570 Introduction to the Study of Science 



IV. BACTERIA 



283. Nature and distribution. Frequent reference has 

 been made to the action of the microscopic plants, called bacteria, 

 in connection with the putrefaction of sewage, with their growth 

 upon the roots of leguminous plants, with their work in making 

 bread dough sour, and in converting alcoholic solutions into 

 acetic acid or vinegar. These are only a few instances to suggest 

 the extent and abundance of bacterial plant life. Like molds 

 and yeasts, bacteria are found to be universal, for they begin 

 growth wherever conditions are favorable. They are found in 

 the air, soil, water, on fruit and vegetables, and in the bodies, 

 tissues, and circulatory fluids of animals and men. That they 

 are abundant may be inferred from the countless number found 

 in a very small amount of decaying plant or animal matter, 

 and from the fact that putrefaction and decay, which are the 

 work of bacteria, are practically everywhere and at all times. 

 Every household furnishes evidence of their presence, and 

 every housekeeper, while perhaps not aware of the nature of the 

 enemies against which war is waged, devotes thought and time 

 to the preservation of food from their action. To defeat or 

 destroy bacteria is the purpose of most of the labor involved in 

 the refrigerating, cooking, canning, smoking, drying, and salting 

 of food materials. 



284. Bacteria and disease. We are familiar with the 

 statement that microbes, germs, or bacteria cause certain 

 diseases. The disease-causing bacteria have been the special 

 subject of scientific study, with the result that it is now well 

 established that many infectious diseases are due to bacterial 

 activities and products. The species of bacteria which cause 

 disease, and are classed therefore as pathogenic, are com- 

 paratively few ; and they are included among the parasites, of 

 which certain fungi or molds have been mentioned (page 554). 

 Of many infectious diseases due to parasitic bacteria the most 

 common are tuberculosis, pneumonia, diphtheria, typhoid fever, 



