The breakdown and utilization of peptides by strains of Fusobacterium nucleatum

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Abstract
It has been claimed that most strains of Fusobacterium nucleatum require peptides rather than free amino acids for growth. In contrast, we have shown that, under continuous culture conditions, all strains tested grow in a chemically defined medium (CDM). The purposes of this study were to determine whether resting cells of F. nucleatum could attack unsubstituted peptides and whether growing cells could utilize a peptide fraction prepared from a commercial peptone. Resting cells cleaved all 19 peptides, containing 3-6 residues, and the 4 key energy-yielding amino acids--Glu, His, Ser and Lys--were rapidly taken up. A hydrophilic Casitone fraction, rich in Glu, promoted growth and peptides < 1 kDa were rapidly utilized. The cleaved residues metabolized were those previously shown to limit growth in CDM: Glu, Ser, His and Lys. The endopeptidase activities of Porphyromonas gingivalis would provide the necessary peptides for the growth of F. nucleatum, which may partly explain why these two organisms frequently coexist in periodontally diseased sites.