506 MANUAL OF MILK PRODUCTS 



absent. Long chains frequently occur and apparently vary 

 with different strains and conditions; pseudobranching has 

 been observed. Very long threads without apparent division 

 are frequently observed in old cultures. Living cells are gram 

 positive; dead cells are gram negative. 



Groivth on artificial media. — One of the most striking fea- 

 tures is its inability to grow on ordinary media. It grows on 

 whey, malt, and slowly on whey agar and certain specially 

 prepared media. The colonies on whey agar are masses of 

 tangled threads resembling colonies of the anthrax bacillus. 

 Gelatin is not liquefied. 



Relation to oxygen. — Most varieties grow equally well in 

 the presence or absence of oxygen. 



Temperature relations. — The maximum temperature is near 

 45° C. (113° F.). The minimum growth temperature varies 

 with different members of the group, but it is always compara- 

 tively high. Most varieties grow very slowly at 25° C. (77° F.), 

 but some grow at 20° C. (68° F.). Hastings and Hammer 

 state that at 20° C. (68° F.) it forms 4 per cent acid in milk 

 as compared with a maximum of 3 per cent at 37° C. (98° F.). 

 According to White and Avery it is killed by an exposure of 15 

 minutes at 60° C. (140° F.). 



Fermentation of sugars. — Many of the sugars are fermented, 

 but statements of different workers are conflicting. It is 

 probable that this property varies in different varieties. 



Milk. — The action of this organism on milk distinguishes 

 it from all other known bacteria. At the optimum tempera- 

 ture milk is curdled in a few hours with a rather soft curd, fre- 

 quently somewhat slimy, which ordinarily does not separate 

 from the whey even on long standing. In twenty-four hours 

 the milk may show acidity equivalent to nearly 2 per cent of 

 lactic acid, and on standing several days this may become 

 about 3 per cent. The most active of the ordinary lactic acid 

 bacteria seldom exceed 1 per cent lactic acid. The more 



