PART V. 



CHAPTER XVII. 

 PARASITIC BACTERIA. 



RESISTANCE AGAINST PARASITIC BACTERIA. 



We have learned that microorganisms may be both useful and 

 harmful. If they grow where they are wanted, they are useful; 

 but if they grow where they are not wanted, they produce many 

 undesirable effects. They spoil foods by causing their putrefaction, 

 they destroy vinegar by consuming the acetic acid. In wines the 

 growth of mischievous microorganisms causes a variety of bad 

 results that are sometimes spoken of as "diseases of wine," and we 

 also hear of "diseases of beer." But there is no good reason for the 

 use of the term here any more than in speaking of the diseases of 

 butter and cheese, when unusual bacteria cause them to ripen 

 abnormally. 



In most of the examples thus far studied the material upon which 

 the microorganisms grow has been supposed to be lifeless, the 

 bacteria existing as saprophytes. There remains the study of these 

 organisms when growing upon the living tissues of animals, thus 

 living the life of parasites. In the latter case they may do injury to 

 the animal or plant upon which they live, thus becoming pathogenic, 

 or disease germs. 



HOW MICROORGANISMS PRODUCE DISEASE. 



When they multiply inside the body, microorganisms show very 

 different habits. Sometimes they become distributed over the 

 whole body, located at no particular spot (blood poisoning), while in 

 other cases they may be definitely localized at some one place 



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