CLASSIFICATION OF BACTERIA. 23 



i. Immunization with a Living Virus. 



This method of immunization simulates most closely the immunity 

 attained spontaneously in overcoming an infection. Although this im- 

 munity is very strong, and lasts for a long period of time, its disadvantages 

 lie in that it is attained with difficulty; frequently the dose of virus injected 

 causes serious symptoms of infection. Various procedures have therefore 

 been advocated to so diminish the toxicity of the immunizing agent that only 

 immunization effects, anoT^no toxic symptoms be obtained. This was 

 attempted either by the reduction of the number of organisms employed, so 

 that very minute doses w r ere inoculated, or by the diminution of the infec- 

 tious nature of these bacteria (virulence so called) . 



The first method, however, was *not found applicable to all ca-ses. The 

 infectious nature of the different bacteria varies markedly. The same 

 bacterium reacts differently with different animals. While some animals 

 possess a natural immunity against certain bacteria, others exhibit a dis- 

 tinct susceptibility to the same micro-organisms. The conceptions there- 

 fore of pathogenicity and virulence are purely of a relative nature. In talking 

 of the pathogenicity of bacteria, one should always mention the class of animal 

 for which these bacterii are pathogenic. 



Bail has used this principle of pathogenicity in classifying bacteria. He 

 Bail's Classi- mentions the following three classes: 

 fication of a. Saprophytes. 



Bacteria. b. Half or partial parasites. 

 c. Whole or pure parasites. 



To the class of saprophytes belong all those bacteria which when injected even in 

 larger doses do not produce any characteristic disease; these are also known as apatho- 

 genic e.g., hen cholera bacilli for human beings. 



Classed as half parasites are those bacteria, according to Bail, the infectious nature 

 of which depends upon the quantity of bacteria injected. While the injection of a 

 rabbit with i/iooo of a loopful of a typhoid culture will produce no evidences of disease, 

 one-tenth of a loopful will result in slight increase in temperature, loss of appetite, and 

 eventually a local redness at the site of the injection. One loopful may bring about 

 the death of the animal. The manifestations are dependent entirely upon the number 

 of bacteria injected. The smaller the number, the milder the symptoms, until one 

 reaches the stage below which no disturbances at all are visible. 



Pure parasites are those which have no subletal dose. Even the smallest amount, 

 when injected, will produce death. As examples, the tubercle bacillus for guinea-pigs, 

 and bacilli belonging to the group of Hemorrhagic Septicemia for rabbits. Of the last 

 mentioned 1/10,000,000,000 of a loopful of some cultures kills a rabbit within twenty- 

 four hours with the symptoms of a septicemia; in other words, the injection of i c.c. of a 

 dilution of one loopful of culture in ten million liters of water suffices to kill the rabbit. 

 Furthermore, the number of bacteria increases so greatly in the body of the rabbit that 

 numerous bacteria can be demonstrated in every drop of blood and in all organs and 

 body fluids. 



The same organism is a saprophyte for the human being and a half parasite for the 

 guinea-pig if injected subcutaneously and a complete parasite by intraperitoneal injection. 

 The conceptions therefore of complete or partial parasite as well as of saprophyte are only 

 relative and are dependent upon the bacteria, the animal species, and the mode of infection. 



