MICROORGANISMS AS A CAUSE OF DISEASE 185 



anaerobes, such as Bacillus putrificus, are sometimes found in large 

 numbers. Amebse and certain other protozoan types are fre- 

 quently found. The intestinal juices, particularly the bile, in- 

 hibit the growth of some species, but the normal inhabitants of 

 the digestive tract will grow and multiply in pure bile. The 

 number of bacteria increases from the stomach on, the greatest 

 numbers being found in the colon. Here the living bacterial 

 cells are frequently present by hundreds of millions to the gram. 

 It has been estimated that human feces sometimes contain as 

 much as 38 per cent, of their bulk in bacteria. The bulk in 

 herbivorous animals is certainly much lower. Some of these 

 bacteria are harmless commensals, but a few are believed sometimes 

 to assume pathogenic roles. It has been claimed that in the case 

 of herbivora certain of these bacterial cells secrete cellulase and 

 assist materially in the digestion of cellulose. This has not been 

 satisfactorily demonstrated, but is not improbable. Whether or 

 not bacteria in the intestines are essential to the maintenance of 

 health is a mooted question. Experimental evidence is conflict- 

 ing. Practically, infection of the intestinal tract occurs very early 

 in life. Metchnikoff and his school have claimed that in man some 

 of the symptoms of old age, such as arteriosclerosis, are due to the 

 absorption of poisonous products of bacterial putrefaction, and 

 has sought to establish an intestinal flora consisting of non-putre- 

 factive types. 



Bacteria of the Organs of Respiration. Air entering the nose 

 is rapidly freed from dust and bacteria by contact with the moist 

 surfaces of the lining membranes, consequently the nose may have 

 many types of organisms present. It is a most efficient filtering 

 device, so that very few bacteria gain entrance to the trachea and 

 the lungs, and these are generally quickly eliminated. 



Bacteria of the Genito-urinary Organs. The secretions of the 

 vagina seem effectually to prevent bacterial growth. The uterus 

 is normally bacteria free, as is also the bladder. The urethra 

 contains few organisms. 



Avenues of Infection. The avenue through which an organism 

 gains entrance to the body is called its portal of entry or its infec- 

 tion atrium. An infection arising from contact with infective 

 external objects is termed exogenous; one caused by organisms 



