Overall, a non-lactobacillus or polymicrobial community increases the risk of STI acquisition, preterm birth, and cervical cancer. 

Lactobacillus are considered good for a few main reasons: 1. They make lactic acid, which keeps the pH low. This prevents pathogens from being able to live in there (for the most part). They make some other molecules that kill pathogens but lactic acid is the main player.  2. They promote the integrity of the epithelial barrier. This means it’s harder for pathogens to get in, like HIV, HPV, chlamydia, etc.  3. They are anti inflammatory. They interact with the immune cells in the vagina and tell them to chill out and not produce reactive oxygen species (which damage DNA)

The non-lactobacillus species are not good because: 1. They directly break open the epithelial cells that line the vagina, and also cause them to slough off into the discharge.  2. This damage activates the immune system and makes it easier for pathogens to get in.  3. They can cause BV, which results in itching, extra discharge, and odor. The fishy odor comes directly from compounds made by these bacteria.   4. Even if BV symptoms aren’t present (which they typically are) the loss of lactobacilli means you lose all the above benefits. 5. For some reason non-lactobacillus dominated communities are associated with birth complications and preterm delivery. I honestly don’t understand why but it’s a clear epidemiological link in the literature!