A native bacterial consortium degrades estriol in domestic sewage and activated sludge via the 4,5- <i>seco</i> pathway and requires estriol to retain its biodegradation phenotype

Estriol (E3) is one of the natural steroid estrogens commonly detected in wastewater. Although microbial biodegradation is a key process for removal of steroid estrogens during wastewater treatment, estriol biodegradation, and biotransformation mechanisms, as well as the involved bacterial consortia remain to be revealed. We enriched three E3-degrading bacterial consortia from raw sewage (inflow wastewater) and sludge samples. These consortia were able to utilize E3; however, individual strains isolated from the consortia could not grow on E3 as the sole carbon source. Instead, they transformed E3 to 16α-hydroxyestrone (16α-OH-E1) as a key product. The E3-transforming bacteria were affiliated with the genera <i>Hydrogenophaga</i>, <i>Microbacterium</i>, and <i>Gordonia</i>. The inflow (IF)-consortium utilized estrone (E1), estradiol (E2), in addition to E3 individually and as a mixture in minimal medium, raw sewage, and activated sludge microcosms, whereas the synthetic estrogen ethinylestradiol (EE2) was not degraded. Degradation of E3 was initiated by transformation to E1 via transient formation of 16α-OH-E1 and then proceeded via the 4,5-<i>seco</i> pathway. The community structure of the IF-consortium strongly shifted toward <i>Croceicoccus estronivorus,</i> which dominated the community after 10 days of incubation. The IF-consortium lost the E3 biodegradation phenotype upon growth on dimethylsulfoxide in the absence of estrogens. We conclude that complete degradation of E3 proceeds via the 4,5-<i>seco</i> pathway and requires concerted action of several community members of the IF-consortium at different time intervals and depending on the substrate and product concentrations. However, functional resilience of the consortium is a crucial factor that needs to be carefully addressed.IMPORTANCEEnvironmental pollution with endocrine-disrupting chemicals, including steroid hormones such as estriol, has become a global concern due to their hazardous impact both on aquatic life and human health. Elimination of estrogens from the environment occurs mainly via microbial biodegradation during wastewater treatment. However, efficient application of bioremediation requires thorough understanding of the structure and dynamics of estrogen-degrading microbial communities in wastewater and the underlying biodegradation and biotransformation mechanisms, which is currently lacking for estriol. In this study, we elucidated the estriol biodegradation pathway adopted by an estriol-degrading consortium obtained from raw domestic sewage. Furthermore, we unraveled how time and substrate type are determinants of the composition and the function of this consortium, which eliminated a mixture of natural estrogens in activated sludge and raw sewage. Hence, our findings constitute a step forward toward developing estrogen-degrading microbial consortia for more efficient removal of these pollutants during wastewater treatment.