Evaluation of Inorganic Phosphate on Growth and Lactose Metabolism of Lactic Streptococci in Batch and Continuous Culture1

Abstract

Addition of 1.9% inorganic phosphates (K2HPO4, 1.33% + KH2PO4, 0.57% wt/vol) in place of 1.9% disodium (β-glycerophosphate (GP) in M17 medium (M17P) resulted in increased buffering capacity. Even at equimolar concentration (88 mM), Na2HPO4 (M17P1) or K2HPO4 (M17P2) showed higher buffering than GP(M17). Cultures consistently showed lower cell density in M17P2 than in other buffered media after 7 h of growth at 32°C, suggesting that buffering media with Na2HPO4 is better than buffering with K2HPO4 for cultivation of lactic streptococci. The effect of buffering media with Na2HPO4 on culture growth and appearance of lactose-negative (Lac−) variants was also tested under continuous culture growth conditions in a chemostat. Growth of Streptococcus lactis C2 continuously at pH 6.8 for 168 h in M17P1, and M17 broths at 32°C failed to yield any lac− strains. Results showed that successful propagation of lactic streptococci in continuous culture growth can be achieved without enriching for cells with undesirable metabolic characteristics.