48Polycystic ovary syndrome (PCOS) is a common endocrine disorder affecting approximately 49 10% of reproductive-aged women worldwide. Diagnosis requires two of the following: 50 hyperandrogenism, oligo/anovulation and polycystic ovaries. In addition to reproductive 51 dysfunction, many women with PCOS display metabolic abnormalities associated with 52hyperandrogenism. Recent studies have reported that the gut microbiome is altered in women 53with PCOS and rodent models of the disorder. However, it is unknown whether the gut 54 microbiome plays a causal role in the development and pathology of PCOS. Given its potential 55role, we hypothesized that exposure to a healthy gut microbiome would protect against 56 development of PCOS. A co-housing study was performed using a letrozole-induced PCOS 57 mouse model that recapitulates many reproductive and metabolic characteristics of PCOS. Since 58 mice are coprophagic, co-housing results in repeated, non-invasive inoculation of gut microbes 59 in co-housed mice via the fecal-oral route. In contrast to letrozole-treated mice housed together, 60letrozole-treated mice co-housed with placebo mice showed significant improvement in both 61reproductive and metabolic PCOS phenotypes. Using 16S rRNA gene sequencing, we observed 62 that the gut microbial composition of letrozole-treated mice co-housed with placebo mice 63 differed from letrozole mice housed together. In addition, our analyses identified several 64 bacterial taxa including Coprobacillus, Dorea and Adlercreutzia associated with the improved 65PCOS phenotype in letrozole-treated mice co-housed with placebo mice. These results indicate 66 that disruption of the gut microbiome may play a causal role in PCOS and that manipulation of 67 the gut microbiome may be a potential treatment option for PCOS. 68 69Significance: 70 71PCOS is a common cause of female infertility and ~80% of women with PCOS have metabolic 72 dysregulation that predisposes them to type 2 diabetes and cardiovascular disease. Since 73 treatment options for the metabolic symptoms of PCOS are limited, there is a need to develop 74 novel therapeutic options. The gut microbiome has emerged as an important player in human 75health and has been shown to play a causal role in obesity. In this study, we found that exposure 76to a healthy gut microbiome through co-housing protected against the development of 77reproductive and metabolic dysregulation in a PCOS mouse model. These results suggest that 78 manipulation of the gut microbiome may be a potential treatment option for women with PCOS. 79 80Introduction: 81 82Polycystic ovary syndrome (PCOS) is a common endocrine disorder affecting approximately 83 10% of women worldwide (1). Diagnosis of PCOS, using the Rotterdam Consensus criteria 84 (2003), requires two of the following: hyperandrogenism, oligo-or amenorrhea and polycystic 85ovaries. PCOS is the leading cause of anovulatory infertility in women, and women with PCOS 86also have an increased likelihood of miscarriage and pregnancy complications. Though ...