This study investigated how Streptococcus sanguis P4A7, an oral streptococcus, utilizes amino acids and casein proteins when grown under glucose-limited conditions in continuous culture. The researchers compared growth on chemically defined medium containing free amino acids versus medium containing casein or its α_s1-casein fraction. Arginine was the most efficiently utilized amino acid, with over 99% consumption from free amino acid medium. When casein served as the nitrogen source, growth was nearly as good as with free amino acids, provided that a reducing agent (cysteine or thioglycollate) was present. However, only 50% of arginine residues from α_s1-casein were utilized compared to 99% of free arginine, suggesting limited accessibility of some casein-bound arginine. Analysis of culture filtrates showed that the bacterium specifically released leucine, phenylalanine, lysine, histidine, arginine, and valine from casein through its endo- and exopeptidase activities. Growth yields were slightly lower on casein medium than on free amino acids, but the organism maintained adequate growth even at high dilution rates, indicating efficient casein degradation.

Key findings

• Arginine was nearly completely utilized (>99%) by S. sanguis P4A7 from free amino acid medium, while only 50% of casein-bound arginine residues were accessible and consumed
• Both casein and α_s1-casein fractions supported growth nearly as well as free amino acids when a reducing agent (cysteine or thioglycollate) was present
• The bacterium specifically released six amino acids (leucine, phenylalanine, lysine, histidine, arginine, and valine) from α_s1-casein through protease activity
• Cysteine functioned primarily as a reducing agent to detoxify hydrogen peroxide rather than as a nutritional source for growth