Enterococcus faecalis thrives in dual-species biofilm models under iron-rich conditions.

Escherichia coli (E. coli) and Enterococcus faecalis (E. faecalis) are pathogenic strains that often coexist in intestinal flora of humans and are prone to cause biofilm-associated infections, such as gastrointestinal tract and urinary tract infections. Earlier studies have demonstrated that E. faecalis biofilm can metabolize ferrous ions in iron-rich environments and promote biofilm growth under in-vivo conditions. However, the influence of iron transporters on dual-species biofilm growth and the nature of molecular-level interactions between iron transporter proteins and Fe remains unknown. Therefore, in this work, co-culture studies were performed and the study indicates that Fe at concentrations of 50-150 µM promotes the colonization of E. coli, and Fe concentrations of 50-200 µM promote the growth of E. faecalis and dual-species colonies. Atomic absorption spectroscopy results reveal that Fe ion augmentation in bacterial cells was increased to 4 folds in the single-species model and 11 folds in the dual-species model under iron-supplemented conditions. Furthermore, Fe augmentation increased the antibiotic resistance of E. faecalis in both single- and dual-species bacterial cultures. In addition, in-silico docking were performed to determine a three-dimensional (3D) structure of ferrous iron-transporter proteins FeoB of E. faecalis and its affinity to extracellular Fe. Our model suggests that the FeoB facilitates the Fe uptake in E. faecalis cells in the absence of iron chelator, 2,2-bipyridyl.