What Causes Bacterial Vaginosis
Bacterial vaginosis is caused by an ecological disruption of the normal vaginal microbiome, characterized by replacement of protective hydrogen peroxide-producing Lactobacillus species with an overgrowth of anaerobic bacteria (particularly Gardnerella vaginalis), Mycoplasma hominis, and other facultative anaerobes—not by acquisition of a single sexually transmitted pathogen. 1, 2
The Microbiological Shift
The fundamental pathophysiology involves:
Loss of protective lactobacilli that normally maintain vaginal health through lactic acid production, hydrogen peroxide generation, and bacteriocin secretion 3
Overgrowth of polymicrobial anaerobic flora including Gardnerella vaginalis, Atopobium vaginae, Mycoplasma hominis, and various anaerobic bacteria that form a biofilm on the vaginal mucosa 4, 5
pH elevation above 4.5 as the acidic environment normally maintained by lactobacilli is disrupted 1
The Sexual Activity Connection (But Not Classic STI Transmission)
The relationship between BV and sexual activity is complex and often misunderstood:
Women who have never been sexually active rarely develop BV, establishing a clear behavioral link 6
Multiple sexual partners increase BV risk, though the exact mechanism remains unclear 6
However, BV is explicitly NOT a classic sexually transmitted infection according to the CDC, as evidenced by the fact that treating male partners does not prevent recurrence or alter clinical course 2, 6
The CDC states "it is unclear whether BV results from acquisition of a sexually transmitted pathogen" despite the strong epidemiological associations with sexual behavior 2
Why the Cause Remains Incompletely Understood
A critical pitfall in understanding BV is assuming it follows traditional infectious disease models:
The exact etiology remains controversial despite decades of research, which has impeded advances in prevention 5
BV may represent an ecological disturbance triggered by multiple factors rather than infection with a specific pathogen 7
Recurrence rates of 50-80% within one year suggest we don't fully understand whether this represents relapse or reinfection 8, 5
Clinical Implications for Understanding Causation
When counseling patients about what causes their BV:
Emphasize the polymicrobial nature rather than suggesting a single "germ" causes it 4, 1
Explain that sexual activity influences development without labeling it as an STI requiring partner notification 2
Acknowledge that the normal protective bacteria have been displaced, which is why simple antibiotic treatment often fails long-term 8
Avoid treating male partners as this represents unnecessary antibiotic exposure without proven benefit 2, 6