Optimal growth of Streptococcus cremoris HP in milk is related to β- and ϰ-casein degradation

Abstract

Initiation of growth and the growth rate of Streptococcus cremoris HP in a complete synthetic medium supplemented with an enzymatic digest of casein appeared to be inhibited by (di)hydrogen phosphate. The inhibitory effect of the inorganic phosphate fraction was counteracted by β-glycerophosphate. Growth experiments involving different caseins or combinations of caseins as the only source of nitrogen (and essential amino acids) were performed in an adapted medium in which optimal growth was expected to depend only on the type of nitrogen source. Maximal growth occurred on a combination of β-casein and a relatively low concentration of χ. It approximated the growth on milk added to the medium. These results and those showing the capacity of the organism to grow on milk-derived fractions suggest that in milk it is mainly the soluble (β-) casein fraction together with the easily accessible hydrophilic part of micellar χ, which maintain optimal growth.