The population of microorganisms that reside in the vagina and cervix of patients with stage 3/4 endometriosis differs from that of healthy controls, suggesting a potential role of microorganism imbalance in this condition, according to a study.

The study, titled “The Endobiota Study: Comparison of Vaginal, Cervical and Gut Microbiota Between Women with Stage 3/4 Endometriosis and Healthy Controls,” was published in the journal Scientific Reports.

Endometriosis is a chronic condition where uterine cells are found outside of the uterus, most commonly in the pelvis. These cells can also spread to other parts such as the intestine.

Researchers believe that endometriosis likely occurs through retrograde menstruation, which causes endometrial cells to leave the uterus and implant in the pelvis. However, all women at some point experience retrograde menstruation, so researchers are looking for other possible causes of endometriosis, which affects about 10% of women of reproductive age.

Microorganisms, such as bacteria, fungi, and viruses, live and play important roles in the body. Together, they form the human microbiota.

Evidence indicates that imbalance in the human microbiota (known as dysbiosis) is associated with disease. For example, disruption of gut microbiota can lead to the development of inflammatory bowel disease as microorganisms help regulate the immune system. This phenomenon has also been associated with other conditions, such as “neuropsychiatric diseases, psoriasis, arthritis, and some cancers, especially colon cancer,” the researchers wrote.

Since inflammation and activation of immune cells have been linked to endometriosis, the researchers hypothesized that dysbiosis might also play a role in the development of the condition, and conducted a study to compare the microbiota between women with and without endometriosis.

The researchers sampled the vaginal, cervical, and gut microbiota from 14 women with stage 3/4 endometriosis and from 14 healthy controls, and they looked for the presence of different microorganisms by sequencing their genomes.

Results indicated that there was an overall similarity between the microbiota composition between the stage 3/4 endometriosis group and controls.

However, there was a difference between the two groups at the genus level, which ranks above species and below family in the hierarchy of biological classification.

There was a total lack of the genus Atopobium in the vaginal and cervical microbiota of the endometriosis group, but the researchers found an increase in the species Gardnerella, Streptococcus, Escherichia, Shigella, and Ureoplasma  —  all potentially pathogenic (able to cause disease) — in the cervical microbiota of stage 3/4 endometriosis patients.

More women in the disease group were also found to have a stool microbiome dominated by Shigella/Escherichia.

“If confirmed in other studies, the complete absence of Atopobium in the vagina and cervix, together with the increased presence of Gardnerella, Escherichia/Shigella and Ureoplasma in cervical microbiome of patients with endometriosis could be a relevant finding of this study,” the authors wrote.

“While overall microbiome composition in the cervix, vagina and gut seems similar between women with stage 3–4 endometriosis and healthy controls, there seems to be some differences at the genus level,” they said.

Further studies are needed to clarify whether the association between dysbiosis and endometriosis is causal, and whether dysbiosis leads to endometriosis or vice versa, the researchers said.