Researchers at the Beijing Friendship Hospital looked at 164 healthy, married women in China, between 18 and 45 years old.
They found that women who had used condoms had larger colonies of beneficial or 'good' microbes in their vaginas compared with women who used other forms of birth control, such as the pill.
The bacteria in question was lactobacillus, a group that is the dominant good bacteria in the vagina and produces lactic acid and hydrogen peroxide, which helps the vagina maintain an average pH of 4.5, comparable to the acidity of beer or tomato juice.
LiveScience said: "This "acidic buffer system," as the researchers called it, is thought to block harmful bacteria from taking up residence and causing infections.
Lactobacillus is thought to help prevent bacterial vaginosis, which is caused by an imbalance of vaginal bacteria and results in itching, unusual discharge and unpleasant odor.
Sexually transmitted diseases are on the rise among young people which indicates that condoms aren't being used widely in the UK - so make sure you put a condom on it!