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PRINCIPLES OF BACTERIOLOGY 



2. Parasites, with possibility of saprophytic growth. These are 

 bacteria which can develop either as parasites or saprophytes. The 

 different varieties vary as to the amount of poison which they produce. 

 Some grow luxuriantly in dead organic material under very diverse 

 conditions, others only under specially favorable conditions. In the 

 body they also vary some grow extensively in the blood, while others 

 are 'limited to one or more tissues, some being widely disseminated 

 throughout the body, while others are localized in or upon a certain 

 portion of it. 



3. Strict parasites, or bacteria which, so far as we know, grow only 

 in the living animal or vegetable organism. These again vary in the 

 amount of poison which they produce and in the local or general infec- 

 tion to which they give rise. 



Adaptation of Bacteria to the Soil upon which They Are Grown. Those 

 bacteria which grow both in living and dead substances vary from time 

 to time as to their readiness to develop in either the one or the other. 

 As a general rule, bacteria grown in any one medium become more and 

 more accustomed to that and other media more or less analogous to it, 

 while, on the other hand, they are less easily cultivated on media widely 

 different from that in which they have developed. Thus we have a 

 culture of tubercle bacilli which, after having grown for three years in 

 the bodies of guinea-pigs, will no longer develop on dead organic matter, 

 while a bacillus which was obtained from the same stock, but grown 

 on bouillon for three years, will no longer increase in the animal body. 

 From the same stock, therefore, two varieties have developed, the one 

 being now practically a saprophyte and the other a parasite. 



Local Effects Produced by Bacteria and Their Products. After the bac- 

 teria gains entrance to a suitable part of the body and find conditions 

 favorable for growth there is a certain lapse of time before sufficient 

 bacterial poisons have accumulated to cause by their action on the tissue 

 noticeable disturbance. This is called the period of incubation. Its 

 length depends on the amount, kind, and virulence of the micro-organ- 

 isms introduced and the tissue invaded. The incubation period over, 

 we note the course of the local and general lesions excited by the specific 

 and general poisons. The extent to which this will progress depends, 

 on the one hand, on the characteristics of the invading micro-organisms; 

 on the other, on the characteristics of the tissues invaded. 



The local effects of the bacterial poisons upon the cells give rise to 

 the various kinds of inflammation, such as serous, fibrin ous, purulent, 

 croupous, hemorrhagic, necrotic, and gangrenous, and also prolifera- 

 tive, as seen, for example, in leprosy. Some bacteria incite specific 

 forms of inflammation along with those common to many bacteria; 

 others produce no peculiar form of lesions. 



Thus inflammation and serous exudation into the subcutaneous tis- 

 sues follow injections of the pneumococcus or anthrax bacillus. The 

 development of the streptococcus or pneumococcus in the endocardium 

 or pleural cavity is followed by a serous exudation, frequently with 

 more or less fibrin production. The formation of pus results more 



