Inhibitor of bacterial growth released by human cells in culture

Used medium from cultured human cells contains a factor that inhibits growth of the "less virulent" strains of pathogenic bacteria, but only retards growth of the "more virulent" strains. The factor is heat-stable, dialyzable, and unaffected by change in pH; it chromatographs as material of molecular weight between 700 and 1500. There is evidence that this factor is an alpha-ketoaldehyde attached to a carrier.