Support of peptide-dependent growth of Bacteroides forsythus by synthetic fragments of haemoglobin or fetuin

Abstract

The putative periodontal pathogen Bacteroides forsythus is a fastidious Gram-negative anaerobe with high proteolytic activity. For growth in a chemically defined medium containing insulin it required serum. Serum could be replaced by human haemoglobin or bovine asialofetuin, or by proteolytic fragments of these two proteins. Four such fragments consisting of from 8 to 18 amino acid residues were isolated and sequenced. Only aspartic acid, threonine, and valine were common to all peptides. An undecapeptide, Hba11, and a dodecapeptide, AsF12, were synthesized and found to be active at micromolar concentrations, but only when presented in combination with insulin. An analysis of amino acid requirements excluded a direct essential role of peptides as sources of amino acids in complete medium, except for valine. Should Bact. forsythus have an essential requirement for this amino acid, it could be satisfied by micromolar concentrations of peptide but not millimolar concentrations of the free amino acid in the absence of peptide. Bact. forsythus could salvage the essential amino acids lysine and isoleucine at 100-fold lower concentrations when presented in peptide-bound form compared to the free amino acids, and at 10-fold lower concentrations of peptide compared to Porphyromonas gingivalis W83, which in contrast to Bact. forsythus grew on free amino acids in the absence of insulin and peptides.