Wearing antiperspirant or deodorant may influence the type and quantity of bacterial life found in the armpit's microbiome, new research suggests.

"Using antiperspirant and deodorant completely rearranges the microbial ecosystem of your skin -- what's living on us and in what amounts," says study author Julie Horvath, PhD, the head of the genomics and microbiology research laboratory at the North Carolina Museum of Natural Sciences and an associate research professor at North Carolina Central University Raleigh, NC. "And we have no idea what effect, if any, that has on our skin and on our health. Is it beneficial? Is it detrimental? We really don't know at this point. Those are questions that we're potentially interested in exploring."

The findings appear in the journal PeerJ.

To learn about the microbial impact of antiperspirant and deodorant, the researchers recruited 17 study participants: three men and four women who used antiperspirant productst; three men and two women who used deodorant; and three men and two women who used neither product. They then launched an eight-day experiment, in which all of the participants had swabs taken of their armpits between 11 a.m. and 1 p.m.

On day one, participants followed their normal hygiene routine in regard to deodorant or antiperspirant use. On days two through six, participants did not use any deodorant or antiperspirant. On days seven and eight, all participants used antiperspirant.

The researchers then cultured all the samples to determine the abundance of microbial organisms growing on each participant and how that differed day to day.

On the first day, people using antiperspirant had fewer microbes in their samples than people who didn't use product at all -- but there was a lot of variability, making it hard to draw firm conclusions, the study authors report. In addition, people who used deodorant actually often had more microbes -- on average -- than those who didn't use product.

By the third day, participants who had used antiperspirant were beginning to see more microbial growth. And by day six, the amount of bacteria for all study participants was fairly comparable, the study showed.

Once all participants began using antiperspirant on days seven and eight, there were very few microbes on any of the participants, verifying that these products dramatically reduce microbial growth.

The researchers also did genetic sequencing on all of the samples from days three and six, to determine how antiperspirant and deodorant might affect the microbial biodiversity -- the composition and variety of types of bacteria -- over time.

They found that, among study participants who hadn't worn deodorant or antiperspirant, 62 percent of the microbes they found were Corynebacteria, followed by various Staphylococcaceae bacteria (21 percent), with a random assortment of other bacteria accounting for less than 10 percent. Corynebacteria are partially responsible for producing the bad smells we associate with body odor, but they are also thought to help us defend against pathogens. Staphylococcaceae are a diverse group of bacteria that are among the most common microbes found on human skin and, while some can pose a risk to human health, most are considered beneficial.

The participants who had been regular antiperspirant users coming into the study had wildly different results. Sixty percent of their microbes were Staphylococcaceae, only 14 percent were Corynebacteria, and more than 20 percent were filed under "other." 

PHOTO CAPTION: Cultured bacteria from one of the samples in the study. Credit: Dawn Stancil, North Carolina Central University