38 INFECTIONS OF THE DIGESTIVE TRACT 



disposal a rough means of differentiation which is often 

 of great service in the microscopical identification (in 

 so far as this is possible) of bacterial types. If this 

 method of study be supplemented by making aerobic and 

 anaerobic cultures on gelatin plates, sugar-agar plates, 

 and blood-agar plates (with or without sugar), one ob- 

 tains a fair idea of the really representative bacteria 

 present in the alimentary tract. Some of the bacteria 

 present may, however, fail to grow on any of these media. 

 Gram-stained microscopical fields prepared from the 

 normal slightly acid, orange-yellow, rather soft fa3ces of 

 a nursling child from any portion of the large intestine, 

 present essentially the same characteristic appearances. 

 In the first place, the bacteria are seen to be extremely 

 abundant more so than in the f seces of normal adults 

 and very much more so than in the meconium. A very 

 large proportion of them are Gram-positive. The 

 great majority of these Gram-positive bacteria present 

 the same appearances and may give to a field the appear- 

 ance of consisting of a nearly pure culture of one bac- 

 terial type. These bacteria are slender, often slightly 

 curved bacilli of moderate size. They constitute the 

 characteristic microorganisms of the human nursling's 

 digestive tract and have been named (for reasons which 

 will later appear) B. bifidus by Tissier, who first described 

 them and took them out of the group of colon bacilli, 

 with which they had been grouped by Escherich. 

 Mixed in with B.bifidus is another Gram-positive bacillus, 

 which is closely related, but grows readily on a strongly 

 acid medium and has hence been designated B. acidophi- 



