PART V 
CHAPTER XVII 
PARASITIC BACTERIA 
RESISTANCE AGAINST PARASITIC BACTERIA 
We have learned that microérganisms 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 putrefac- 
tion, they destroy vinegar by consuming the acetic acid. In wines 
the growth of mischievous microdrganisms 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 microdrganisms 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, microérganisms show 
very different habits. Sometimes they become distributed over 
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