VIKOLOGICAL METHODS 251 



taining one's own colony has much to recommend it. Even when a 

 colony is maintained locally, certain pitfalls must be guarded against. 

 It should be remembered that wild rodents, usually mice, if given the 

 opportunity may carry diseases to your stock. Feeds not properly 

 stored can become contaminated with excretions from wild mice. Carry- 

 ing cages returned to the colony room without first having been sterilized 

 may bring infection to the stock animals. It becomes apparent that 

 maintaining a colony of animals gives control over many factors, but 

 good judgment must still be exercised in their handling. 



Inoculum. In general the routes and methods of inoculation described 

 in Chap. X suffice where viruses and rickettsiae are concerned. There 

 are, however, some exceptions and additions that should be noted. 



The investigator of viral and rickettsial diseases cannot be too careful 

 about the inoculum to be used for the animals. Several precautions are 

 necessary in this regard. While laboratory animals under certain condi- 

 tions can resist the introduction of a few bacteria of mediocre patho- 

 genicity or a considerable number of nonpathogenic bacteria, usually it is 

 wise to introduce no bacteria along with the viral or rickettsial agents. 

 Under certain conditions the animals are inoculated with an exudate con- 

 taining bacteria as well as viruses, and the animals succeed in separat- 

 ing the virus by overcoming the bacteria. It can be readily understood, 

 however, that bacteria may overgrow the virus, causing failure in the 

 attempt to propagate the virus. It should become almost second nature 

 to the investigator to culture the inoculum aerobically and anaerobi- 

 cally to check its bacterial sterility. Bacterially contaminated, viral 

 preparations inoculated intracranially will almost invariably result in the 

 bacterial infection predominating. 



Various exudates containing bacteria may be received in the viro- 

 logical laboratory, and occasionally pure cultures of viruses or rickettsiae 

 become bacterially contaminated. There are several methods for remov- 

 ing the bacterial contaminants. Differential centrifugation at speeds 

 that will sediment them but not the virus will permit the removal of msiny 

 bacteria. Almost never will this method render the material free from 

 bacteria. Filtering the specimen through a filter known to be fine enough 

 to remove the bacteria but not so fine as to remove the viruses or rickettsiae 

 may be the only successful method available. Where larger viruses and 

 rickettsiae are concerned, this method may fail. Not uncommonly 

 one sacrifices half the virus content or more because virus adheres to the 

 filter. Filtration will not remove bacterial protoplasm which has gone 

 into solution. For example, a bacteriologically sterile filtrate may pro- 

 duce toxic manifestations w^hen introduced intranasally into mice because 

 of soluble bacterial products rather than the accompanying virus or its 

 products. 



