670 GASTRO-INTESTINAL BACTERIOLOGY 
Significance of Intestinal Bacteria.— The striking differences in 
morphology, chemistry and in cnltural characters between the intes- 
tinal floras characteristic respectively of nurslings, artificially-fed 
infants and adults suggest at once that nutritional stimuli may be an 
important factor in determining the dominance of types of bacteria. 
An intestinal flora does not appear to be essential for the well-being 
of mammals in the Arctic regions; Levin^ 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,^ turtles,^ tadpoles^ and guinea-pigs'^ 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.*^ 
The normal intestinal flora may be regarded as intestinal parasites 
just as the various bacteria which occur commonly in the skin are 
regarded as cutaneous parasites. It is important to realize that the 
normal intestinal organisms, like the cutaneous organisms, are "oppor- 
tunists," potentially capable of becoming invasive whenever the bar- 
riers which ordinarily suffice to limit their development to the lumen 
of the alimentary canal become impaired, giving rise to endogenous 
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^ 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. Pasteur, ISQO, 13, 558: Skandinaviscl:K?s Arch. f. Physiol., 1904, 16, 249. 
•- Schottehus: Arch. f. Hyg., 1902, 42, 48. 
3 Moro: Jahrb. f. Kinderheilk., 1905, 12, 467. 
4 Metchnikoff: Ann. Inst. Pasteur", 1901, 15, 631. 
6 Nuttall and Thicrfeldcr: Ztschr. f. phvsiol. Chem., 1895, 21, 109; 1896, 22, 62; 
1897, 23, 231. 
« Hilgermann (Arch. f. Hyg., 1905, 54, 335) and others have produced experimental 
evidence in favor of the view that the immature intestinal epithelium 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. 
' Centralbl. f. Bakteriol., 1887, 2, 633; also Jahrb. f, Kinderheilk., 1900, 52, 1. 
