Galactose Metabolism and Capsule Formation in a Recombinant Strain of Streptococcus thermophilus with a Galactose-Fermenting Phenotype

Sugar and organic acid contents of M17 culture media containing 5% lactose (LM17) during fermentation at 40°C. The LM17 medium was inoculated with Streptococcus thermophilus MR-1C (open symbols) and MR-AAC (filled symbols). Galactose = —○—, —●—); lactose = —□—, —■—; and lactic acid = —▵—, —▴— contents in g/L; absorbance (optical density) at 600nm (OD₆₀₀; ---♢---, ---♦---).

Sugar and organic acid contents of milk during fermentation at 40°C. Reconstituted milk (10% total solids) was inoculated with Streptococcus thermophilus MR-1C (open symbols) or MR-AAC (filled symbols). Concentrations of galactose (—○—, —●—), glucose (—□—, —■—), lactic acid (—▵—, —▴—), and citric acid (—♢—, —♦—) in g/L; lactic acid/galactose molar ratio [---×--- (MR-1C); ---⊗---(MR-AAC)].

Bacterial capsular exopolysaccharide production during the fermentation process as estimated from the relative fluorescence of bacterial cells after probing with fluorescein isothiocyanate-conjugated peanut agglutinin. Fermentation was carried out at 40°C in reconstituted milk (10% solids) inoculated with Streptococcus thermophilus MR-1C (—□—) and MR-AAC (—■—), and in LM17 inoculated with Strep. thermophilus MR-1C (—▵—) and MR-AAC (—▴—). As a control, fluorescein isothiocyanate-conjugated wheat germ agglutinin was used to probe the bacterial cell wall during milk fermentation with MR-1C (—○—) or MR-AAC (—●—) as starters. Fermentation was tracked in milk based on pH changes (---×---) and in LM17 based on absorbance (optical density) at 600nm (OD₆₀₀; ---+---).