There are present in commercial peptones substances which exhibit bacteriostatic properties for certain bacterial species. These substances are bacteriostatic in the oxidized form, but not in the reduced form. Their bacteriostatic action can be overcome, and their concentration titrated, by the addition of reduced thiol compounds to the media in which they are present.
Different brands of peptone differ greatly in the amount of bacteriostatic substances they contain; these differences account, in part at least, for the fact that media prepared from the same meat infusion, but with different kinds of peptone, vary in their ability to support bacterial growth.
The bacteriostatic fraction of a certain peptone solution can be completely removed by precipitation with acid and acetone. A peptone which has thus been purified becomes capable of supporting the growth of very small inocula of Pneumococcus.
The significance of the sensitiveness of certain bacterial species to substances which are bacteriostatic in the oxidized but not in the reduced form is considered with reference to (a) the mechanism of bacteriostasis, (b) the growth of bacterial species in artificial media, (c) the problem of infection.