THE GASTRO-INTESTINAL FLORA OF NORMAL INFANTS 589 



of mammals in the Arctic regions; Levin 1 has found that the feces 

 of polar bears are practically sterile. It must be remembered, however, 

 that similar animals kept in captivity in more temperate climates 

 exhibit a very definite intestinal and fecal flora. Attempts to rear 

 chicks, 2 turtles, 3 tadpoles 4 and guinea-pigs 5 in a sterile environment 

 have not added materially to available knowledge of the physiological 

 significance of the intestinal flora, partly because the rigorous con- 

 ditions under which such observations must be made interfere greatly 

 with the normality of the animals' environment. It is probable that 

 the significance of the intestinal flora lies rather in its potential antag- 

 onism to alien bacteria which certainly gain entrance to the alimentary 

 canal from time to time, than in any specific participation in the 

 normal digestive process of the host. 6 



The normal intestinal flora may be regarded as intestinal parasites 

 just as the various bacteria which occur commonly on the skin are 

 regarded as cutaneous parasites. It is important to realize that the 

 normal intestinal organisms, like the cutaneous organisms, are 

 "opportunists," potentially capable of becoming invasive whenever 

 the barriers which ordinarily suffice to limit their development to the 

 lumen of the alimentary canal become impaired, giving rise to endo- 

 genous infections. 



Unlike the cutaneous parasitic flora or that of other surfaces of the 

 body which does not appear to vary materially from infant to adult 

 life, the intestinal flora changes in a most definite and striking manner 

 as the individual develops from infancy to senescence. This change 

 does not appear to depend fundamentally upon bacteria ingested 

 with the food, for Escherich 7 and many others have shown that steri- 

 lization of the food does not cause a noteworthy reduction in the 

 number of types of fecal bacteria in young children. 



The most important normal factor in determining the intestinal 



1 Ann. Inst. Past., 1899, xiii, 558; Skandinavisches Arch. f. Physiol., 1904, xvi, 249. 



2 Schottelius, Arch. f. Hyg., 1902, xlii, 48. 



3 Moro, Jahrb. f. Kinderheilk., 1905, xii, 467. 



4 Metchnikoff, Ann. Inst. Past., 1901, xv, 361. 



5 Nuttall and Thierfeldef, Ztschr. f. physiol. Chem., 1895, xxi, 109; 1896, xxii, 62; 

 1897, xxiii, 231. 



6 Hilgermann (Arch. f. Hyg., 1905, liv, 335) and others have produced experimental 

 evidence in favor of the view that the immature intestinal tract of the young infant is 

 more permeable to bacteria than that of adolescents and adults. It may be inferred 

 from these observations that the normal nursling intestinal flora is somewhat protec- 

 tive in its relation to the host, in that the normal fermentative activities of the organisms 

 comprising the intestinal flora create conditions throughout the alimentary canal which 

 are inimical to the development of alien proteolytic and fermentative bacteria. 



7 Centralbl. f. Bakt., 1887, ii, 633; also, Jahrb. f. Kinderheilk., 1900, lii, 1. 



