In the presence of animal fluids or their protein constituents, Type I Pneumococcus survived and multiplied at acid hydrogen ion concentrations which in the plain broth were bactericidal for these organisms. Minimal numbers of these cells readily produced pneumococcus growth in serum broth when adjusted at hydrogen ion concentration as great as pH 5.5 with hydrochloric, or to pH 6.5 with acetic acid. Growth of the pneumococci could be demonstrated in serum broth adjusted to pH 5.0 with hydrochloric, or to pH 5.5 with acetic acid although at these hydrogen ion concentrations, large amounts of inoculum were necessary. Similar results were obtained with broth to which certain animal proteins were added and in serum broth which was previously heated by autoclave at 20 pounds pressure for 20 minutes.
Pneumococcus growth proceeded at a more rapid rate in serum dextrose broth at pH 6.5 than in dextrose broth alone at the optimal hydrogen ion concentration of pH 7.8. At pH 6.0 large numbers of pneumococci failed to produce the same amount of growth in serum dextrose broth as at pH 6.5 or in dextrose broth alone at pH 7.8. It is of interest that in serum dextrose broth cultures, the stationary and decline phases of pneumococcus growth were prolonged and cell death delayed in comparison with cultures in dextrose broth alone.