Sexually active women in the study who used condoms had larger colonies of beneficial microbes in their vaginas compared with women who used other forms of birth control, the researchers found.
The scientists focused on lactobacillus, a group of bacteria that dominates the natural flora of the vagina for many women. The microbes, which produce lactic acid and hydrogen peroxide, help the vagina maintain an average pH of 4.5, comparable to the acidity of beer or tomato juice. This "acidic buffer system," as the researchers called it, is thought to block harmful bacteria from taking up residence and causing infections.
Though there may not be a "normal" microbiome for a healthy vagina, the presence of lactobacillus is thought to help prevent bacterial vaginosis, which is an imbalance of vaginal bacteria that causes itching, unusual discharge and unpleasant odor. Beneficial bacteria have even been linked to a decreased risk of human immunodeficiency virus (HIV) infection.
In the new study, the researchers at Beijing Friendship Hospital recruited 164 healthy, married women in China, between 18 and 45 years old, who were not using hormonal birth control, such as the pill, as their regular method of contraception.
Among the participants, 72 were using condoms, 57 were using an intrauterine device (IUD), and 35 were using the so-called rhythm method, in which a couple abstains from sex on the days pregnancy is mostly likely to occur. The researchers found that the population of lactobacillus was significantly higher in the condom group.
Sexual activity can disrupt the balance of the vagina's ecosystem, especially when semen (which has a pH of 7.0 to 8.0) enters the mix, the researchers said.
The results suggest that condoms can help the vagina maintain its natural acidic defenses, the researchers said.
However, the researchers warned that condoms might not be the best way to prevent unwanted pregnancy for everyone.
Condoms have a failure rate of 15 percent with "average use," which takes into account human errors in using them. In contrast, IUDs have a failure rate of 0.6 percent to 0.8 percent in the first year, and can be effective for more than a decade after insertion, the researchers said.
The new study was published online this week in the journal PLOS One.