38 BIOLOGY OF PNEUMOCOCCUS 



horse muscle, freed from fat, furnish a better base than the com- 

 mercial meat extracts. Peptone preparations containing the higher 

 proportion of proteose nitrogen, mainly because of the stronger 

 buffering action, are to be preferred to preparations in which the 

 bulk of the nitrogen is in the form of peptones and amino acids. 

 The latter, however, because of their lower cost may be substi- 

 tuted for routine purposes. In different formulas the amount of 

 peptone varies from 0.1 to 2 per cent, depending upon the pro- 

 teose nitrogen content of the preparation. Felton and Dougherty 425 

 claimed that 2 per cent of peptone preserves and increases viru- 

 lence. 



Kruse and Pansini 763 first drew attention to the fact that certain 

 preparations of peptone hindered full growth, a problem later 

 studied by Wright 1549 in 1933, who pointed out the fact that one 

 of the difficulties in preparing suitable broth was largely due to in- 

 complete reduction of peptone. This obstacle could be overcome by 

 adding the peptone to the broth before heat is applied, thus ex- 

 posing it to the reducing action of meat or meat infusion during 

 the steaming process. The inhibitory action of peptone may be fur- 

 ther diminished by the reducing action of sugars and an alkaline 

 reaction (Wright). Dubos 3334 also found that some commercial 

 peptones contain bacteriostatic substances. These may be removed 

 by precipitation with acid and acetone, or their inhibitory action 

 may be neutralized by the addition of reduced thiol compounds 

 (0.0003 per cent thiogly collie acid). Reduced cysteine increases 

 the rate of growth. 



A small amount of sodium chloride, usually 0.5 per cent, ap- 

 pears to be a necessary ingredient, although sodium phosphate 

 may be substituted. An excess of salt retards or prevents the 

 growth of Pneumococcus (Wright 1548 ), and its concentration, 

 whether as chloride or phosphate, should not exceed 0.1 molar 

 (Dernby and Avery 313 ). Glucose stimulates and enhances initial 

 growth, and quantities ranging from 0.1 to 4.0 per cent have been 

 advised, while Turro 1430 preferred 8 per cent ! Amounts higher than 



