The utilization of casein and amino acids by Streptococcus sanguis P4A7 in continuous culture

Streptococcus sanguis P4A7 was grown in glucose limited conditions in continuous culture at pH 7.0 in a chemically defined medium containing either free amino acids or casein as the organic nitrogen source. Apart from aspartate and threonine, which were poorly utilized at the higher dilution rates, all amino acids in the free-amino-acid medium were utilized to various extents. At the higher dilution rates, aspartate actually increased in concentration, probably due to deamidation of asparagine. The amino acid most utilized at all dilution rates was arginine, with up to 99% of the amino acid being consumed. Both casein and its alpha s1-casein fraction supported growth at a level only slightly lower than that obtained with the free-amino-acid medium, provided that either cysteine or thioglycollate was present. With the exception of tyrosine, nearly all of the amino acyl residues of alpha s1-casein were utilized to some degree. In general, the higher the concentration of each amino acid in the medium (whether free or as part of alpha s1-casein) the higher the extent of utilization by S. sanguis P4A7. Only 50% of the arginyl residues (0.16 mM) of alpha s1-casein were utilized compared with 99% of free arginine (1.5 mM) under similar conditions, suggesting that only 50% of the alpha s1-casein arginine was accessible to the organism.(ABSTRACT TRUNCATED AT 250 WORDS)