureus (MRSA). And when those compounds mixed with antimicrobial compounds made by the human skin, the compounds worked synergistically—that is, together they were more deadly to S. aureus than the sum of their independent killing abilities. Yet, they still left other benign and beneficial microbes unharmed.

Scanning the arms of five AD patients with S. aureus, the researchers could find a few drug-pumping S. epidermidis and S. hominis colonies—but they were rare. Nevertheless, the researchers grew the strains in lab, creating vast quantities. Then they mixed the microbe slurry with a standard moisturizer. The result was five patient-specific lotions brimming with helpful, personally derived microbes.

Next, the five patients slathered one of their arms with plain lotion and the other with their personalized probiotic lotion. The final concentration of microbes from the slathered lotion was around 100,000 colony-forming units per square centimeter of skin. This is about the same density of bacteria that you’d find on healthy skin.

After 24 hours—with no bathing—S. aureus levels dropped significantly on the arms of three patients with microbe-laden lotion. The harmful germ disappeared completely on the treated limbs of the other two patients.

Overall, the authors conclude these findings not only demonstrate the therapeutic potential of our own microbiomes, but also the damage that an out-of-whack community can cause.

Science Translational Medicine, 2017. DOI:10.1126/scitranslmed.aah4680 (About DOIs).