The biofilms were oxygenated due to both the high CVt= t ⁄ × 100% (Eqn. 2) t gas permeability of the PDMS and the natural oxygenation of the medium. For experiments assessing the effect of additional Mg 2+ , 20 mM of MgCl2 was added to the nutrient broth. LB 1/10 medium contains less than 0.03 mM of Mg 2+ . All experiments were conducted at room temperature.