Galactose utilization profiles in Streptococcus thermophilus strains
Carlo Perla1, Antonio Francioso2, Remo Ferri1, Alessio Pio Rossetti2, Grazia Di Campli1, Giorgia Perpetuini2, Nunzio Lobefaro1, Antonio Spinosi1, Rosanna Tofalo2
1 Dalton Biotecnologie s.r.l., Italy, 2 University of Teramo, Italy
Streptococcus thermophilus is widely in the dairy industry. While glucose is rapidly consumed, many S. thermophilus strains lack efficient galactose (GAL) uptake or catabolism, leading to its accumulation in the final product. High residual galactose is undesirable due to potential implications for product quality; in fact, its occurrence is correlated with browning of Mozzarella cheese upon heating. In this study, the galactose phenotype of 100 presumptive S. thermophilus strains from dairy origin has been studied. The strains were identified through a species-specific PCR. Moreover, strains were typed by RAPD-PCR using GTG5 primer. The content of lactose, glucose, and galactose was determined by UPLC. All strains were identified as S. thermophilus. The analysis of galactose residues among S. thermophilus strains revealed substantial variability in galactose utilization efficiency, suggesting the presence of distinct fermentation phenotypes. Residual galactose concentrations ranged from 0.65 to 4.16 g/L, with the majority of strains (92 out of 100) falling between 1 and 2 g/L. In 4 samples, the content of residual galactose was <1 g/L. Based on these values, strains were categorized into three phenotypic groups: low-GAL (≤1.3 g/L), intermediate-GAL (1.31–2 g/L), and high-GAL (>2 g/L) users. Approximately one-third of the strains exhibited low residual galactose levels, indicating efficient uptake and metabolism of galactose. Conversely, several strains—most notably those with GAL values exceeding 3 g/L—demonstrated limited galactose utilization, potentially reflecting a gal⁻ phenotype. The glucose was depleted by all strains, suggesting a preferential glucose consumption consistent with carbon catabolite repression. These results showed a galactose fermentation diversity among S. thermophilus strains, which could be exploited to develop tailored starter cultures.
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