Probiotics and fermented foods are considered by many to be nutritional fixtures – Whole Foods aisles are stacked with artisanal versions of fermented staples like sauerkraut, kimchi and kefir, and no respectable supplement regimen is complete without a good probiotic. As digestive issues have become a more prevalent concern among Americans, an increased awareness about the importance of stomach health has followed suit.

"Because the skin is the most permeable organ, it reflects the state of our gut," says Jelena Nikolajevic of Mainstay Acupuncture, who frequently puts her patients on oral probiotics and a diet rich in fermented foods to reduce inflammation that can lead to skin conditions like acne, rosacea, dermatitis and psoriasis. Now beauty brands say there is also a beneficial effect when probiotics and fermented ingredients are applied topically. Just as stomachs need to be repopulated with good bacteria to run smoothly, the argument goes, so too does our skin. "What these products are attempting to do is rebalance the bacterial ecosystem of the skin," says facialist Kristina Holy.

While the probiotics in food and supplements use live bacteria, niche brands like Tula and Aurelia utilize bioactive molecules – a technology that allows their products to be effective and have a longer shelf life. As for fermented ingredients, they've been prized for years in Korea and continue to appear stateside. "Fermentation means allowing for the growth of good bacteria," explains dermatologist Craig Kraffert. "They work on a molecular level versus a bacterial level like probiotics." Often, though, half the battle skin-care companies face isn't finding these miracle ingredients but convincing a squeamish public to embrace them. "First there was an ick factor," says dermatologist Dendy Engelman, "but now people are more comfortable with the idea of probiotics and fermented ingredients in their skin care because they are more familiar with the terminology and the benefits."

Fermentation means allowing for the growth of good bacteria.

— Dr. Craig Kraffert