**Steroids as Environmental Compounds Recalcitrant to Degradation: Genetic Mechanisms of Bacterial Biodegradation Pathways**

#### **Elías R. Olivera \* and José M. Luengo**

Departamento Biología Molecular ( Área Bioquímica y Biología Molecular), Universidad de León, 24007 León, Spain

**\*** Correspondence: erodo@unileon.es; Tel.: +34-987-29-1229

Received: 2 June 2019; Accepted: 3 July 2019; Published: 6 July 2019

**Abstract:** Steroids are perhydro-1,2-cyclopentanophenanthrene derivatives that are almost exclusively synthesised by eukaryotic organisms. Since the start of the Anthropocene, the presence of these molecules, as well as related synthetic compounds (ethinylestradiol, dexamethasone, and others), has increased in di fferent habitats due to farm and municipal e ffluents and discharge from the pharmaceutical industry. In addition, the highly hydrophobic nature of these molecules, as well as the absence of functional groups, makes them highly resistant to biodegradation. However, some environmental bacteria are able to modify or mineralise these compounds. Although steroid-metabolising bacteria have been isolated since the beginning of the 20th century, the genetics and catabolic pathways used have only been characterised in model organisms in the last few decades. Here, the metabolic alternatives used by di fferent bacteria to metabolise steroids (e.g., cholesterol, bile acids, testosterone, and other steroid hormones), as well as the organisation and conservation of the genes involved, are reviewed.

**Keywords:** sterols; bile acids; steroid hormones; biodegradation; 9,10-*seco* pathway; 4,5-*seco* pathway; 2,3-*seco* pathway
