Background: Cervicitis is associated with serious gynaecologic and obstetric sequelae, including pelvic inflammatory disease and infertility. Cervicitis is usually caused by a sexually transmitted infection, however, more than 50% of cases are idiopathic i.e. no known causative pathogen is identified. This poses considerable challenges for the clinical management of women with cervicitis, and contributes to overuse of antibiotics. To provide new insights into the infectious causes of cervicitis, inform clinical care, reduce morbidity and improve antimicrobial prescribing, we compared the vaginal microbiota of women with and without idiopathic cervicitis. Methods: This was a nested case-control study of women with idiopathic cervicitis. Participants were derived from a cross-sectional study conducted at the Melbourne Sexual Health Centre between April 2017-April 2019 that investigated clinical indications for testing for Mycoplasma genitalium. Idiopathic cervicitis cases (n=64) comprised symptomatic women who had cervicitis on clinical examination and tested negative for known causes of cervicitis (Chlamydia trachomatis, M. genitalium, Trichomonas vaginalis, Neisseria gonorrhoeae). Controls were asymptomatic women (n=155) who tested negative for each of these organisms. The vaginal microbiota was characterised using 16S-rRNA gene sequencing. Community state types (CSTs) were defined using VALENCIA, and compared between women with and without cervicitis using logistic regression. Associations between bacterial taxa abundance and cervicitis were investigated using ANCOM-BC. Results: The vaginal microbiota was categorised into three broad CSTs: optimal Lactobacillus (non L.iners)- dominant microbiota (mostly reflecting L. crispatus dominant communities; present in 84/161 [52%] women), L. iners-dominant microbiota (present in 77/161 [48%] women), and non-optimal microbiota deficient in lactobacilli (present in 58/161 [36%] women). Women with a Lactobacillus-deficient nonoptimal microbiota were more likely to have cervicitis compared to women with an optimal Lactobacillus-dominant microbiota (Odds ratio [OR] = 2.98, 95%CI: 1.44, 6.16, p=0.003). The mean abundance of Fannyhessea vaginae, Coriobacteriales bacterium DNF00809 and Prevotella bivia were increased in women with cervicitis compared to women without cervicitis (p< 0.05). Conclusion: Our findings suggest that a vaginal microbiota deficient in Lactobacillus spp. may contribute to the development of cervicitis. Of note, specific anaerobic bacteria including F. vaginae, DNF00809 and P. bivia, may represent infectious causes of cervicitis. [ABSTRACT FROM AUTHOR]