288 Life and Health 



lac t is act upon the milk sugar present in milk and con- 

 vert it into lactic acid, thus bringing about the souring 

 of milk. 



447, The Place of Bacteria in the Economy of Nature. 

 The myriads of bacteria, with their marvelous rapidity of 

 propagation, would devastate the earth were it not for the 

 winds, rains, melting snow and ice which scatter them far 

 and wide and destroy them. 



Like countless other species of living organisms, bacteria 

 are subject to the relentless law which allows only the fittest 

 to survive. The bacteria of higher and more complex types 

 devour those of a lower type. Myriads perish in the diges- 

 tive tract of man and other animals. The excreta of some 

 species of bacteria act as poison to destroy other species. 



448. Disease-Producing Bacteria. Now, while many spe- 

 cies of bacteria are harmless, some may be the cause of 

 sickness and death when they gain admittance to the body 



under certain con- 

 ditions. These 

 disease-producing, or 

 pathogenic, bacteria, 

 when established in 



FIG. 157 .^Two Forms of Pathogenic Bacteria. the tlsSUeS f the 



(Drawn from phbtographs.) b d y> ma 7 Pduce 



A, spiral form of bacteria found in cholera. (Magni- a particular disease. 



fied about 1000 diameters.) B, rod-shaped bacteria It has been proved 



(called bacilli) from a culture obtained in anthrax, -\ -, -,-, -, , . 



or malignant pustule, of the face. Diseased hides beyond all ^ doilbt 



carry this micro-organism and thus may occasion that Certain dis- 



disease among those who handle hides and wool. ' e<; arp nrOf li]red 



(Magnified about 1000 diameters.) 



through the agency 



of bacteria. In yellow fever, diphtheria, typhoid fever, and 

 consumption the causal relation of a particular kind of bac- 

 teria to each disease has been definitely established. 



