Microorganisms in the healthy upper reproductive tract: from denial to beneficial assignments for reproductive biology

Contrary to the traditional assumption of a sterile uterus, the number of studies characterizing microbial entities in the healthy upper reproductive tract (endometrial cavity, including follicular fluid and placenta) have been on the increase. Substantial data has been accumulated correlating microbial composition with fertility outcome. In this context, the presence of certain taxa was associated to an improved reproductive success. A summarization for the evidence of these molecular mechanisms through which bacteria may affect developmental processes during pregnancy is presented and discussed with special focus placed upon the immunological aspects.