Preparation of the Peptide Based Media (PBM)
Growth in peptide-based media (PBM) is magnesium-limited (Christensenet al. , 2017). The previously described PBM recipe requires adding 0.4% of glucose (4 g/L) as a carbon source. We prepared the PBM via dissolving 10g/L Peptone, 1.5% casein hydrolysate (casamino acids) and 40 mM MOPS (3-(N-morpholino) propane sulfonic acid) buffer pH 7.4. The 50x MOPS buffer stock solution contained 2M MOPS and 0.2M Tricine pH 7.4 set with concentrated KOH. We achieved extremely high cell densities, OD600 = 10-13, when 2x PBM were used in the presence of 1-10 mM MgCl2 or MgSO4.