68 PATHOGENIC BACTERIA. 



desirable to secure an attenuated culture considerable 

 pains must be taken to cultivate it at certain maximum 

 temperatures or in media containing small amounts of 

 antiseptic substances. 



When the virulence has been destroyed by experi- 

 mental manipulations or by natural attenuation, it is 

 sometimes difficult, sometimes impossible, to secure its 

 return. The method usually adopted is that of rapid 

 passage from animal to animal of the same kind, then to 

 more resistant kinds, hoping to accustom it to life in the 

 animal once more. It may be necessary to kill the 

 animal to do this, inoculating beneath the skin or into 

 the peritoneum, allowing some hours or a day for some 

 of the bacteria to grow, killing the animal, and transfer- 

 ring the secretions containing the survived bacteria to 

 another animal, and so on. In this manner there is an 

 artificial selection carried on by which there are secured 

 from each animal those bacteria best qualified to live in 

 that kind of animal. 



Another method frequently employed for exalting the 

 virulence of bacteria is that of growing them in collo- 

 dion sacs placed in the abdominal or other cavity of the 

 body, or beneath the skin, where they receive by osmosis 

 the fluids of the body and become accustomed to them. 



Muir and Ritchie state that attenuated diphtheria cult- 

 ures may have their virulence raised by being injected 

 into an animal together with the Streptococcus pyogenes; 

 an attenuated culture of the bacillus of malignant edema 

 by being injected with the Bacillus prodigiosus; an atten- 

 uated streptococcus by being injected with the Bacillus 

 coli communis, etc. A culture of typhoid fever bacillus 

 may also be increased in virulence by being injected along 

 with a dead culture of the Bacillus coli communis. 



These observations call attention to the results of mixed 

 infections. While we are so trained by our laboratory ex- 

 periments and our bacteriologic reading that we think of 

 streptococcus, diphtheria, and tetanus infection as being 

 caused by the streptococcus, diphtheria, and tetanus 



