Ii6 PATHOGENIC BACTERIA. 



era spirillum and plague bacillus, and Wright, 1 of 

 typhoid fever bacilli, in vaccinating against the respec- 

 tive diseases. 



In his experiments upon rabies, Pasteur 2 found that 

 drying the tissues in which they were enclosed was a 

 satisfactory method of attenuating the germs of hydro- 

 phobia, and that after a certain period of exposure to a 

 dehydrating substance they ceased to be pathogenic, 

 though their inoculation was succeeded by immunity. 



Thus, from the original observations in which the cow 

 was the important factor, we now reach a time when 

 vaccines, viruses, or attenuated cultures are prepared in 

 the laboratory by a variety of methods. Some of the 

 germs are dried, some heated, some grown upon media 

 containing antiseptics, some are deprived of their spore- 

 producing capacity, some washed free of their toxic prod- 

 ucts, some are combined with bacteria of other species, 

 some entirely killed in order that desired results shall fol- 

 low from their inoculation, and sometimes the essential 

 toxin is separated from the culture and used for immuniz- 

 ing purposes, all these substances being known as vac- 

 cines. 



It is particularly interesting to observe that dead cult- 

 ures can be made use of for the production of immunity, 

 probably because of the toxin which they contain. 



In some cases saprophytic bacteria may be made use 

 of in producing immunity or increasing resistance to dis- 

 ease. Hueppe and Wood claim that inoculation with 

 saprophytic bacteria derived from water and the soil may 

 protect animals against the pathogenic bacteria, and are 

 said to have produced immunity to anthrax in this man- 

 ner. Pawlowski has also found that the influence of one 

 bacterium upon another, or the influence of one bacte- 

 rium upon an animal infected with another, sometimes 

 affords a protection similar to that of vaccination. Thus, 

 he asserts that if rabbits are infected with anthrax, to 



1 Brit. Med. Journ., Jan. 30, 1897, p. 256. 



2 Compte-rendu de la Soc. Biol, de Paris, 1S89, cviii., p. 1228. 



