Vaginal microbiota changes during a natural menstrual cycle in the Women4Health cohort highlights specific features of the Italian population

Large sex hormonal fluctuations are thought to influence vaginal microbiota, but little is known about the impact of small, physiological variations. Here we tracked changes in vaginal microbiota during four key menstrual cycle phases in 61 healthy Italian women from the Women4Health cohort. The microbiota was primarily composed of Lactobacillus , with L. iners being the most abundant. Noteworthy, the high abundance of L. iners contrasts with previous studies in European populations, challenging its proposed pathogenic role, and suggesting distinct microbiota profiles within Europe. Individual microbiotas were generally stable, but beta diversity was higher during the follicular phase. Only 11 women exhibited compositional shifts and those mostly occurred between follicular and ovulatory phases. Finally, among the hormones evaluated, 17-beta estradiol had the largest impact on taxa abundance variations. Our study highlights specific features of the Italian population and points to the resilience of the vaginal microbiota to physiological hormonal changes.