Galactose metabolism and capsule formation in a recombinant strain of Streptococcus thermophilus with a galactose-fermenting phenotype

The capsule-producing, galactose-negative Streptococcus thermophilus MR-1C strain was first transformed with a low-copy plasmid containing a functional galK gene from Streptococcus salivarius to generate a recombinant galactose-fermenting Strep. thermophilus strain named MR-AAC. Then, we compared the functional properties of Strep. thermophilus MR-AAC with those of the parent MR-1C strain when used as starter for fermented products and cheese. In lactose-supplemented laboratory medium, MR-AAC metabolized galactose, but only when the amount of lactose was less than 0.1% (wt/vol). After 7 h of fermentation, the medium was almost depleted of galactose. The parent strain, MR-1C, showed the same pattern, except that the concentration of galactose decreased by only 25% during the same period. It was found that, during milk fermentation and Mozzarella cheese production, the galactose-fermenting phenotype was not expressed by MR-AAC and this strain expelled galactose into the medium at a level similar to the parent MR-1C strain. In milk and in lactose-supplemented medium, capsular exopolysaccharide production occurred mainly during the late exponential phase and the stationary growth phase with similar kinetics between MR-1C and MR-AAC.