In another study published late last year, Dr. Gallo and his colleagues injected a beneficial strain of Staphylococcus epidermidis, along with some food that only it could digest, into the ears of mice. The combination treatment, known as a synbiotic, encouraged the growth of S. epidermidis, which in turn reduced both the number of P. acnes and level of inflammation in the mice.

Other scientists have been reporting similar findings. In 2014, a team in South Korea and the United States showed that an extract from Helicobacter pylori — a common resident of the human stomach — also can inhibit P. acnes and decrease skin inflammation in mice.

Scientists in Canada have demonstrated that people who take both probiotics and antibiotics have significantly fewer acne lesions after 12 weeks, compared with people who take only one or the other.

Several private companies are racing to capitalize on a growing consumer appetite for probiotic cosmetics, toiletries and topical treatments. The biotech company AOBiome offers a “live probiotic spray,” for instance, that is meant to replenish populations of beneficial skin bacteria.

Many microbiologists worry, however, that the science is nowhere near advanced enough to justify the proliferation of these products. Scientists still have a lot to learn about what microbial ecosystems look like on healthy skin, how they change during illness, and how to safely interfere.

Topical probiotics can easily rub off and be transferred to other parts of the body or other people, Dr. Grice pointed out. Just because a microbe kills one species of pathogen does not mean it is unwaveringly “good” or peaceful.

And what if the bacteria in a lotion or spray were to infiltrate the body via a cut or scratch?

Dr. Grice agreed, however, that the idea is intriguing. Whereas typical antibiotics and antiseptics indiscriminately kill all kinds of bacteria throughout the body and drive the evolution of highly dangerous microbes impervious to existing drugs, probiotics may be much more selective.