BIOLOGY OF CLOSTRIDIUM WELCHII 403 



vation on the thermal death point of vegetative forms of Clos- 

 tridium Welchii. Of 20 strains isolated from market milk and 

 tested after four months' cultivation on artificial media, all were 

 negative at 61°, two survived 60°, nine survived 59° and all 

 survived 58°. Of 24 strains from market milk grown for three 

 months all failed to survive 60°, two survived 59°, 14 survived 

 58° and all survived 57°. These cultures were kept in the spore 

 form in peptone egg for a month and a half before testing, whereas 

 the first 20 strains were transferred daily or every forty-eight hours 

 to fresh sugar media during the four months. Twenty-five freshly 

 isolated strains grown for two weeks gave the following results : — 

 all survived 56°, 18 survived 57° and eight survived 58°. All 

 cultures after four months survived 58°, after three months 57°, 

 while the corresponding value for freshly isolated strains was 

 only 56°. 



Of the spore forms seven out of 20 (35 per cent) failed to survive 

 boiling after four months' growth, eight out of 24 (33| per cent) 

 failed to survive boiling after three months' growth and 15 out 

 of 25 (60 per cent) after two weeks' growth. 



f . Relation of age of cultures and kind of medium to thermal 

 death point. Tables 5 and 6 show the greater resistance to heat 

 of cultures from milk and feces when grown in glucose-liver broth 

 for more than eight hours before testing. In 13 out of 20 cultures 

 from market milk, growth for twenty-four hours increases the 

 thermal death point 1°. In the other cases no difference was 

 observed under identical conditions. In 19 cultures from human 

 feces, growth for forty-eight hours increased the thermal death 

 point 1° in every case. 



Table 5 gives also the results of 19 cultures grown in sterile 

 milk for sixty-six hours in comparison to the growth in glucose- 

 liver broth and shows variations in resistance to heat. Seven 

 of the cultures had the same thermal death point as in the eight- 

 hour glucose-liver broth cultures, six cultures resisted 1°C. 

 higher while the remaining six survived a rise of 2°C. above the 

 eight-hour cultures Probably cultures isolated from feces and 

 grown in sterile milk for eight hours approximate the same 

 resistance to heat as the milk cultures themselves. 



