Marilen Benner

CHAPTER 6 142 Assessing commensal uterine colonization Not only theoretically does the endometrium offer an environment suited for co-existence with bacteria but also, in accordance with earlier hypotheses, such as Viniker’s theory of the “bacteria endometrialis” inhabiting the uterine cavity (14), various approaches have now provided evidence for uterine colonization, as outlined below. Initial assessment of endometrial colonization was based on observations of cells in culture. It has long been thought that the few observed growing species from endometrial cultures after hysterectomy resulted from the sterile character of the uterus, and the bacteria that were found were ascribed to a pathogenic condition (32, 33). However, women with no sign of infection also showed some bacterial growth in samples maintained in vitro . Species such as Lactobacillus (34, 35), Gardnerella vaginalis , Enterobacter , and Streptococcus agalactiae (33), Escherichia coli and Enterococcus faecalis were typically found in patients with uterine pathologies, such as endometritis (36). Other studies used catheter tips that were used in embryo transfer for microbial assessment based on culturing. Bacterial growth was associated with outcomes of IVF treatment, with varying results. If bacterial growth was observed upon culturing the catheter tips used for embryo transfer during IVF, live birth rates were found to be either decreased (37-40), increased or not affected at all, depending on the study or bacterial species cultured (41, 42). This underlines the need to carefully assess the outcomes and limitations of such an approach, as discussed below. However, even if smears led to growing cultures, pregnancy was possible (e.g. clinical pregnancy rate 28% versus 17% in the positive culture group, n= 279) (38). Also, using isolates other than from catheter tips, endometrial colonization could be shown for asymptomatic as well as symptomatic women (43- 45). Taken together, microbial presence could not be ascribed to a certain pregnancy outcome based on culture-dependent methods. As we now know, culture-dependent characterization of microbial communities is associated with limitations (Supplementary Table I). In culture-based approaches, rapidly growing, aerobic species dominate, leaving rare species that demand specific culture conditions undetected (e.g. (46, 47). Molecular approaches allow detection of species that will not be revealed by culture-dependent techniques. Mitchell and colleagues examined uterine swabs and EF from hysterectomies by quantitative PCR (qPCR) for 12 bacterial species, including Atopobium vaginae, Prevotella spp. , Lactobacillus crispatus, Lactobacillus iners, G. vaginalis, and bacterial vaginosis - associated bacterium 1 (BVAB1) (48). All of the selected species could be detected in vaginal samples and to a varying extent in the endometrium. Clear differences could be found between vaginal and endometrial samples. While A. vaginae was more commonly detected in vagina, L. iners and BVAB1 were more likely to be detected in endometrial samples. Of note, 95% of hysterectomy samples showed the presence of bacterial DNA. Also based on selected targets, Swidsinski and colleagues used fluorescent in-situ hybridization probes for G. vaginalis, A. vaginae, Lactobacillus, Bacteroides, Prevotella, Enterobacteriaceae, and Eubacteria (49). Again, the endometrial microbiotic environment was shown to be different than that of the