196 



PASTEURISED OR HEATED MILK 



tures may be Gram negative. Spore formation is never ob- 

 served and they ferment carbohydrates with the production of 

 acid but do not form gas. Milk coagulation is produced by some 

 members of the group and not by others. 



For the isolation of this group there is no better method 

 than that used by Heyman in 1898, viz., the use of a meat pep- 

 tone broth containing 2 per cent dextrose and 0.3 per cent 

 acetic acid. After incubation at 37° C. for forty-eight hours, 

 a portion of the culture is seeded into another broth tube and 

 the process repeated until only aciduric bacilli remain. For 

 further isolation dextrose agar containing 1.5 per cent agar 

 and 2 per cent dextrose without any adjustment of the acidity 

 may be used. According to Rahe ^ the addition of 0.2 per cent 

 of sodium oleate as recommended by Salge ^^ is productive of 

 good results. By this method Rahe {vide supra) investigated 

 a number of the aciduric bacteria, and divided them into three 

 groups according to their biochemical properties. 



Group A, which is the B. bulgaricus group, is characterised 

 by a rapid clotting of milk and its usual inability to ferment 

 carbohydrates other than lactose and dextrose. 



Group B also clots milk but ferments maltose, saccharose, 

 and laevulose in addition to lactose and dextrose, and usually 

 also mannite and raffinose. 



Group C does not clot milk and ferments maltose even 

 more vigorously than group B. Saccharose and Isevulose are 

 fermented and usually raffinose, but mannite is not acted upon. 



