176 



The positive reactions in the distillates from the agar cul- 

 tures were not therefore due to this method giving different 

 results to those obtained with the direct reaction on the broth 

 cultures, but owed their origin to the fact that the strain was 

 impure or a mutation had taken place. 



These investigations into the formation of indol show that 

 in almost all cases Pfeiffer's bacillus is either distinctly indol- 

 positive or negative and that this property is not as a rule 

 influenced by cultivation for a long time or by fairly wide 

 variations* in the experimental conditions. They also show 

 that media containing trypsin-digested casein is specially 

 suitable for this reaction. The question of the connection 

 between indol formation and the other characters will be 

 discussed later. 



Agglutination. 



For the preparation of agglutinating sera rabbits were injected 

 intravenously with living cultures grown on haemoglobin or Fildes 

 agar for about 24 hours and then suspended in salt solution. As 

 a rule 5—7 injections were given in increasing doses at intervals 

 of 5 oi- 6 days. The rabbits were killed and the blood collected 

 7 or 8 days after the last injection. The sera were kept in a 

 frozen condition. 



Sera were prepared with the following strains, and were used 

 extensively in tbe agglutination experiments: 



Strain isolated in pure culture Serum prepared Titre (esti 

 I 6 July 1918 | 



mated several times) 

 800-1600 

 400 

 200 



400—800 

 200 

 200 



200—400 

 800 

 200 

 400 

 400 

 400 



400—800 

 200 



