Casein utilization by Streptococcus thermophilus results in a diauxic growth in milk

Abstract

In milk, Streptococcus thermophilus displays two distinct exponential growth phases, separated by a nonexponential one, during which proteinase synthesis was initiated. During the second exponential phase, utilization of caseins as the source of amino acids resulted in a decrease in growth rate, presumably caused by a limiting peptide transport activity.

FIG. 1.

Growth of Prt⁻ (a) and Prt⁺ (b) strains of S. thermophilus in milk. Prt⁻ strains were ST7 (•) and ST18 prtS (▴); Prt⁺ strains were ST1 (•) and ST18 (▴).

FIG. 2.

Deviations of the experimental data from the regression lines during growth of S. thermophilus ST18 (mean of nine independent repetitions with confidence limits at P values of 0.95). From top to bottom: first exponential, intermediate, and second exponential growth phases. Dotted lines indicate the standard errors of the spiral plating method.

FIG. 3.

Emission of light during growth of S. thermophilus ST18P_prtS::P_prtS -lux in milk. •, CFU per milliliter; ▴, light units per milliliter (arbitrary units). The dotted line indicates the amount of light produced per CFU.

FIG. 4.

Final population of S. thermophilus ST7 grown in milk conditioned with a first culture of S. thermophilus ST16 (•) or ST18 (▴), as a function of the cell density at the end of the milk conditioning.