Effect of proteinases of starter bacteria on the growth and proteolytic activity of Lactobacillus plantarum DPC2741

The effects of partially purified proteinases (PrtP) of Streptococcus thermophilus ST8, Lactobacillus helveticus PR4, Lb. delbrueckii subsp. bulgaricus B397, Lactococcus lactis subsp. cremoris ST2 or Brevibacterium linens 9 on the growth and proteolytic activity of Lb. plantarum DPC2741 were studied. In sterile reconstituted skim milk (RSM), almost all the PrtP promoted higher cell numbers compared with the pure bacterial culture. The enzymes of Lb. helveticus PR4 and Lc. lactis subsp. cremoris ST2 were the most effective in increasing the maximum growth rate (μ_max), the time in which μ_max was held constant and in decreasing the lag phase of growth. Compared with the pure bacterial culture, higher concentrations of free amino acids were found when Lb. plantarum DPC2741 was cultivated in RSM with the PrtP of starter bacteria; the profiles of free amino acids and peptides depended on the type of PrtP. Lb. plantarum DPC2741 did not grow when α_s1-, β-, or κ-casein (CN) fractions were substituted for amino acids in the chemically defined medium (CDM). When PrtP were added to CDM, bacterial growth was restored to an extent that depended on the substrate specificity of the enzyme and CN fraction used. Experiments in which combinations of the three essential amino acids (Glu, Val and Ile) were added to CDM containing CN fractions made it possible to identify some amino acid deficiency which was responsible for the limited growth of Lb. plantarum DPC2741 in combination with PrtP of starter bacteria. To simulate cheese ripening conditions, Lb. plantarum DPC2741 alone and in combinations with PrtP of Str. thermophilus ST8, Lb. delbrueckii subsp. bulgaricus B397 or Lb. helveticus PR4 was grown into a model cows' milk curd system for 10 d at 12°C. All the PrtP enhanced the bacterial growth, increased the concentration of free amino acids and promoted profiles of amino acids and peptides qualitatively different.