100 BACTERIOLOGY FOR NURSES 



ing cocci has an injurious effect on anthrax bacilli. On the other 

 hand, aerobic bacilli make possible the growth of anaerobes by ab- 

 sorbing free oxygen. Tetanus bacilli and their spores would be 

 less likely to develop in wounds were it not for the presence of 

 aerobic bacteria introduced with them. Blood infections are us- 

 ually due to one form of bacteria only. Even when several varie- 

 ties are introduced only one as a rule survives and multiplies. It 

 has been stated that the presence of one organism may increase 

 the virulence of another; for example, the scarlet fever virus is 

 said to favor the development of streptococci. It is rather more 

 probable that the factor favoring the growth of the latter organisms 

 is the reduced resistance of the tissues due to the poison produced 

 by the former. On the other hand, the products of certain bacteria 

 may rid the body of certain other forms. Pasteur was able with 

 attenuated chicken cholera cultures to produce immunity against 

 anthrax. The ingestion of soured milk with its enormous numbers 

 of lactic acid bacteria is advocated in order that a harmless variety 

 may crowd out in the intestines more dangerous organisms. 



Pathogenic Effects Produced by Bacteria. As already stated, 

 bacteria may be roughly divided into two classes: saprophytes 

 and parasites. No strict dividing line can be drawn since many 

 species may enter one or the other class according to conditions. 

 A similar statement applies to the terms pathogenic and non-patho- 

 genic. No known organism will under all circumstances produce 

 disease in all animals ; conversely, ordinary saprophytes may de- 

 velop both parasitic and pathogenic powers when the body resist- 

 ance is sufficiently reduced by fatigue, exposure, or another infec- 

 tion. Again an organism that is highly pathogenic for one animal 

 may be quite harmless for another. The terms then are relative. 

 A microorganism is pathogenic only when the defenses of the body 

 are not strong enough to resist it. 



Bacteria may be termed pathogenic when they produce one 

 or more of the following conditions: (1) mechanical injury to the 

 tissues; (2) disintegration of tissues to furnish themselves with 

 food, or (3) irritation and destruction of tissues by poisons. 



Bacteria possessing, great vitality may, as already stated, pass 



