CLASSIFICATION OF BACTERIA 



I. Immunization with a Living Virus. 



2 3 



This method of immunization simulates most closely the immunity 

 attained spontaneously in overcoming an infection. This immunity 

 is very strong and lasts for a long period of time, but it is attained with 

 difficulty; frequently the dose of virus injected causes serious symptoms of 

 infection. Various procedures have therefore been advocated so to di- 

 minish the toxicity of the immunizing agent that only immunization 

 effects, and no toxic symptoms be obtained. This was attempted either 

 by the reduction of the number of organisms employed, so that very 

 minute doses were inoculated, or by the diminution of the infectious 

 nature of these bacteria (virulence so called). 



The first method, however, was not found applicable to all cases. 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 

 distinct susceptibility to the same micro-organisms. The conceptions 

 therefore 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 bacteria 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 sublethal 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.cm. 

 of a dilution of , one loopful of culture in ten million liters of water suffices to kill the 

 rabbit. Furthermore, the bacteria increase so greatly in the body of the rabbit that 

 they 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 injec- 



