Preparation of the Peptide Based Medium (PBM)
Growth in a peptide-based medium (PBM) is magnesium-limited (Christensenet al. , 2017). The previously described PBM recipe requires extra glucose (4 g/L) as a carbon source. Instead, we prepared the PBM by dissolving 10g/L Peptone, 1.5% casein hydrolysate (casamino acids) and 40 mM MOPS buffer pH 7.4. The 50x MOPS buffer stock solution contained 2 M MOPS (3-(N-morpholino) propane sulfonic acid) and 0.2 M tricine pH 7.4 set with concentrated KOH. We achieved very high cell densities, OD600 = 12-13, when 2x PBM was supplemented with 1-10 mM MgCl2 or MgSO4.